anita bean-the complete guide to sports nutrition (complete guides) -a & c black publishers ltd...

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Other titles in THE COMPLETE GUIDE series

The Complete Guide toEndurance Trainingby Jon Ackland

The Complete Guide toPostural Trainingby Kesh Patel

The Complete Guide toCore Stabilityby Matt Lawrence

The Complete Guide toStudio Cyclingby Rick Kiddle

The Complete Guide toExercise to Musicby Debbie Lawrence

The Complete Guide toPostnatal Fitnessby Judy DiFiore

The Complete Guide toStrength Trainingby Anita Bean

The Complete Guide toStretchingby Christopher Norris

The Complete Guide toSports Massageby Tim Paine

The Complete Guide toExercising Away Stressby Debbie Lawrence

The Complete Guide toSports Motivationby Ken Hodge

The Complete Guide toCircuit Trainingby Debbie Lawrence, Bob Hope

A & C Black LondonTHE

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SPORTSNUTRITION

6th

edition

Anita Bean

Published by A & C Black Publishers Ltd36 Soho Square, London W1D 3QYwww.acblack.com

Sixth edition 2009; reprinted 2010Fifth edition 2006Fourth edition 2003Third edition 2000; reprinted 2001Second edition 1996; reprinted 1997 (twice), 1998, 1999, 2000First edition 1993; reprinted 1994, 1995

Copyright 2009, 2006, 2003, 2000, 1996, 1993 Anita Bean

ISBN 978 14081 0538 2

All rights reserved. No part of this publication may be reproduced in any form or by any means graphic, electronic or mechanical, including photocopying, recording, taping or information storage and retrieval systems without the prior permission in writing of the publishers.

Anita Bean has asserted her right under the Copyright, Design and Patents Act, 1988, to be identified as the authorof this work.

A CIP catalogue record for this book is available from the British Library.

Cover photograph courtesy of Steve Baccon/Getty ImagesPhotographs ShutterstockAuthor photograph Grant Pritchard

A & C Black uses paper produced with elemental chlorine-free pulp, harvested from managed sustainable forests.

Typeset in 1012 on 12pt Baskerville BE Regular by Palimpsest Book Production Ltd, Grangemouth, Stirlingshire.

Printed and bound in China byC&C Offset Printing Co.

NoteWhilst every effort has been made to ensure that thecontent of this book is as technically accurate and as soundas possible, neither the author nor the publishers canaccept responsibility for any injury or loss sustained as aresult of the use of this material

CONTENTS

Acknowledgements viForeword viiPreface to the sixth edition viii

1 An Overview of Sports Nutrition 12 Energy for exercise 73 Fuelling before, during and after exercise 214 Protein requirements for sport 445 Vitamins and minerals 536 Sports supplements 647 Hydration 858 Fat: body fat and dietary fat 1019 Weight loss 117

10 Weight gain 13911 The female athlete 14512 The young athlete 15913 The vegetarian athlete 18014 Competition nutrition 18815 Your personal nutrition programme 19816 The recipes 237

APPENDICES

1 The glycaemic index and glycaemic load 2552 Glossary of vitamins and minerals 259

List of abbreviations 272List of weights and measures 272References 273Further reading 289Useful addresses 290On-line resources 291Index 292

Many people have contributed directly andindirectly to this book. These include the manysportspeople, coaches and scientists whom I havehad the privilege to meet and work with over theyears. They have provided me with inspiration,knowledge and precious insights into sport.I value their suggestions, comments and honesty.

I would also like to thank Simon, myhusband, for his patience; and Chloe and Lucymy two wonderful (and sporty) daughters formaking me believe that anything is possible.

Finally, this book would not have beenwritten without the vision and enthusiasm of theeditorial team at A & C Black. I am grateful fortheir diligence and support over the last 16 years.

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ACKNOWLEDGEMENTS

I know from first hand experience just howimportant good nutrition is for sports per -formance. Its always been a crucial part of mytraining strategy and has, undoubtedly, helpedme achieve the success Ive enjoyed. Ive learnedover the years that I have to fuel my bodyproperly otherwise I wouldnt have the energyor the strength to push my body throughgruelling workouts and races.

My biggest nutritional challenge has alwaysbeen eating enough food. In training, I burn50006000 calories a day, which is a vastamount of food! And definitely not easy to fit inaround training and everything else. Ive workedout often through trial and error how much Ihave to eat, the right times to eat and which arethe best foods for fast recovery.

There are so many things to think aboutbefore a big race but, for me, nutrition is right upthere near the top. I have to plan what Im goingto eat and drink beforehand and make sure Ihave the right amounts of carbs, protein and fats.Its not always easy, especially when Imtravelling or competing in other countries I

have to check beforehand that Ill be able to getall the food and drink I need.

Thats why this book is such a useful resourceto me. It explains clearly and concisely thescience of nutrition for sport. Its helped me withmy training and competitions. And its answeredloads of questions Ive had about my diet. Anitahas managed to make a complex subjectaccessible and exciting.

Her advice is accurate and, importantly, itsalso realistic and achievable. So its hardlysurprising that, since it was first published in1993, The Complete Guide to Sports Nutrition hasbecome the top-selling book on sports nutritionin the UK. I would thoroughly recommend it toanyone who wants to get more out of their sport.Ive learned a lot from this book and Imconfident it will help you, whether youre justtraining for fitness or getting ready for the nextOlympics.

James Cracknell OBE, MSc,British international rowing double

Olympic champion and world record holder.

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FOREWORD

viii

I am delighted to be able to tell you that, sincethe first edition was published in 1993, theComplete Guide to Sports Nutrition has become abestselling book in its field. It is a long-standingrecommended text on many higher educationcourses, and is frequently quoted in the media.My intention has always been to cut throughthe hype and provide sound advice thatsportspeople can follow. You wont get lost withtechnical jargon!

As more studies are carried out, ourunderstanding of athletes nutritional needsgrows. In this edition, you will find the mostup-to-date and practical advice on sportsnutrition. It includes new recommendations onhydration, carbohydrate intake and the use ofsports supplements. It also provides referencesfor the studies cited in the text so you mayobtain more detailed information on particulartopics if you wish.

In the last fifteen years, Ive receivedcountless emails and letters from ordinarypeople as well as competitive athletes runners, weight lifters, personal trainers,cyclists, triathletes, rugby players, footballers,

sports coaches, swimmers who havefollowed the dietary advice in my book,improved their personal bests and then wonraces or matches they had never thoughtpossible. Many readers have thanked me forsaving them money by advising against buyingsupplements that, despite the serious claims,dont work.

During my competitive years as a naturalbodybuilder (I won the British championshipsin 1991), I experienced first hand the challengesof combining eating, training and resting. Thisis never easy, but hopefully I have managed topass on some of my experience to you in thisbook. Nowadays I practise Ashtanga yoga, aswell as swim, walk and run to keep fit. Needlessto say, I stick to a healthy diet!

Read this book from cover to cover or dipinto the sections that interest you most. Ibelieve that this sixth edition brings you themost complete guide to sports nutrition yet!

Anita Bean September 2008

PREFACE TO THE SIXTH EDITION

There is universal scientific consensus that dietaffects performance. A well-planned eatingstrategy will help support any trainingprogramme, whether you are training forfitness or for competition; promote efficientrecovery between workouts; reduce the risk ofillness or overtraining, and help you to achieveyour best performance.

Of course, everyone has different nutritionalneeds and there is no single diet that fits all.Some athletes require more calories, protein orvitamins than others; and each sport has itsunique nutritional demands. But it is possible tofind broad scientific agreement as to whatconstitutes a healthy diet for sport generally.The following guidelines are based on theconclusions of the 2003 International OlympicCommittee (IOC) Consensus Conference onNutrition and Sport and the 2007 consensusstatement of the International Association ofAthletic Federations (IAAF).

1. CALORIES

Your daily calorie needs will depend on yourgenetic make-up, age, weight, body composition,your daily activity and your training programme.It is possible to estimate the number of caloriesyou need daily from your body weight (BW)and your level of daily physical activity.

Step 1: Estimate your basal metabolic rate (BMR)

As a rule of thumb, BMR uses 22 calories forevery kg of a womans body weight and 24calories per kg of a mans body weight.

Women: BMR = weight in kg x 22Men: BMR = weight in kg x 24

For a more accurate method for calculatingBMR, see page 131.

Step 2: Work out your physical activity level (PAL)

This is the ratio of your overall daily energyexpenditure to your BMR a rough measure ofyour lifestyle activity. Mostly inactive or sedentary (mainly sitting):

1.2Fairly active (include walking and exercise 12

x week): 1.3Moderately active (exercise 23 x weekly):

1.4Active (exercise hard more than 3 x weekly):

1.5Very active (exercise hard daily): 1.7

Step 3: Multiply your BMR by your PAL towork out your daily calorie needs

Daily calorie needs = BMR x PALThis figure gives you a rough idea of yourdaily calorie requirement to maintain yourweight. If you eat fewer calories, you willlose weight; if you eat more then you will gainweight.

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AN OVERVIEW OF SPORTS NUTRITION 1

Your BMR is the number of calories you burn atrest (to keep your heart beating, your lungsbreathing, to maintain your body temperature,etc). It accounts for 6075% of the calories youburn daily. Generally, men have a higher BMRthan women.

2. CARBOHYDRATE

Carbohydrate is an important fuel for exercise. Itis stored as glycogen in your liver and muscles,and must be re-stocked each day. Approximately100 g glycogen (equivalent to 400 kilocalories)may be stored in the liver, and up to 400 gglycogen (equivalent to 1600 kilocalories) inmuscle cells. The purpose of liver glycogen is tomaintain steady blood sugar levels. When bloodglucose dips, glycogen in the liver breaks down torelease glucose into the bloodstream. The purposeof muscle glycogen is to fuel physical activity.

