glycemic index and sport nutrition

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Review

Glycemic Index in Sport Nutrition

Luca Mondazzi, MD, Enrico Arcelli, MDMapei Sport Service and Research Centre, Castellanza (VA) (L.M.), School of Exercise Science, University of Milan (E.A.), Milan,

ITALY

Key words: glycemic index, sport nutrition, glycogen recovery, glycogen loading, lipid oxidation, CHO oxidation

Carbohydrates (CHO) can be classified on the basis of their glycemic index (GI), and the use of this

classification has been increasingly supported by science. Because of its impact on blood glucose and insulin

responses following the ingestion of CHO foods, the GI has been studied in many fields of medicine, including

sport nutrition. As a new tool in sport nutrition, glycemic index manipulation has been evaluated to improve the

first and second phases of glycogen recovery, glycogen load, and exercise metabolism, including control of

rebound hypoglycemia and, it is interesting to note, stimulation of lipid oxidation for longer availability of

glucose sources during endurance exercise.

Although attractive, the use of GI in sport nutrition has received only partial support from available

experimental evidence. At the biochemical level, consistent evidence has been attained to suggest that GI

manipulation can determine variations in adipocyte lipolysis, plasma free fatty acids levels, and lipid and CHO

oxidation rates during exercise. However, when the effects of GI manipulation have been assessed at the

functional level, the results have been inconsistent, with evidence of improved exercise performance in some

studies, but not in many others. The purpose of the current article is to review the effects and limits of GI

manipulation in sport nutrition, and to propose an overall strategy for its application.

Key teaching points:

N The GI concept may be important in sport nutrition because of its relevance to postprandial blood glucose and, more important,

insulin responses.

N The modulation of GI may determine variations in many processes involved in sport nutrition, including glycogen recovery and

load and exercise metabolism.

N The use of low GI foods/meals has been associated with increased adipocyte lipolysis, increased plasma free fatty acids levels,increased lipid oxidation, and decreased CHO oxidation during endurance exercise.

N Although the effects of GI modulation on exercise performance have been inconsistent, the possible value of the GI concept in sport

nutrition warrants further evaluation.

INTRODUCTION

In 1981, Jenkins et al. [1] published the first index of the

relative glycemic effects of carbohydrates (CHO) from foods.

Per gram of CHO, foods with a high glycemic index (GI) cause

a higher level of postprandial blood glucose and a greater

overall blood glucose response during the first 2 hours after

consumption than foods containing low GI CHO. Despite some

initial controversies, it is now accepted that different foods

containing equal amounts of CHO can lead to a range of blood

glucose responses; therefore, the concept of GI has been

widely recognized as a reliable classification of foods

according to their postprandial glycemic effects, and it has

received attention in a number of fields in medicine,

Address correspondence to: Luca Mondazzi, MD, Mapei Sport Service and Research Centre, Via Don Minzoni 34, 21053 Castellanza (VA), ITALY. E-mail:

[email protected]

Developed from presentations made by the 2 authors at the Science in Nutrition 1st International Congress, organized by the Paolo Sorbini Foundation for Nutrition

Science and held in Rome, March 78, 2008.

Financial disclosure: This study was not supported by any grant or corporate sponsorship. Luca Mondazzi, MD, has no personal financial interest in the work. Enrico

Arcelli, MD, has no personal financial interest in the work.

Journal of the American College of Nutrition, Vol. 28, No. 4, 455S463S (2009)

Published by the American College of Nutrition

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encompassing diabetology, cardiovascular disease, diabetol-

ogy, and oncology [27].

The purpose of the current article is to review the effects

and limitations of the use of the GI in sport nutrition, and to

propose an overall strategy for its application. The review of

the literature will be organized into 4 main phases based on the

athletes position in the nutritional cycle: (1) first phase of

recovery, (2) second phase of recovery, (3) last meal before

race, (4) race (Fig. 1).

DESCRIPTION OF SUBJECT

First Phase of Recovery

When demanding exercise bouts follow one another at short

time intervals, it is useful to replenish glycogen deposits

between bouts. The first phase of recovery could have a major

role in this process.

When dietary intake during exercise and recovery was

restricted to water only, the first phase of recovery after

demanding exercise sessions that depleted muscle glycogen to

25% of resting level was characterized by a high glycogen-

synthesis rate, with maximal activity during the first 30 minutes

and then a rapid decline to about one fifth by 60 minutes and to

about one ninth by 120 minutes after the end of exercise [8].This high glycogen-synthesis rate is sustained by 2 mechan-

isms: increased glucose uptake and increased glycogen

synthase activity. During this short-lasting phase of recovery,

the muscle uptake of glucose can proceed independently of the

presence of insulin and is more active when muscle glycogen

at the end of exercise is low [8]. The insulin-independent

uptake of glucose by muscle fibers is mediated by the

translocation of glucose transporter carrier protein-4 (GLUT4)

to their surface, which is a well-known effect of insulin that is

stimulated also by muscle contractions via other, although notwell-defined, mechanisms [9]. In addition to their effects on

glucose uptake, low glycogen concentrations and muscle

contractions stimulate glycogen synthase activity indepen-

dently of insulin, which remains a main activator of this

enzyme [10].

Because the activation of glucose uptake and glycogen

synthase by muscle contractions is very short lived, the high

glycogen-synthesis rate of the first phase of recovery should be

supported by nutritional strategies aimed at obtaining the

fastest rate of glucose delivery to muscles. Therefore, it should

be preferable to choose high GI CHO, which can guarantee a

faster and greater increase in blood glucose levels comparedwith low GI CHO (Fig. 2). However, this is not the case. The

choice of a high GI CHO postexercise meal made no

difference in the rate of muscle glycogen synthesis at 8 and

24 hours post exercise when immediate (0, 2, 4, 8, and

22 hours) and delayed (2, 4, 6, 8, and 22 hours) feedings of

high GI meals were compared. According to the results of this

study, the 2 hours delayed feeding should not cause any

worsening of the postexercise glycogen resynthesis rate when

compared with immediate feeding of the same amount of high

GI CHO. On this basis, it should be reasonable to speculate

that there is no difference between high and low GI CHO

administration during the first 2 hours after the end of aglycogen depleting exercise, provided that there is sufficient

time to recover and the sufficient CHO is ingested during the

whole recovery period. In contrast, when the recovery period is

very short and less than 8 hours is available between 2 exercise

sessions, the choice of high GI CHO could actually make

muscle glycogen reach higher concentrations by the start of the

following bout of exercise. Indeed, in a study by Ivy et al. [11],

when CHO was ingested immediately or 2 hours post exercise

and muscle biopsies were taken no longer than 4 hours after

Fig. 1. Proposed phases of the nutrition cycle of athletes as identified

for description of glycemic index effects in sport nutrition.

Fig. 2.Comparison of typical blood glucose variations after high GI or

low GI CHO ingestion. After high GI CHO ingestion, blood glucose

concentration shows a steeper increase and reaches higher values than

after low GI CHO ingestion.


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the end of exercise, the delay in ingestion of CHO was

associated with lower muscle glycogen-synthesis rates. When

recovery time is very short and/or muscle glycogen depletion

is profound because of very demanding exercise, high GI

natural foods can be replaced by CHO supplements made up of

glucose or maltodextrins solutions, which allow the fastest

possible rates of absorption and delivery to the muscle.

In conclusion, although the physiological backgrounds

favor the choice of high GI CHO during the first phase of

recovery after glycogen depleting exercise, experimental

evidence is very scarce and supports this concept only when

the recovery time is very short.

Second Phase of Recovery

The second phase of recovery after glycogen depleting

exercise bouts can last from several hours to days, extending

from the end of the first phase of recovery to the start of the

last meal before exercise (Fig. 1). It requires insulin, is

associated with increased insulin sensitivity, and is character-ized by a much lower rate of glycogen synthesis when

compared with the first phase [8]. It persists for many hours

but tends to be shorter when CHO intake is high, glycogen

synthesis is more active, and muscle glycogen levels are

increased [12].

Because this phase of recovery requires insulin for

glycogen synthesis and uptake of glucose by muscle fibers,

from a theoretical point of view, higher insulin secretion is

ideal. Therefore, the choice of high GI CHO should be more

effective for replenishing glycogen stores than is low GI

CHO. Evidence suggests that in contrast to the first phase of

recovery, where the GI of CHO makes little or no differencein insulin levels, during the second phase of recovery

substantial differences in insulin levels can be observed

between high and low GI CHO feedings [13,14]. Despite this

substantial difference in insulin levels, the second phase of

recovery insulin levels are still lower than those produced by

ingestion of the same amount and type of CHO at rest,

because of the increased insulin sensitivity induced by

exercise [13].

The rate of glycogen synthesis in the first 24 hours after

exercise was evaluated in a study by Burke et al. [14]. In this

study, for the 24 hours following glycogen depleting exercise,

subjects rested and consumed 4 high CHO meals of high GI orlow to moderate GI. The increase in muscle glycogen

concentration from the end of exercise to the end of the

24 hour recovery, as assessed by muscle biopsy, was greater by

approximately 50% (106.1 6 11.7 mmol/kg wet weight vs.

71.5 6 6.5 mmol/kg wet weight;p 5 0.02) when foods with a

high GI were consumed.