The more active you are, the higher yourcarbohydrate needs. Guidelines for daily intakesare about 57 g per kg of body weight per day formoderate duration/low intensity daily training.Those who do moderateheavy endurancetraining should consume 710 g per kg bodyweight per day; and those training more than 4hours per day are advised to consume 10 g ormore per kg body weight per day.

To promote post-exercise recovery, the2003 IOC Consensus conference recommendsconsuming 1 g per kg BW per hour during thefirst four hours following exercise.

If you plan to train again within 8 hours, it isimportant to begin refueling as soon as possibleafter exercise. Moderate and high glycaemicindex (GI) carbohydrates (see page 36) willpromote faster recovery during this period.However, for recovery periods of 24 hours orlonger, the type and timing of carbohydrateintake is less critical, although you should choosenutrient-dense sources wherever possible.

During exercise lasting longer than 60minutes, consuming 2060 g carbohydrate perhour helps maintain your blood glucose level,delay fatigue and increase your endurance,according to studies at the University of Texas,US. Choose high GI carbohydrates (e.g. sportsdrinks, energy gels and energy bars, bananas,fruit bars, cereal or breakfast bars), whichconvert into blood sugar rapidly.

3. PROTEIN

Amino acids from proteins form the buildingblocks for new tissues and the repair of bodycells. They are also used for making enzymes,hormones and antibodies. Protein also providesa (small) fuel source for exercising muscles.

Athletes have higher protein requirementsthan non-active people. Extra protein is neededto compensate for the increased musclebreakdown that occurs during and after intenseexercise, as well as to build new muscle cells.The IOC and IAAF both recommend between1.2 and 1.7 g protein/kg BW/day for athletes, or84119 g daily for a 70 kg person. This isconsiderably more than a sedentary person,who requires 0.75 g protein/kg BW daily.

Some athletes eat high protein diets in thebelief that extra protein leads to increasedstrength and muscle mass, but this isnt true itis stimulation of muscle tissue through exercise,not extra protein that leads to muscle growth.As protein is found in so many foods, mostpeople including athletes eat a little moreprotein than they need. This isnt harmful theexcess is broken down into urea (which isexcreted) and fuel, which is either used forenergy or stored as fat if your calorie intakeexceeds your output.

Several studies have found that carbohydrateand protein eaten together immediately afterexercise enhances recovery and promotes

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Physical activity includes all activities from doingthe housework to walking and working out inthe gym. The number of calories you burn in anyactivity depends on your weight, the type ofactivity and the duration of that activity.

muscle building. This does not mean additionalfood or supplements. It means that you shouldspace out some of the protein and carbohydratesyou currently have in your diet and consume itafter workouts.

4. FAT

Some fat is essential it makes up part of thestructure of all cell membranes, your braintissue, nerve sheaths, bone marrow and itcushions your organs. Fat in food also providesessential fatty acids, the fat-soluble vitamins A,D and E, and is an important source of energyfor exercise. The IOC does not make a specificfat recommendation, but the American Collegeof Sports Medicine (ACSM) and AmericanDietetic Association recommend fat provides2025% of calorie intake for athletes comparedwith the UK government recommendation of33% for the general population. Therefore,about 2033% of the calories in your dietshould come from fat.

Bad fats (saturated and trans fats) should bekept to a minimum (the UK governmentrecommends less than 10% of calories), with themajority coming from good (unsaturated) fats.Omega-3s may be particularly beneficial forathletes as they help increase the delivery ofoxygen to muscles, improve endurance andmay speed recovery, reduce inflammation andjoint stiffness.

5. HYDRATION

You should ensure you are hydrated beforestarting training competition and aim tominimise dehydration during exercise.Dehydration can result in reduced enduranceand strength, and heat related illness. The IOCadvises matching your fluid intake to your fluid

losses as closely as possible and limitingdehydration to no more than 2% loss of bodyweight (e.g. a body weight loss of no more than1.5 kg for a 75 kg person).

Additionally, the IAAF cautions against over-hydrating yourself before and during exercise,particularly in events lasting longer than4 hours. Constantly drinking water may diluteyour blood so that your sodium levels fall.Although this is quite rare it is potentially fatal.The American College of Sports Medicine andUSA Track & Field advise drinking when yourethirsty or drinking only to the point at whichyoure maintaining your weight, not gainingweight.

Sports drinks are better than water duringintense exercise lasting more than 60 minutesbecause their sodium content will promotewater retention and prevent hyponatraemia.

6. VITAMINS AND MINERALS

While intense exercise increases therequirement for several vitamins and minerals,there is no need for supplementation providedyou are eating a balanced diet. The IOC andIAAF believe most athletes are well able to meettheir needs from food rather than supplements.Theres scant proof that vitamin and mineralsupplements improve performance, althoughsupplementation may be warranted in athleteseating a restricted diet.

Similarly, there is insufficient evidence torecommend antioxidant supplementation forathletes. Low intakes of iron and calcium arerelatively more common among female athletes a deficiency of these nutrients may impair healthand performance. The IOC cautions against theindiscriminate use of supplements and warns ofthe risk of contamination with banned substances.For more detail on vitamin, mineral andantioxidant requirements, see pages 5363.

AN OVERVIEW OF SPORTS NUTRITION

3

7. PRE-COMPETITION DIET

What you eat and drink during the week beforea competition can make a big difference to yourperformance, particularly for endurance eventsand competitions lasting more than 90 minutes.The aim of your pre-competition eatingstrategy is to maximise muscle glycogen storesand ensure proper hydration.

This can be achieved by tapering yourtraining while maintaining or increasingcarbohydrate (710 g/kg/BW/day). Smallfrequent meals are better than big meals. Makesure that you drink at least 2 litres per day.Avoid unfamiliar foods and drinks and stick toa well-rehearsed eating plan on the day of theevent.

HOW TO PLAN YOURTRAINING DIET

Use this fitness food pyramid as a base fordeveloping your daily training diet. It dividesfood into seven categories: fruit; vegetables;carbohydrate-rich foods; calcium-rich foods;protein-rich foods; healthy fats and junk foods.

The foods in the lower layers of the pyramidshould form the main part of your diet whilethose at the top should be eaten in smallerquantities.

Include foods from each group in thepyramid each day.

Make sure you include a variety of foodswithin each group.

Aim to include the suggested number ofportions from each food group each day.


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What counts as one portion?

Food Group Number of portions Food Portion sizeeach day

Vegetables 35 1 portion = 80 g (aboutthe amount you can holdin the palm of your hand)Broccoli, 23 spears/ floretscauliflowerCarrots 1 carrotOther vegetables 2 tablespoonsTomatoes 5 cherry tomatoes

Fruit 24 1 portion = 80 g(about the size of atennis ball)Apple, pear, peach, 1 medium fruitbananaPlum, kiwi fruit, satsuma 12 fruitStrawberries 810Grapes 1216Tinned fruit 3 tablespoonsFruit juice 1 medium glass

Grains and Potatoes 46 1 portion = about the sizeof your clenched fistBread 2 slices (60 g)Roll/ bagel/ wrap 1 item (60 g)Pasta or rice 5 tablespoons (180 g)Breakfast cereal 1 bowl (4050 g)Potatoes, sweet 1 fist-sized (150 g)potatoes, yams

Calcium-rich foods 24 1 portion = 200 ml milkMilk (dairy or calcium- 1 medium cup (200 ml)fortified soya milk)Cheese Size of 4 dice (40 g)Tofu Size of 4 dice (60 g)Yoghurt/ fromage frais 1 pot (150 ml)

Protein-rich foods 24 1 portion = size of adeck of cards (70 g)Lean meat 3 slicesPoultry 2 medium slices/ 1 breastFish 1 fillet (115140 g)Egg 2Lentils/ beans 5 tablespoons (150 g)Tofu/ soya burger or 12sausage

Healthy fats and oils 12 1 portion = 1 tablespoonNuts and seeds 2 tablespoons (25 g)Seed oils, nut oils 1 tablespoon (15 ml)Avocado Half avocadoOily fish* Deck of cards (140 g)

*Oily fish is very rich in essential fats so just 1 portion a week would cover your needs

Table 1.1

Fruit and vegetables 59 portions a dayFruit and vegetables contain vitamins, minerals,fibre, antioxidants and other phytonutrients,which are vital for health, immunity andoptimum performance.

Grains and potatoes46 portions a dayA diet rich in wholegrain foods bread, breakfastcereals, rice, pasta, porridge oats beans, lentilsand potatoes maintains high glycogen (storedcarbohydrate) levels, needed to fuel hardtraining. Aim for at least half of all grains eaten tobe wholegrains. Note: portion sizes here (60 gbread) are twice that recommended by the FoodStandards Agency (25 g bread) as these are morerealistic for active people.

Calcium-rich foods24 portions a dayIncluding dairy products, nuts, pulses andtinned fish in your daily diet is the easiest way toget calcium, which is needed for strong bones.

Protein-rich foods24 portions a dayRegular exercisers need more protein thaninactive people (see pages 4748), so include

lean meat, poultry, fish, eggs, soya or Quorn inyour daily diet. Beans, lentils, dairy foods andprotein supplements can also be countedtowards your daily target.

Healthy fats12 portions a dayThe oils found in nuts, seeds, rapeseed oil,olive oil, flax seed oil, sunflower oil, andoily fish may improve endurance and recoveryas well as protect against heart disease (seepages 111114).

Discretionary caloriesThese are the calories that you have left afteryou have eaten all the fruit, vegetables, grains,protein-rich foods, calcium-rich foods andhealthy fats recommended for the day. Themore active you are, the more discretionarycalories are allowed. For most regularexercisers this is likely to be around 200300calories worth of treats such as biscuits, cakes,puddings, alcoholic drinks, chocolate orcrisps, but these extra calories also need toaccount for any added sugar in sports drinksand energy bars, or the jam you spread onyour toast, or sugar you add to coffee or tea.


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When you exercise, your body must startproducing energy much faster than it does whenit is at rest. The muscles start to contract morestrenuously, the heart beats faster to pump bloodaround the body more rapidly, and the lungswork harder. All these processes require extraenergy. Where does it come from, and how canyou make sure you have enough to last througha training session?

Before we can fully answer such questions, itis important to understand how the bodyproduces energy, and what happens to it. Thischapter looks at what takes place in the bodywhen you exercise, where extra energy comesfrom, and how the fuel mixture used differsaccording to the type of exercise. It explainswhy fatigue occurs, how it can be delayed, andhow you can get more out of training bychanging your diet.

What is energy? Although we cannot actually see energy, we cansee and feel its effects in terms of heat andphysical work. But what exactly is it?

Energy is produced by the splitting of achemical bond in a substance called adenosinetriphosphate (ATP). This is often referred to asthe bodys energy currency. It is produced inevery cell of the body from the breakdown ofcarbohydrate, fat, protein and alcohol fourfuels that are transported and transformed byvarious biochemical processes into the same endproduct.

What is ATP?ATP is a small molecule consisting of anadenosine backbone with three phosphategroups attached.

Energy is released when one of the phosphategroups splits off. When ATP loses one of itsphosphate groups it becomes adenosinediphosphate, or ADP. Some energy is used tocarry out work (such as muscle contractions), butmost (around three-quarters) is given off as heat.This is why you feel warmer when you exercise.Once this has happened, ADP is converted backinto ATP. A continual cycle takes place, in whichATP forms ADP and then becomes ATP again.

The inter-conversion of ATP andADPThe body stores only very small amounts of ATPat any one time. There is just enough to keep upbasic energy requirements while you are at rest sufficient to keep the body ticking over. Whenyou start exercising, energy demand suddenlyincreases, and the supply of ATP is used upwithin a few seconds. As more ATP must beproduced to continue exercising, more fuel mustbe broken down.

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ENERGY FOR EXERCISE 2

Figure 2.1 ATP

P

P

P

A

Where does energy come from?There are four components in food and drinkthat are capable of producing energy: carbohydrate protein fat alcohol.

When you eat a meal or have a drink, thesecomponents are broken down in the digestivesystem into their various constituents or buildingblocks. Then they are absorbed into thebloodstream. Carbohydrates are broken downinto small, single sugar units: glucose (the mostcommon unit), fructose and galactose. Fats arebroken down into fatty acids, and proteins intoamino acids. Alcohol is mostly absorbed directlyinto the blood.

The ultimate fate of all of these components isenergy production, although carbo hydrates,proteins and fats also have other importantfunctions.

Carbohydrates and alcohol are used mainlyfor energy in the short term, while fats are used asa long-term energy store. Proteins can be used toproduce energy either in emergencies (forinstance, when carbohydrates are in short supply)or when they have reached the end of their usefullife. Sooner or later, all food and drinkcomponents are broken down to release energy.But the body is not very efficient in convertingthis energy into power. For example, duringcycling, only 20% of the energy produced isconverted into power. The rest becomes heat.

How is energy measured? Energy is measured in calories or Joules. Inscientific terms, one calorie is defined as theamount of heat required to increase thetemperature of 1 gram (or 1 ml) of water by1 degree centigrade (C) (from 14.5 to 15.5 C).The SI (International Unit System) unit forenergy is the Joule ( J). One Joule is defined asthe work required to exert a force of one Newtonfor a distance of one metre.

As the calorie and the joule represent verysmall amounts of energy, kilocalories (kcal orCal) and kilojoules (kJ) are more often used. Astheir names suggest, a kilocalorie is 1000 caloriesand a kilojoule 1000 joules. You have probablyseen these units on food labels. When wemention calories in the everyday sense, we arereally talking about Calories with a capital C, orkilocalories. So, food containing 100 kcal hasenough energy potential to raise the temperatureof 100 litres of water by 1 C.

To convert kilocalories into kilojoules, simplymultiply by 4.2. For example:

1 kcal = 4.2 kJ10 kcal = 42 kJ

To convert kilojoules into kilocalories, divide by4.2. For example, if 100 g of food provides 400kJ, and you wish to know how many kilocaloriesthat is, divide 400 by 4.2 to find the equivalentnumber of kilocalories:

400 kJ 4.2 = 95 kcal

Why do different foods provide different amounts of energy? Foods are made of different amounts ofcarbohydrates, fats, proteins and alcohol. Eachof these nutrients provides a certain quantity ofenergy when it is broken down in the body. For


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Figure 2.2 The relationship between ATPand ADP

ATP ADP + P + ENERGY

instance, 1 g of carbohydrate or protein releasesabout 4 kcal of energy, while 1 g of fat releases 9kcal, and 1 g of alcohol releases 7 kcal.

The energy value of different foodcomponents 1 g provides: carbohydrate 4 kcal (17 kJ) fat 9 kcal (38 kJ) protein 4 kcal (17 kJ) alcohol 7 kcal (29 kJ).

Fat is the most concentrated form of energy,providing the body with more than twiceas much energy as carbohydrate or protein, andalso more than alcohol. However, it is notnecessarily the best form of energy forexercise.

All foods contain a mixture of nutrients, andthe energy value of a particular food depends on

the amount of carbohydrate, fat and protein itcontains. For example, one slice of wholemealbread provides roughly the same amount ofenergy as one pat (7 g) of butter. However, theircomposition is very different. In bread, mostenergy (75%) comes from carbohydrate, while inbutter, virtually all (99.7%) comes from fat.

How does my body storecarbohydrate? Carbohydrate is stored as glycogen in the musclesand liver, along with about three times its ownweight of water. Altogether there is about threetimes more glycogen stored in the muscles thanin the liver. Glycogen is a large molecule, similarto starch, made up of many glucose units joinedtogether. However, the body can store onlya relatively small amount of glycogen there isno endless supply! Like the petrol tank in a car,the body can only hold a certain amount.

The total store of glycogen in the averagebody amounts to about 500 g; with approx -imately 400 g in the muscles and 100 g in theliver. This store is equivalent to 16002000 kcal enough to last one day if you were to eatnothing. This is why a low-carbohydrate diettends to make people lose quite a lot of weight inthe first few days. The weight loss is almostentirely due to loss of glycogen and water.Endurance athletes have higher muscle glycogenconcentrations compared with sedentary people.Increasing your muscle mass will also increaseyour storage capacity for glygocen.

The purpose of liver glycogen is to maintainblood glucose levels at rest and duringprolonged exercise.

Small amounts of glucose are present in theblood (approximately 15 g, which is equiv alentto 60 kcal) and in the brain (about 2 g or 8 kcal)and their concentrations are kept within a verynarrow range, both at rest and during exercise.This allows normal body functions to continue.

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Metabolism

Metabolism is the sum of all the biochemicalprocesses that occur in the body. There are twodirections: anabolism is the formation of largermolecules; catabolism is the breakdown of largermolecules into smaller molecules. Aerobicmetabolism includes oxygen in the processes;anaerobic metabolism takes place withoutoxygen. A metabolite is a product of metabolism.That means that anything made in the body is ametabolite.

The bodys rate of energy expenditure iscalled the metabolic rate. Your basal metabolic rate(BMR) is the number of calories expended tomaintain essential processes such as breathingand organ function during sleep. However, mostmethods measure the resting metabolic rate(RMR), which is the number of calories burnedover 24 hours while lying down but not sleeping.

How does my body store fat? Fat is stored as adipose (fat) tissue in almost everyregion of the body. A small amount of fat, about300400 g, is stored in muscles this is calledintramuscular fat but the majority is storedaround the organs and beneath the skin. Theamount stored in different parts of the bodydepends on genetic make-up and individualhormone balance. The average 70 kg personstores 1015 kg fat. Interestingly, people whostore fat mostly around their abdomen (theclassic pot-belly shape) have a higher risk ofheart disease than those who store fat mostlyaround their hips and thighs (the classic pearshape).

Unfortunately, there is little you can do tochange the way that your body distributes fat.But you can definitely change the amount of fatthat is stored, as you will see in Chapter 7.

You will probably find that your basic shape issimilar to that of one or both of your parents.Males usually take after their father, and femalesafter their mother. Female hormones tend tofavour fat storage around the hips and thighs,while male hormones encourage fat storagearound the middle. This is why, in general,women are pear shaped and men are appleshaped.

How does my body store protein? Protein is not stored in the same way ascarbohydrate and fat. It forms muscle and organtissue, so it is mainly used as a building materialrather than an energy store. However, proteinscan be broken down to release energy if need be,so muscles and organs represent a large source ofpotential energy.

Which fuels are most importantfor exercise? Carbohydrates, fats and proteins are all capableof providing energy for exercise; they can all betransported to, and broken down in, musclecells. Alcohol, however, cannot be used directlyby muscles for energy during exercise, no matterhow strenuously they may be working. Only theliver has the specific enzymes needed to breakdown alcohol. You cannot break down alcoholfaster by exercising harder either the livercarries out its job at a fixed speed. Do not thinkyou can work off a few drinks by going for a jog,or by drinking a cup of black coffee!

Proteins do not make a substantialcontribution to the fuel mixture. It is only duringvery prolonged or very intense bouts of exercisethat proteins play a more important role ingiving the body energy.

The production of ATP during most forms ofexercise comes mainly from broken downcarbohydrates and fats.

Table 2.1 illustrates the potential energyavailable from the different types of fuel that arestored in the body.