The greater efficacy of high GI CHO meals in replenishing

muscle glycogen stores during the second phase of recovery

was confirmed in a more recent study by Wee et al. [15]. In

this study, after an overnight fast, subjects consumed a high ora low GI breakfast, and 3 hours later muscle samples were

obtained for glycogen evaluation. Muscle glycogen concentra-

tion increased by 15% after the high GI breakfast and remained

unchanged in the other case.

In conclusion, available experimental evidence indicates

that glycogen stores may be replenished more rapidly during

the second phase of recovery following demanding exercise

with high GI CHO meals. From a practical point of view, the

length of this phase would make a great difference in the

choice of the most appropriate nutritional strategy. In fact,

when the time between 2 bouts of demanding exercise is

measured by days, compared with hours, no evidence suggests

that high GI meals replenish glycogen stores any better than

low GI meals, providing that an adequate amount of CHO is

ingested. Moreover, the choice of low GI foods may offer

advantages over that of high GI foods when energy metabolism

during subsequent exercise is considered, as is discussed in the

following section.

Last Meal before Exercise

The nutritional choices made at the last meal before

exercise could have consequences for the replenishment of

glycogen stores and for energy metabolism following exercise.The choice of low GI CHO food for the last meal has been

suggested to increase the contribution of lipid oxidation to

energy production with consequent sparing and extended

availability of glucose sources during exercise. As is shown in

Fig. 3, the evidence regarding this strategy can be divided into

(1) biochemical (blood glucose, insulin and free fatty acids

levels, lipid oxidation and CHO oxidation rates, and muscle

glycogen content) and (2) functional (exercise performance)

types.

Fig. 3. Low GI meal(s) before exercise: effects of metabolism and

exercise performance. Proposed rationale for choosing low GI meal(s)

before exercise: biochemical and functional levels of evidence.


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Glucose. As expected, following the ingestion of high GI

preexercise meals, blood glucose concentrations rise and

decline more sharply and reach higher peak levels than after

low GI feedings that contain the same amount of CHO. Serum

insulin concentrations tend to parallel those of blood glucose,

with significantly higher values noted after high CHO

preexercise feedings. This has been found in virtually all

studies in which blood glucose and insulin responses were

assessed before the start of exercise [16,17]. In many studies,

blood glucose levels were also assessed during exercise. In the

first few minutes of exercise, blood glucose levels can drop

dramatically and, in some cases, fall to below normal levels(3.5 mmol/L or 63 mg/dL); this is referred to as rebound

hypoglycemia (Fig. 4). It usually lasts no longer than a few

minutes and is irrelevant to the availability of glycogen stores

for exercise. Although blood glucose values as low as

3.0 mmol/L very often cause symptoms of neuroglycopenia

in resting people, during exercise this seems to be quite

unusual, with little or no relevance for athletic performance

[18,19]. Nevertheless, some athletes might have high sensitiv-

ity to low blood glucose levels during exercise, and exercise-

induced rebound hypoglycemia might be a phenomenon that

affects their performance [19]. Of course, a progressive warm-

up can prevent such an unfavorable outcome.The reduction in blood glucose levels that occurs in the

early phase of exercise and, in its most severe form, rebound

hypoglycemia are probably caused by the combination of

accelerated exercise- and insulin-induced glucose uptake by

muscles, in conjunction with depressed glucose production by

the liver [19]. Therefore, insulin levels before the start of

exercise may be of importance in the pathogenesis of blood

glucose reduction in its early phase. Accordingly, in many

studies, mean blood glucose levels at 15 to 30 minutes during

exercise were lower after high GI than after low GI preexercise

feeding [1517,2028].

Hypoglycemia that occurs during the latter phase of

prolonged exercise appears to be caused by the inability of

the liver to maintain blood glucose levels and is associated

with depletion of liver and muscle glycogen stores [29].

During the latter phase of prolonged exercise, blood glucose

levels were reported to be higher when subjects were fed low

GI compared with high GI preexercise meals [26,3034].

However, the data have been inconsistent [16,17,20,22

25,28,3538].

In conclusion, there is good agreement that rebound

hypoglycemia can be prevented by the use of low GI

preexercise CHO meals, although this has not resulted in

improved athletic performance in most cases. On the contrary,

little evidence indicates that low GI CHO preexercise meals

are associated with higher blood glucose levels in the latter

phases of endurance exercise.

Insulin.As expected, after the last meal before the start of

exercise, serum insulin concentrations showed higher peak

levels and larger incremental area under the curve over the

postprandial period when high GI meals were compared with

low GI meals. However, within minutes of the start of exercise,

insulin levels rapidly fall and reach the same low values,

regardless of the GI of the preexercise meal. Low insulin levels

then may be maintained throughout exercise or may decrease

even further. However, this effect is independent of the GI of

the preexercise meal [1517,2026,32,3437].