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When is protein used for energy? Protein is not usually a major source of energy,but it may play a more important role during thelatter stages of very strenuous or prolongedexercise as glycogen stores become depleted. Forexample, during the last stages of a marathon ora long distance cycle race, when glycogen storesare exhausted, the proteins in muscles (andorgans) may make up around 10% of the bodysfuel mixture.

During a period of semi-starvation, or if aperson follows a low-carbohydrate diet,glycogen would be in short supply, so moreproteins would be broken down to provide thebody with fuel. Up to half of the weight lost bysomeone following a low-calorie or low-carbohydrate diet comes from protein (muscle)loss. Some people think that if they deplete theirglycogen stores by following a low-carbohydratediet, they will force their body to break downmore fat and lose weight. This is not the case:you risk losing muscle as well as fat, and thereare many other dis advantages, too. These arediscussed in Chapter 9.

HOW IS ENERGY PRODUCED?

The body has three main energy systems it canuse for different types of physical activity. Theseare called:

1 the ATPPC (phosphagen) system

2 the anaerobic glycolytic, or lactic acid,system

3 the aerobic system comprising of the gly -colytic (carbohydrate) and lipolytic (fat)systems.

At rest, muscle cells contain only a very smallamount of ATP, enough to maintain basicenergy needs and allow you to exercise atmaximal intensity for about 1 second. Tocontinue exercising, ATP must be regeneratedfrom one of the three energy systems, each ofwhich has a very different biochemical pathwayand rate at which it produces ATP.

How does the ATPPC systemwork?This system uses ATP and phosphocreatine (PC)that is stored within the muscle cells, to generateenergy for maximal bursts of strength and speedthat last for up to 6 seconds. The ATPPCsystem would be used, for example, during a 20-metre sprint, a near-maximal lift in the gym, or asingle jump. Phosphocreatine is a high-energycompound formed when the protein, creatine, islinked to a phosphate molecule (see box What iscreatine?). The PC system can be thought of asa back-up to ATP. The job of PC is to regenerateATP rapidly (see Fig. 2.3). PC breaks down intocreatine and phosphate, and the free phosphatebond transfers to a molecule of ADP forming anew ATP molecule. The ATPPC system can

ENERGY FOR EXERCISE

11

Fuel reserves in a person weighing 70 kg

Fuel stores Potential energy available (kcal)Glycogen Fat Protein

Liver 400 450 400Adipose tissue (fat) 0 135,000 0Muscle 1200 350 24,000Source: Cahill, 1976.

Table 2.1

release energy very quickly, but, unfort unately, itis in very limited supply and can only provide34 kcal. After this the amount of energyproduced by the ATPPC system fallsdramatically, and ATP must be produced fromother fuels, such as glycogen or fat. When thishappens, other systems take over.


12

Figure 2.3 PC splits to release energy toregenerate ATP rapidly

P

P

P

AA

P

P

P C P C

P+

+ +

phosphocreatine phosphatetransfer

creatine

energy

adenosinediphosphate adenosine

triphosphate

phosphate

What is creatine?

Creatine is a compound thats made naturally inour bodies to supply energy. It is mainlyproduced in the liver from the amino acidsglycine, arginine and methionine. From the liver itis transported in the blood to the muscle cellswhere it is combined with phosphate to makephosphocreatine (PC).

The muscle cells turnover about 23 g ofcreatine a day. Once PC is broken down intoATP (energy), it can be recycled into PC orconverted into another substance calledcreatinine, which is then removed via the kidneysin the urine.

Creatine can be obtained in the diet from fish(tuna, salmon, cod), beef and pork (approx. 35 gcreatine/kg uncooked fish or meat). That meansvegetarians have no dietary sources. However, tohave a performance-boosting effect, creatine hasto be taken in large doses. This is higher than youcould reasonably expect to get from food. Youwould need to eat at least 2 kg of raw steak aday to load your muscles with creatine.

The average-sized person stores about 120 gcreatine, almost all in skeletal muscles (higherlevels in fast-twitch muscle fibres, see p. 14). Ofthis amount, 6070% is stored as PC, 3040% asfree creatine.

How does the anaerobic glycolyticsystem work?This system is activated as soon as you beginhigh-intensity activity. It dominates in eventslasting up to 90 seconds, such as a weighttraining set in the gym or a 400800 m sprint. Inorder to meet sudden, large demands for energy,glucose bypasses the energy pro ducing pathwaysthat would normally use oxygen, and follows adifferent route that does not use oxygen. Thissaves a good deal of time. After 30 seconds ofhigh-intensity exercise this system contributes upto 60% of your energy output; after 2 minutes itscontribution falls to only 35%.

The anaerobic glycolytic system usescarbohydrate in the form of muscle glycogen orglucose as fuel. Glycogen is broken down toglucose, which rapidly breaks down in theabsence of oxygen to form ATP and lactic acid(see Fig. 2.4). Each glucose molecule producesonly two ATP molecules under anaerobicconditions, making it a very inefficient system.The bodys glycogen stores dwindle quickly,proving that the benefits of a fast delivery servicecome at a price. The gradual build-up of lactic acid will eventually cause fatigue and prevent

further muscle contrac tions. (Contrary topopular belief, it is not lactic acid, but the buildup of hydrogen ions and acidity that causes theburning feeling during or immediately aftermaximal exercise see p. 18.)

How does the aerobic systemwork?The aerobic system can generate ATP from thebreakdown of carbohydrates (by glycolysis) andfat (by lipolysis) in the presence of oxygen (seeFig. 2.5). Although the aerobic system cannotproduce ATP as rapidly as can the other twoanaerobic systems, it can produce largeramounts. When you start to exercise, youinitially use the ATPPC and anaerobic

ENERGY FOR EXERCISE

13

Figure 2.4 Anaerobic energy system

GLYCOGEN

LACTIC ACID

ENERGY

ATP ATP

GLUCOSE[NO OXYGEN,VERY RAPID]

What happens to the lactic acid?

Lactic acid produced by the muscles is not awasted by-product. It constitutes a valuable fuel.When the exercise intensity is reduced or youstop exercising, lactic acid has two possible fates.Some may be converted into another substancecalled pyruvic acid, which can then be brokendown in the presence of oxygen into ATP. Inother words, lactic acid produces ATP andconstitutes a valuable fuel for aerobic exercise.Alternatively, lactic acid may be carried awayfrom the muscle in the bloodstream to the liverwhere it can be converted back into glucose,released back into the bloodstream or stored asglycogen in the liver (a process called gluconeo -genesis). This mechanism for removing lactic acidfrom the muscles is called the lactic acid shuttle.

This explains why the muscle soreness andstiffness experienced after hard training is notdue to lactic acid accumulation. In fact, the lacticacid is usually cleared within 15 minutes ofexercise.

glycolytic systems, but after a few minutes yourenergy supply gradually switches to the aerobicsystem.

Most of the carbohydrate which fuels aerobicglycolysis comes from muscle glycogen.Additional glucose from the bloodstreambecomes more important as exercise continuesfor longer than 1 hour and muscle glycogenconcentration dwindles. Typically, after 2 hoursof high-intensity exercise (greater than 70%VO2max), almost all of your muscle glycogenwill be depleted. Glucose delivered from thebloodstream is then used to fuel your muscles,along with increasing amounts of fat (lipolyticglycolysis). Glucose from the bloodstream maybe derived from the breakdown of liver glycogenor from carbohydrate consumed during exercise.

In aerobic exercise, the demand for energy isslower and smaller than in an anaerobic activity,so there is more time to transport sufficientoxygen from the lungs to the muscles and forglucose to generate ATP with the help of theoxygen. Under these circumstances, onemolecule of glucose can create up to 38molecules of ATP. Thus, aerobic energy

production is about 20 times more efficient thananaerobic energy production.

Anaerobic exercise uses only glycogen,whereas aerobic exercise uses both glycogen andfat, so it can be kept up for longer. Thedisadvantage, though, is that it produces energymore slowly.

Fats can also be used to produce energy in theaerobic system. One fatty acid can producebetween 80 and 200 ATP molecules, dependingon its type (see Fig. 2.5). Fats are therefore aneven more efficient energy source thancarbohydrates. However, they can only bebroken down into ATP under aerobic conditionswhen energy demands are relatively low, and soenergy production is slower.

Muscle fibre types and energyproductionThe body has several different muscle fibretypes, which can be broadly classified intofast-twitch (FT) or type II, and slow-twitch (ST)or type I (endurance) fibres. Both muscle fibretypes use all three energy systems to produceATP but the FT fibres use mainly the ATPPCand anaerobic glycolytic systems, while the STfibres use mainly the aerobic system.

Everyone is born with a specific distributionof muscle fibre types; the pro portion of FT fibresto ST fibres can vary quite considerably betweenindividuals. The pro portions of each musclefibre type you have has implications for sport.For example, top sprinters have a greaterproportion of FT fibres than average and thuscan generate explosive power and speed.Distance runners, on the other hand, haveproportionally more ST fibres and are betterable to develop aerobic power and endurance.


14

Figure 2.5 Aerobic energy system

GLYCOGEN

FATTY ACID

ENERGY

80200 MOLECULES ATP

+ OXYGEN

GLUCOSE

+ OXYGEN

FAT

38 MOLECULES ATP

How do my muscles decidewhether to use carbo hydrateor fat during aerobic exercise?During aerobic exercise the use of carbo hydraterelative to fat varies according to a number offactors. The most important are:

1 the intensity of exercise2 the duration of exercise3 your fitness level4 your pre-exercise diet.

Intensity

The higher the intensity of your exercise, thegreater the reliance on muscle glycogen (see Fig.2.6). During anaerobic exercise, energy isproduced by the ATPPC and anaerobicglycolytic systems. So, for example, duringsprints, heavy weight training and inter mittentmaximal bursts during sports like football andrugby, muscle glycogen, rather than fat, is themajor fuel.

During aerobic exercise you will use amixture of muscle glycogen and fat for energy.Exercise at a low intensity (less than 50% ofVO2max) is fuelled mainly by fat. As youincrease your exercise intensity, for example, asyou increase your running speed, you will use ahigher proportion of glycogen than fat. During

moderate-intensity exercise (5070% VO2max),muscle glycogen supplies around half yourenergy needs; the rest comes from fat. Whenyour exercise intensity exceeds 70% VO2max,fat cannot be broken down and transported fastenough to meet energy demands so muscleglycogen provides at least 75% of your energyneeds.

Duration

Muscle glycogen is unable to provide energyindefinitely as it is stored in relatively smallquantities. As you continue exercising, yourmuscle glycogen stores become progressivelylower (see Fig. 2.7). Thus, as muscle glycogenconcentration drops, the contribution that bloodglucose makes to your energy needs increases.The proportion of fat used for energy alsoincreases but it can never be burned without thepresence of carbohydrate.

On average, you have enough muscleglycogen to fuel 90180 minutes of enduranceactivity; the higher the intensity, the faster yourmuscle glycogen stores will be depleted. Duringinterval training, i.e. a mixture of endurance and

ENERGY FOR EXERCISE

15

Source: Costill, 1986.

Figure 2.6 Fuel mixture/exercise intensity

Figure 2.7 Fuel mixture/exercise duration

100% 100%

0% 0%

1800Time (minutes)

Fat

Mus

cle

glyc

ogen

Fat

Muscle glycogen

anaerobic activity, muscle glycogen stores willbecome depleted after 4590 minutes. Duringmainly anaerobic activities, muscle glycogen willdeplete within 3045 minutes.

Once muscle glycogen stores are depleted,protein makes an increasing contribution toenergy needs. Muscle proteins break down toprovide amino acids for energy production andto maintain normal blood glucose levels.

Fitness level

As a result of aerobic training, your musclesmake a number of adaptations to improve yourperformance, and your bodys ability to use fatas a fuel improves. Aerobic training increases thenumbers of key fat-oxidising enzymes, such ashormone-sensitive lipase, which means yourbody becomes more efficient in breaking downfat into fatty acids. The number of bloodcapillaries serving the muscle increases so youcan transport the fatty acids to the muscle cells.The number of mitochondria (the sites of fattyacid oxidation) also increases which means youhave a greater capacity to burn fatty acids ineach muscle cell. Thus, improved aerobic fitnessenables you to break down fat at a faster rate atany given intensity, thus allowing you to spareglycogen (see Fig. 2.8). This is important becauseglycogen is in much shorter supply than fat. Byusing proportion ally more fat, you will be able toexercise for longer before muscle glycogen isdepleted and fatigue sets in.

Pre-exercise diet

A low-carbohydrate diet will result in low muscleand liver glycogen stores. Many studies haveshown that initial muscle glycogen concentrationis critical to your performance and that lowmuscle glycogen can reduce your ability tosustain exercise at 70% VO2max for longer than1 hour (Bergstrom et al., 1967). It also affects yourability to perform during shorter periods ofmaximal power output.

When your muscle glycogen stores are low,your body will rely heavily on fat and protein.However, this is not a recommended strategy forfat loss, as you will lose lean tissue. (See Chapter9 for appropriate ways of reducing body fat.)

WHICH ENERGY SYSTEMS DO IUSE IN MY SPORT?

Virtually every activity uses all three energysystems to a greater or lesser extent. No singleenergy system is used exclusively and at anygiven time energy is being derived from each ofthe three systems (see Fig. 2.9). In every activity,ATP is always used and is replaced by PC.Anaerobic glycolysis and aerobic energy pro -duction depend on exercise intensity.

For example, during explosive strength andpower activities lasting up to 5 seconds, such as asprint start, the existing store of ATP is theprimary energy source. For activities involvinghigh power and speed lasting 530 seconds, suchas 100200 m sprints, the ATPPC system is theprimary energy source, together with some


16

Figure 2.8 Trained people use less glycogenand more fat

muscle glycogen broken down throughanaerobic glycolysis. During power enduranceactivities such as 400800 m events, muscleglycogen is the primary energy source andproduces ATP via both anaerobic and aerobicglycolysis. In aerobic power activities, such asrunning 510 km, muscle glycogen is the

primary energy source producing ATP viaaerobic glycolysis. During aerobic events lasting2 hours or more, such as half- and full marathons,muscle glycogen, liver glycogen, intra-muscularfat and fat from adipose tissue are the main fuelsused. The energy systems and fuels used forvarious types of activities are summarised inTable 2.2.

What happens in my body when Istart exercising? When you begin to exercise, energy is producedwithout oxygen for at least the first few seconds,before your breathing rate and heart can catchup with energy demands. Therefore, a build-upof lactic acid takes place. As the heart and lungswork harder, getting more oxygen into your

ENERGY FOR EXERCISE

17

Figure 2.9 Percentage contribution ofenergy systems during exercise of differentdurations

The main energy systems used during different types of exercise

Type of exercise Main energy system Major storage fuels used

Maximal short bursts ATPPC (phosphagen) ATP and PClasting less than 6 sec

High intensity lasting ATPPC ATP and PC up to 30 sec Anaerobic glycolytic Muscle glycogen

High intensity lasting Anaerobic glycolytic Muscle glycogenup to 15 min Aerobic

Moderatehigh intensity Aerobic Muscle glycogenlasting 1560 min Adipose tissue

Moderatehigh intensity Aerobic Muscle glycogenlasting 6090 min Liver glycogen

Blood glucoseIntra-muscular fatAdipose tissue

Moderate intensity lasting Aerobic Muscle glycogenlonger than 90 min Liver glycogen

Blood glucoseIntra-muscular fatAdipose tissue

Table 2.2

body, carbo hydrates and fats can be brokendown aerobically. If you are exercising fairlygently (i.e. your oxygen supply keeps up withyour energy demands), any lactic acid thataccumulated earlier can be removed easily sincethere is now enough oxygen around.

If you continue to exercise aerobically, moreoxygen is delivered around the body and morefat starts to be broken down into fatty acids.They are taken to muscle cells via thebloodstream and then broken down with oxygento produce energy.

In effect, the anaerobic system buys time inthe first few minutes of an exercise, before thebodys slower aerobic system can start tofunction.

For the first 515 minutes of exercise(depending on your aerobic fitness level) themain fuel is carbohydrate (glycogen). As timegoes on, however, more oxygen is delivered tothe muscles, and you will use proportionally lesscarbohydrate and more fat.

On the other hand, if you begin exercisingvery strenuously (e.g. by running fast), lactic acidquickly builds up in the muscles. The delivery ofoxygen cannot keep pace with the huge energydemand, so lactic acid continues to accumulateand very soon you will feel fatigue. You mustthen either slow down and run more slowly, orstop. Nobody can main tain a fast run for verylong.

If you start a distance race or training run toofast, you will suffer from fatigue early on and beforced to reduce your pace consider ably. A headstart will not necessarily give any benefit at all.Warm up before the start of a race (by walking,slow jogging, or performing gentle mobilityexercises), so that the heart and lungs can start towork a little harder, and oxygen delivery to themuscles can increase. Start the race at amoderate pace, gradually building up to anoptimal speed. This will prevent a large oxygendebt and avoid an early depletion of glycogen.

In this way, your optimal pace can be sustainedfor longer.

The anaerobic system can also cut in tohelp energy production, for instance when thedemand for energy temporarily exceedsthe bodys oxygen supply. If you run uphillat the same pace as on the flat, your energydemand increases. The body will generate extraenergy by breaking down glycogen/glucoseanaerobically. However, this can only be keptup for a short period of time, because therewill be a gradual build-up of lactic acid. Thelactic acid can be removed aerobicallyafterwards, by running back down the hill, forexample.

The same principle applies during fast burstsof activity in interval training, when energy isproduced anaerobically. Lactic acid accumulatesand is then removed during the rest interval.

WHAT IS FATIGUE?

In scientific terms, fatigue is an inability tosustain a given power output or speed. It is amismatch between the demand for energy bythe exercising muscles and the supply of energyin the form of ATP. Runners experience fatiguewhen they are no longer able to maintain theirspeed; footballers are slower to sprint for the balland their technical ability falters; in the gym, youcan no longer lift the weight; in an aerobics class,you will be unable to maintain the pace andintensity. Subjectively, you will find that exercisefeels much harder to perform, your legs may feelhollow and it becomes increasingly hard to pushyourself.

Why does fatigue develop duringanaerobic exercise?During explosive activities involving maximalpower output, fatigue develops due to ATP and


18

PC depletion. In other words, the demand forATP exceeds the readily available supply.

During activities lasting between 30 secondsand 30 minutes, fatigue is caused by a differentmechanism. The rate of lactic acid removal inthe bloodstream cannot keep pace with the rateof lactic acid production. So during high-intensity exercise lasting up to half an hour thereis a gradual increase in muscle acidity, whichreduces the ability of the muscles to maintainintense contractions. It is not possible tocontinue high-intensity exercise indefinitelybecause the acute acid environment in yourmuscles would inhibit further contractions andcause cell death. The burning feeling youexperience when a high concentration of lacticacid develops is a kind of safety mechanism,preventing the muscle cells from destruction.

Reducing your exercise intensity will lowerthe rate of lactic acid production, reduce thebuild-up, and enable the muscles to switch to theaerobic energy system, thus enabling you tocontinue exercising.

Why does fatigue develop duringaerobic exercise?Fatigue during moderate and high-intensityaerobic exercise lasting longer than 1 houroccurs when muscle glycogen stores aredepleted. Its like running out of petrol in yourcar. Muscle glycogen is in short supplycompared with the bodys fat stores. Liverglycogen can help maintain blood glucose levelsand a supply of carbohydrate to the exercisingmuscles, however stores are also very limitedand eventually fatigue will develop as a result ofboth muscle and liver glycogen depletion andhypoglycaemia (see Fig. 2.10).

During low to moderate-intensity exerciselasting more than three hours, fatigue is causedby additional factors. Once glycogen stores havebeen exhausted, the body switches to the aerobic

lipolytic system where fat is able to supply most(not all) of the fuel for low-intensity exercise.However, despite having relatively large fatreserves, you will not be able to continueexercise indefinitely as fat cannot be convertedto energy fast enough to keep up with thedemand by exercising muscles. Even if youslowed your pace to enable the energy suppliedby fat to meet the energy demand, other factorswill cause you to fatigue. These include a rise inthe concen tration of the brain chemicalserotonin, which results in an overall feeling oftiredness, acute muscle damage, and fatigue dueto lack of sleep.

ENERGY FOR EXERCISE

19

Source: Costill, 1988.

Figure 2.10 The increase in perceivedexertion as glycogen stores becomedepleted

How can I delay fatigue?Glycogen is used during virtually every type ofactivity. Therefore the amount of glycogenstored in your muscles and, in certain events,your liver, before you begin exercise will have adirect affect on your performance. The greateryour pre-exercise muscle glycogen store thelonger you will be able to maintain your exerciseintensity, and delay the onset of fatigue.Conversely, sub-optimal muscle glycogen storescan cause earlier fatigue, reduce your endurance,reduce your intensity level and result in smallertraining gains.

You may also delay fatigue by reducing therate at which you use up muscle glycogen. Youcan do this by pacing yourself, graduallybuilding up to your optimal intensity.

SUMMARY OF KEY POINTS

The body uses three energy systems: (1) theATPPC, or phosphagen, system; (2) theanaerobic glycolytic, or lactic acid, system;(3) the aerobic system, which comprises bothglycolytic (carbohydrate) and lipolytic (fat)systems.

The ATPPC system fuels maximal bursts ofactivity lasting up to 6 seconds.

Anaerobic glycolysis provides energy forshort-duration high-intensity exercise lastingfrom 30 seconds to several minutes. Muscleglycogen is the main fuel.

The lactic acid produced during anaerobicglycolysis is a valuable fuel for further energy

production when exercise intensity isreduced.

The aerobic system provides energy fromthe breakdown of carbohydrate and fat forsub-maximal intensity, prolonged exercise.

Factors that influence the type of energysystem and fuel usage are exercise intensityand duration, your fitness level and your pre-exercise diet.

The proportion of muscle glycogen used forenergy increases with exercise intensity anddecreases with exercise duration.

For most activities lasting longer than 30seconds, all three energy systems are used toa greater or lesser extent; however, onesystem usually dominates.

The main cause of fatigue during anaerobicactivities lasting less than 6 seconds is ATPand PC depletion; during activities lastingbetween 30 seconds and 30 minutes it islactic acid accumulation and muscle cellacidity.

Fatigue during moderate and high-intensityexercise lasting longer than 1 hour is usuallydue to muscle glycogen depletion. For eventslasting longer than 2 hours fatigue isassociated with low liver glycogen and lowblood sugar levels.

For most activities, performance is limited bythe amount of glycogen in the muscles. Lowpre-exercise glycogen stores lead to earlyfatigue, reduced exercise intensity andreduced training gains.


20

Carbohydrate is needed to fuel almost everytype of activity and the amount of glycogenstored in your muscles and liver has a directeffect on your exercise performance. A highmuscle-glycogen concentration will allow you totrain at your optimal intensity and achieve agreater training effect. A low muscle-glycogenconcentration, on the other hand, will lead toearly fatigue, reduced training intensity and sub-optimal performance.

Clearly, then, glycogen is the most importantand most valuable fuel for any type of exercise.This chapter explains what happens if you fail toeat enough carbo hydrate and glycogen levelsbecome depleted. It shows you how to calculateyour precise carbohydrate requirements andconsiders the latest research on the timing ofcarbohydrate intake in relation to training.

Each different carbohydrate produces adifferent response in the body, so this chaptergives advice on which types of carbohydratefoods to eat. It presents comprehensiveinformation on the glycaemic index (GI), a keypart of every athletes nutritional tool box.Finally, it considers the current thinking oncarbohydrate loading before a competition.

The relationship between muscleglycogen and performanceThe importance of carbohydrates in relation toexercise performance was first demonstrated in1939. Christensen and Hansen, found that ahigh-carbohydrate diet significantly increasedendurance. However, it wasnt until the 1970sthat scientists discovered that the capacity forendurance exercise is related to pre-exercise

glycogen stores and that a high-carbohydratediet increases glycogen stores.

In a pioneering study, three groups of athleteswere given a low-carbohydrate diet, a high-carbohydrate diet or moderate-carbohydratediet (Bergstrom et al., 1967). Researchersmeasured the concentration of glycogen in theirleg muscles and found that those athletes eatingthe high-carbohydrate diet stored twice as muchglycogen as those on the moderate-carbohydratediet and 7 times as much as those eating the low-carbohydrate diet. Afterwards, the athletes wereinstructed to cycle to exhaustion on a stationarybicycle at 75% of VO2max. Those on the high-carbohydrate diet managed to cycle for 170minutes, considerably longer than those on themoderate-carbohydrate diet (115 minutes) or thelow-carbohydrate diet (60 minutes) (see Fig 3.1).

21

FUELLING BEFORE, DURINGAND AFTER EXERCISE 3

Figure 3.1 The effect of carbohydrateintake on performance

NORMAL MIXED DIET

LOW-CARBOHYDRATE DIET

HIGH-CARBOHYDRATE DIET

HOW MUCH CARBOHYDRATESHOULD I EAT PER DAY?

Sports nutritionists and exercise physiologistsconsistently recommend that regular exercisersconsume a diet containing a relatively high

percentage of energy from carbohydrate and arelatively low percentage of energy from fat(ACSM/ADA/DC, 2000; American DieteticAssociation, 1993). There is plentiful evidencethat such a diet enhances endurance andperformance for exercise lasting longer thanone hour.

This recommendation is based on the factthat carbohydrate is very important forendurance exercise since carbohydrate stores as muscle and liver glycogen are limited.Depletion of these stores results in fatigue andreduced performance. This can easily happenif your pre-exercise glycogen stores are low. Inorder to get the most out of your trainingsession, you should ensure your pre-exerciseglycogen stores are high. This will help toimprove your endurance, delay exhaustionand help you exercise longer and harder(Coyle, 1988; Costill & Hargreaves, 1992).Previously, researchers recommended a dietproviding 6070% energy from carbohydratebased on the consensus statement from theInternational Conference on Foods, Nutrition& Performance in 1991 (Williams & Devlin,1992).

However, this method is not very user-friendly and can be misleading as it assumes anoptimal energy (calorie) intake. It does notprovide optimal carbohydrate for those withvery high or low energy intakes. For example,for an athlete consuming 40005000 caloriesdaily, 60% energy from carbohydrate (i.e. 600g+) would exceed their glycogen storagecapacity (Coyle, 1995). Conversely, for athletesconsuming 2000 calories daily, a dietproviding 60% energy from carbohydrate (i.e.300g) would not contain enough carbohydrateto maintain muscle glycogen stores.

Scientists recommend calculating yourcarbohydrate requirement from your bodyweight and also your training volume (IAAF,2007; Burke et al., 2004; IOC, 2004; Burke, 2001;


22

Can high fat diets increaseendurance?

While most of the research on diet and endurancehas focused on the role of carbohydrate, a numberof studies have considered whether a high fat dietmight enhance the muscles ability to burn fat. Thethinking behind this research is that since fat is amajor fuel during prolonged endurance exercise, ahigh fat diet may be able to train the muscles toburn more fat during exercise, conserving preciousglycogen and giving muscles greater access to amore plentiful supply of energy in the body. Indeed,it appears that increasing fat intake enhances thestorage and burning of intramuscular fat as well asimproving the ability of the muscles to take up fatfrom the blood stream (Muoio et al., 1994; Helgeet al., 2001; Lambert et al., 1994). However, theseeffects are observed only in elite or well-conditioned athletes and the performanceadvantage only applies at relatively low exerciseintensities. For less conditioned athletes or un -trained individuals, or those exercising above 65%VO2 max, high fat diets have no performanceadvantage (Burke et al., 2004). Whats more, a highfat intake may increase body fat % (if calorie intakeexceeds calorie burning) and, if the diet containsexcessive saturated fat, you risk high bloodcholesterol levels. US researchers analysed 20studies that looked at the high fat diets andperformance (Erlenbusch et al., 2005). Theyconcluded that high fat diets have no performanceadvantage for non-elite athletes but all athletes(especially non-elite) benefited from a highcarbohydrate diet.

Schokman, 1999), since your glycogen storagecapacity is roughly proportional to your musclemass and body weight, i.e. the heavier you are,the greater your muscle mass and the greateryour glycogen storage capacity. The greater yourtraining volume, the more carbohydrate youneed to fuel your muscles. It is more flexible as ittakes account of different training requirementsand can be calculated independent of calorieintake.

Table 3.1 indicates the amount ofcarbohydrate per kg of body weight needed perday according to your activity level. Mostathletes training for up to two hours daily requireabout 57 g/ kg body weight, but during periodsof heavy training requirements may increase to710 g/ kg BW.

For example, for a 70 kg athlete who trains for12 hours a day:

Carbohydrate need = 67 g /kg of body weightDaily carbohydrate need = Between (70 6) =

420g and (70 7) = 490g

i.e. Daily carbohydrate need = 420490g

FUELLING BEFORE, DURING AND AFTER EXERCISE

23

How much carbohydrate?

Activity level* g carbohydrate/kg body weight/day

35 hours/week 45

57 hours/week 56

12 hours/day 67

24 hours/day 78

More than 4 hours/day 810

*Number of hours of moderate intensity exercise or sport

Table 3.1

Is a high carbohydrate dietpractical?

In practice, eating a high carbohydrate diet can bedifficult, particularly for those athletes with highenergy needs. Many complex carbohyd rate foods,such as bread, potatoes and pasta are quite bulkyand the diet quickly becomes very filling,particularly if whole grain and high fibre foodsmake up most of your carbohydrate intake.Several surveys have found that enduranceathletes often fail to consume the recommendedcarbohydrate levels (Frentsos, 1999; Jacobs &Sherman, 1999). Most get between 45 and 65% oftheir calories from carbohydrate. This may bepartly due to the large number of calories neededand therefore the bulk of their diet, and partly dueto lack of awareness of the benefits of a highercarbohydrate intake. It is interesting that most ofthe studies upon which the carbohydraterecommendations were made, used liquid carbo -hydrates (i.e. drinks) to supplement meals. Tour deFrance cyclists and triathletes consume up to onethird of their carbohydrate in liquid form. If you arefinding a high carbo hydrate diet impractical, tryeating smaller more frequent meals andsupplementing your food with liquid forrns ofcarbohydrate such as meal replacement shakes(see p. 79) and glucose polymer drinks (see p. 95).

WHICH CARBOHYDRATES AREBEST?

Carbohydrates are traditionally classifiedaccording to their chemical structure. The mostsimplistic method divides them into twocategories: simple (sugars) and complex (starchesand fibres). These terms simply refer to thenumber of sugar units in the molecule.

Simple carbohydrates are very smallmolecules consisting of one or two sugar units.They comprise the monosaccharides (1-sugarunits): glucose (dextrose), fructose (fruit sugar)and galactose; and the disaccharides (2-sugarunits): sucrose (table sugar, which comprises aglucose and fructose molecule joined together)and lactose (milk sugar, which comprises aglucose and galactose molecule joined together).

Complex carbohydrates are much largermolecules, consisting of between 10- and severalthousand-sugar units (mostly glucose) joinedtogether. They include the starches, amylose andamylopectin, and the non-starch polysaccharides(dietary fibre), such as cellulose, pectin andhemicellulose.

In between simple and complex carbo -hydrates are glucose polymers and malto dextrin,which comprise between 3- and 10-sugar units.They are made from the partial breakdown ofcorn starch in food processing, and are widelyused as bulking and thickening agents inprocessed foods, such as sauces, dairy desserts,baby food, puddings and soft drinks. They arepopular ingredients in sports drinks andengineered meal-replacement products, owingto their low sweetness and high energy densityrelative to sucrose.

In practice, many foods contain a mixture ofboth simple and complex carbohydrates, makingthe traditional classification of foods into simpleand complex very confusing. For example,biscuits and cakes contain flour (complex) and

sugar (simple), and bananas contain a mixture ofsugars and starches depending on their degree ofripeness.

Not all carbohydrates are equalIts tempting to think that simple carbo hydrates,due to their smaller molecular size, are absorbedmore quickly than complex carbohydrates, andproduce a large and rapid rise in blood sugar.Unfortunately, its not that straightforward. Forexample, apples (containing simple carbo -hydrates) produce a small and prolonged rise inblood sugar, despite being high in simple carbo -hydrates. Many starchy foods (complexcarbohydrates), such as potatoes and bread, aredigested and absorbed very quickly and give arapid rise in blood sugar. So the old notion aboutsimple carbohydrates giving fast-released energyand complex carbo hydrates giving slow-releasedenergy is incorrect and misleading.

What is more important as far as sportsperformance is concerned is how rapidly thecarbohydrate is absorbed from the small intestineinto your bloodstream. The faster this transfer,the more rapidly the carbo hydrate can be takenup by muscle cells (or other cells of the body) andmake a difference to your training and recovery.

THE GLYCAEMIC INDEX

To describe more accurately the effect differentfoods have on your blood sugar levels, scientistsdeveloped the glycaemic index (GI). While theGI concept was originally developed to helpdiabetics control their blood sugar levels, it canbenefit regular exercisers and athletes too. It is aranking of foods from 0 to 100 based on theirimmediate effect on blood sugar levels, ameasure of the speed at which you digest foodand convert it into glucose. The faster the rise inblood glucose the higher the rating on the index.


24

To make a fair comparison, all foods arecompared with a reference food, such as glucose,and are tested in equivalent carbohydrateamounts. The GI of foods is very useful to knowbecause it tells you how the body responds tothem. If you need to get carbohydrates into yourbloodstream and muscle cells rapidly, forexample immediately after exercise to kick-startglycogen replen ishment, you would choose highGI foods. In 1997 the World Health Organization(WHO) and Food and Agriculture Organization(FOA) of the United Nations endorsed the use ofthe GI for classifying foods, and recommendedthat GI values should be used to guide peoplesfood choices.

How is the GI worked out?The GI value of food is measured by feeding 10or more healthy people a portion of foodcontaining 50 g carbo hydrate. For example, totest baked potatoes, you would eat 250 gpotatoes, which contain 50 g of carbohydrate.Over the next two hours, a sample of blood istaken every 15 minutes and the blood sugar levelmeasured. The blood sugar level is plotted on agraph and the area under the curve calculatedusing a computer programme (see Fig. 3.2). Onanother occasion, the same 10 people consume a

50 g portion of glucose (the reference food). Theirresponse to the test food (e.g. potato) is comparedwith their blood sugar response to 50 g glucose(the reference food). The GI is given as apercentage which is calculated by dividing thearea under the curve after youve eaten potatoesby the area under the curve after youve eaten theglucose. The final GI value for the test food is theaverage GI value for the 10 people. So, the GI ofbaked potatoes is 85, which means that eatingbaked potato produces a rise in blood sugarwhich is 85% as great as that produced aftereating an equivalent amount of glucose.

Appendix 1 (The glycaemic index andcarbohydrate content of foods) gives the GIcontent of many popular foods. Most values liesomewhere between 20 and 100. Sportsnutritionists find it useful to classify foods as highGI (71100), medium GI (5670) and low GI(055). This simply makes it easier to select theappropriate food before, during and afterexercise. In a nutshell, the higher the GI, thehigher the blood sugar levels after eating thatfood. In general, refined starchy foods, includingpotatoes, white rice and white bread, as well assugary foods, such as soft drinks and biscuits arehigh on the glycaemic index. For example,baked potatoes (GI 85) and white rice (GI 87)produce a rise in blood sugar almost the same aseating pure glucose (yes, you read correctly!).Less refined starchy foods porridge, beans,lentils, muesli as well as fruit and dairyproducts are lower on the glycaemic index. Theyproduce a much smaller rise in blood sugarcompared with glucose.

Only a few centres around the world providea legitimate GI testing service. The HumanNutrition Unit at the University of Sydney inAustralia has been at the forefront of GIresearch for over two decades, and hasmeasured the GI of hundreds of foods.International Tables of Glycaemic Index havebeen published by the American Journal of


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Figure 3.2 Measuring the GI of food

Bloo

d su

gar

leve

l

Bloo

d su

gar

leve

l

Glucose (reference food) Potato (test food)

1 hour 2 hours 1 hour 2 hours

Time Time

100% 85%


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Factors that influence the GI of a food

Factor How it works Examples of foods

Particle size Processing reduces particle size and Most breakfast cereals,makes it easier for digestive enzymes e.g. cornflakes and rice to access the starch. The smaller the crispies have a higher GI particle size (i.e. the more processed than muesli or porridge.the food), the higher the GI.

Degree of starch The more gelatinised (swollen Cooked potatoes (high GI);gelatinisation with water) the starch, the greater biscuits (lower GI).

the surface area for enzymes to attack, the faster the digestion and rise in blood sugar, i.e. higher GI.

Amylose to amylopectin There are two types of starch: Beans, lentils, peas and ratio amylose (long straight molecule, basmati rice have high

difficult access by enzymes) and amylose content, i.e. low GI; amylopectin (branched molecule, wheat flour and productseasier access by enzymes). The containing it have high more amylose a food contains the amylopectin content, i.e.slower it is digested, i.e. lower GI. high GI.

Fat Fat slows down rate of stomach Potato crisps have a loweremptying, slowing down digestion GI than plain boiledand lowering GI. potatoes; adding butter or

cheese to bread lowers GI.

Sugar (sucrose) Sucrose is broken down into Sweet biscuits, cakes,1 molecule of fructose and 1 molecule sweet breakfast cereals, of glucose. Fructose is converted honey.into glucose in the liver slowly, giving a smaller rise in blood sugar.

Soluble fibre Soluble fibre increases viscosity of Beans, lentils, peas, oats,food in digestive tract and slows porridge, barley, fruit.digestion, producing lower blood sugar rise, i.e. lowers GI.

Protein Protein slows stomach emptying Beans, lentils, peas, pastaand therefore carbohydrate (all contain protein as welldigestion, producing a smaller blood as carbohydrate). Eating sugar rise, i.e. lowers GI. chicken with rice lowers

the GI.

Table 3.2

Clinical Nutrition (Foster-Powell and Brand-Miller, 1995; Foster-Powell et al., 2002). But newand revised data are constantly being added tothe list as commercial foods are reformulatedand these are available on the websitewww.glycemicindex.com (note the US spelling ofthis website not to be confused withwww.glycaemicindex.com)

What makes one food have a highGI and another food a low GI?Factors that influence the GI of a food includethe size of the food particle, the biochemicalmake-up of the carbohydrate (the ratio ofamylose to amylopectin), the degree of cooking(which affects starch gelatinisation), and thepresence of fat, sugar, protein and fibre. Howthese factors influence the GI of a food issummarised in Table 3.2.

How can you calculate the GI of ameal?To date, only the GIs of single foods have beendirectly measured. In reality, it is more useful toknow the GI of a meal, as we are more likelyto eat combinations of foods. It is possible toestimate the GI of a meal by working out its totalcarbohydrate content, and then the contribution

of each food to the total carbohydrate content.Table 3.3 shows how to calculate the overall GIof a typical breakfast.

For a quick estimate of a simple meal, such asbeans on toast, you may assume that half thecarbohydrate is coming from the bread and halffrom the beans. So you can add the GI values ofthe two foods together and divide by 2: (70 + 48) 2 = 59.

If you have uneven proportions of two foods,for example 75% milk: 25% muesli, then 75% ofthe GI of milk can be added to 25% of the GI ofmuesli.

What are the drawbacks of the GI?The key to efficient glycogen refuelling andminimal fat storage is to maintain steady levels


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How to calculate the GI of a meal

Food Carbohydrate % total GI Contribution(g) carbohydrate to meal GI

Orange juice (150 ml) 12.5 26 46 26% 46 = 12

Weetabix (30 g) 21 43 69 43% 69 = 30

Milk (150 ml) 7 15 27 15% 27 = 4

1 slice toast 13 27 70 27% 70 = 19

Total 48 100 Meal GI = 65

Source: Leeds et al., 2000.

Table 3.3

Why does pasta have a low GI?

Pasta has a low GI because of the physicalentrapment of ungelatinised starch granules in asponge-like network of protein (gluten)molecules in the pasta dough. Pasta cooked aldente has a slighter lower GI than pasta that hasbeen cooked longer until it is very soft. Pasta isunique in this regard, and as a result, pastas of anyshape and size have a fairly low GI (30 to 60).

of blood glucose and insulin. When glucoselevels are high (for example, after consuminghigh GI foods), large amounts of insulin areproduced, which shunts the excess glucose intofat cells. However, it is the combined effect of alarge amount of carbohydrate as well a foods GIvalue that really matters.

The biggest drawback of the GI is that itdoesnt take account of the portion size you areeating. For example, watermelon has a GI of 72 and is therefore classified as a high GI food which puts it off the menu on a low GI diet.However, an average slice (120g) gives you only6 g carb ohydrate, not enough to raise yourblood glucose level significantly. You would

need to eat at least 6 slices (720 g) to obtain 50 gcarbohydrate the amount used in the GI test.

Similarly, many vegetables appear to have ahigh GI, which means they may be excludedon low GI diet. However, their carbohydratecontent is low and therefore their effect onblood glucose levels would be small. Sodespite having a high GI the glycaemic load(GI x g carbohydrate per portion divided by100) is low.

Another drawback is that some high fat foodshave a low GI, which gives a falsely favourableimpression of the food. For example, the GI ofcrisps or chips is lower than that of bakedpotatoes. Fat reduces the rate at which food is


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Low GI diet at a glance

In essence, a low GI diet comprises carbohydratefoods with a low GI as well as lean protein foodsand healthy fats: Fresh fruit the more acidic the fruit the lower

the GI. Apples, pears, oranges, grapefruit,peaches, nectarines, plums and apricots have thelowest GI values while tropical fruits such aspineapple, papaya and watermelon have highervalues. However, as average portion size is small,the GL would be low.

Fresh vegetables most vegetables have a verylow carbohydrate content and dont havea GI value (you would need to eat enormousamounts to get a significant rise in bloodglucose). The exception is potatoes, which havea high GI. Eat them with protein/ healthy fat orreplace with low GI starchy vegetables (seebelow)

Low GI starchy vegetables these includesweetcorn (GI 4648), sweet potato (GI 46)and yam (GI 37)

Low GI breads these include stonegroundwholemeal bread (not ordinary wholemeal

bread), fruit or malt loaf, wholegrain bread withlots of grainy bits, breads containing barley, rye,oats, soy and cracked wheat or those containingsunflower seeds or linseeds; chapati and pittabreads (unleavened), pumpernickel (rye kernel)bread, sourdough bread

Low GI breakfast cereals these includeporridge, muesli and other oat or rye basedcereals, and high bran cereals (e.g. All Bran)

Low GI grains these include bulgar wheat,noodles, oats, pasta, basmati (not ordinarybrown or white ) rice

Beans and lentils chick peas, red kidneybeans, baked beans, cannellini beans, mungbeans, black-eyed beans, butter beans, split peasand lentils

Nuts and seeds almonds, brazils, cashews,hazelnuts, pine nuts, pistachios, peanuts;sunflower, sesame, flax and pumpkin seeds

Fish, lean meat, poultry and eggs thesecontain no carbohydrate so have no GI value

Low fat dairy products milk, cheese andyoghurt are important for their calcium andprotein content. Opt for lower fat versionswhere possible.

digested but saturated and trans fats (see pages114115) can push up heart disease risk. Itsimportant you dont select foods only by theirGI check the type of fat (i.e. saturated orunsaturated) and avoid those that contain largeamounts of saturated or trans fats.

What is the glycaemic load?You can gain a more accurate measure of the risein your blood glucose (and insulin level) by usingthe glycaemic load (GL). This concept is derivedfrom a mathematical equation developed byProfessor Walter Willett from the HarvardMedical School in the US. It is calculated simplyby multiplying the GI of a food by the amountof carbohydrate per portion and dividing by 100.One unit of GL is roughly equivalent to theglycaemic effect of 1 g of glucose. It gives you agood estimate of both the quality (GI) andquantity of carbohydrate:

GL = (GI x carbohydrate per portion) 100

So, for watermelon:

GL = (72 x 6) 100 = 4.3

A high glycaemic load can result from eatinga small quantity of a high-carbohydrate high GIfood (e.g. white bread) or a larger quantity of alow GI food (e.g. pasta). This results in a largesurge in blood glucose and insulin.

Conversely, eating smaller amounts of a low-carbohydrate high GI food (e.g. watermelon) or a

larger quantity of a low GI food (e.g. beans)produces a low glycaemic load. This results in asmaller and more sustained rise in blood glucose.

To optimise glycogen storage and minimisefat storage, aim to achieve a small or moderateglycaemic load eat little and often, avoidoverloading on carbohydrates, and stick tobalanced combinations of carbo hydrate,protein and healthy fat.

Theres no need to cut out high glycaemicfoods. The key is to eat them either in smallamounts or combined with protein and/or alittle healthy fat. This will evoke lower insulinlevels and less potential fat storage. Forexample, have a baked potato (high GI food)with a little margarine and baked beans or tuna(both low GI foods). Both protein and fat put abrake on the digestive process, slowing downthe release of glucose.

Should I use GI or GL?GI remains the best-researched and one of themost reliable indicators of health risk. In studiesat Harvard University a low GI diet has beencorrelated with a low risk of chronic diseaseslike heart disease, type II diabetes and cancer


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GI value GL value Daily GL total

Low 055 010 080

Medium 5670 1119 80120

High 71100 > 20 > 120

Glycaemic response in athletes

Scientists say that high GI foods have a smallereffect on blood glucose and insulin in regularexercisers compared with non-exercisers. Thatsbecause exercise modifies the glycaemicresponse. Studies at the University of Sydney inAustralia have found that when athletes are fedhigh GI foods, they produce much less insulinthan would be predicted from GI tables. In otherwords, they dont show the same peaks andtroughs in insulin as sedentary people do. Usethe GI index only as a rough guide to howvarious foods are likely to behave in your body.

of the bowel, upper gastro-intestinal tract andpancreas. In particular, low GI is linked to highlevels of HDL (good) cholesterol (see page115). So if you have a low GI diet the chancesare you have a high good cholesterol level anda lower risk of heart disease. In 1999 theWorld Health Organisation (WHO) and Foodand Agriculture Organisation (FAO) recom -mended that people base their diets on low GIfoods in order to prevent chronic diseases.

However, the risk of disease is also predictedby the GL of the overall diet. In other words,GL simply strengthens the relationship, whichsuggests that the more frequently people eathigh GI foods, the greater their health risk.

The downside to GL is that you could end upeating a low carbohydrate diet with a lot of fatand/ or protein. Use the GI table (AppendixOne) to compare foods within the same category(e.g. different types of bread) and dont worry

about the GI of those foods with a very lowcarbohydrate content (e.g. watermelon).

BEFORE EXERCISE

What, when and how much you eat beforeexercise will effect your performance, strengthand endurance. Paradoxically, consumingcarbohydrate increases carbohydrate burning inthe muscle cells yet still delays the onset offatigue. Numerous studies have concluded thatconsuming carbohydrate before exercise resultsin improved performance when comparedwith exercising on an empty stomach(Chryssanthopoulos et al., 2002; Neufer et al.,1987; Sherman et al., 1991; Wright et al., 1991).


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Does exercise first thing in themorning burn more body fat?

If fat loss is your main goal, exercising on anempty stomach such as first thing in themorning may encourage your body to burnslightly more fat for fuel. According to Universityof Connecticut researchers, insulin levels are attheir lowest and glucagen levels are at theirhighest after an overnight fast. This increases theamount of fat that leaves your fat cells and travelsto your muscles, where the fat is burned. On thedownside, you may fatigue sooner or drop yourexercise intensity and therefore end up burningfewer calories and less body fat! If performanceis your main goal, exercising in a fasted state willalmost certainly reduce your endurance. And ifstrength and muscle mass are important goals,you will be better off exercising after a light meal.After an overnight fast, when muscle glycogenand blood glucose levels are low, your muscleswill burn more protein for fuel. So you could endup losing hard-earned muscle!

How much fibre?

Dietary fibre is the term used to describe thecomplex carbohydrates found in plants that areresistant to digestion. It includes cellulose, pectins,glucans, inulin and guar. The Department ofHealth recommends between 18 g and 24 g aday. The average intake in the UK

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