Free Fatty Acids. Because insulin is an inhibitor of

lipolysis and a stimulator of lipogenesis, the higher the serum

insulin levels, the lower the possibility of releasing free fatty

acids (FFAs) from adipose tissue into the bloodstream [39].

When the effect of high GI or low GI meals on the postprandial

decrease in plasma FFA levels was evaluated, in some studies

this decrease was reported to be greater in the former than in

the latter cases [15,17,21,32], although this observation was

not confirmed by other studies [16,2326,28,30,34,35,37,40].

During exercise, while insulin levels decreased, plasma

FFA concentrations gradually increased, after a low GI and

after a high GI meal, but evidence suggested a greater increase

after a low GI meal in most [1517,20,21,23,26,28,30,32,33,3537], but not all [24,25,34,38], studies. Plasma glycerol

concentrations showed a similar response to that of plasma

FFAs, indicating that the increase in plasma FFA availability

during exercise was due to lipolysis [15,16,21,23,26,32,37,38].

Therefore, convincing evidence indicates that a preexercise

low GI meal can induce greater plasma FFA concentrations

through stimulation of lipolysis both during the resting

postprandial period and, more important, during the subse-

quent exercise session. On the contrary, high insulin levels

Fig. 4. Typical behavior of blood glucose levels in reactive

hypoglycemia and their correlations with symptoms in health

and disease.


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Exercise Performance. Effects on exercise performance

were evaluated by measuring time to exhaustion during steady-

state running or cycling [16,21,30,31,36], by time trial (TT)

running or cycling tests [22,2426], by 15 to 30 minute

performance cycle tests [17,20,35], by incremental exercise

tests to exhaustion [27], and, finally, by alternate sprinting and

jogging to fatigue [38]. As a whole, the effects on exercise

performance were inconsistent, with evidence of improvement

in some studies [16,26,30,31,36] but not in others [17,20

22,24,25,27,35,38] when low GI preexercise meals were

compared with high GI ones. The most supportive results for

performance-enhancing effects of low GI meals were obtained

when exercise was performed at submaximal intensity through

to the end, which occurred in 3 [16,30,36] out of 4 [21] studies.

On the contrary, when the last part of the test was a 15 to

40 minute bout of high intensity exercise, the results did not

support low GI meals [17,20,22,35]. For instance, in a 10 km

TT run preceded by 1 hour of submaximal steady-state

exercise, the results did not favor low GI diets [24,25], but

when the distance of the TT run was longer (16 km) and the

intensity of the exercise lower, a low GI preexercise meal was

associated with improved performance [26].

When plasma FFA levels were considered, it was found

that FFA levels and performance were evaluated in 10 studies:

performance improved in 4 [16,26,30,36] out of 7 [17,21,35]

studies reporting higher lipid oxidation rates during exercise

after a low GI meal. The remaining 3 studies did not report

higher FFA levels during exercise and reported no improve-

ment in performance [24,25,38].

When lipid oxidation rate was considered, it was found that

this rate and performance had been evaluated in 7 studies:

performance improved in 3 [16,26,36] out of 5 [21,24] studies

that showed higher lipid oxidation rates during exercise after a

low GI meal. The remaining 2 studies did not find higher lipid

oxidation rates during exercise and reported no improvement

in performance [22,25]. Whether or not these weak correla-

tions between GI of the preexercise meal, lipid metabolism,

and performance hold true, the whole picture remains puzzling

and the possible effects of GI on performance are unclear.

Nevertheless, in no case was the choice of a high GI

preexercise meal suggested to better modulate energy

metabolism during exercise.

In some of the aforementioned studies, preexercise mealswere supplied in the evening and exercise tests were

administered after an overnight fast [32,36,38]. This latency

did not prevent the observation of differences in plasma FFA

levels [32,36] or reduction in average RER values, lipid

oxidation rate, and time to exhaustion [36] after low GI in

comparison with high GI preexercise meals. Nevertheless,

exercising in the fasted state is usually not advisable. In recent

years, it was observed that a single low GI meal could improve

glucose tolerance and then reduce insulin release at a second

meal. This second meal effect was observed at lunch

following a low GI breakfast [49] or even at breakfast

following a low GI evening meal [50]. Therefore, it was tested

whether the effects of a low GI evening meal on exercise

metabolism could be maintained the following day after a

commonly used high GI breakfast with skimmed milk and

cornflakes, white bread and jam, and a CHO beverage,

providing 2 g CHO per kg body mass [51,52]. As expected,

during the postprandial period following ingestion of a high GI

breakfast, plasma glucose and insulin concentrations were

higher in the high GI than in the low GI trial, but no

differences in blood glucose, plasma FFA and glycerol

concentrations, RER, and estimated lipid and CHO oxidation

rates were observed during subsequent exercise [51,52].

Therefore, it seems that a low GI evening meal can improve

glucose tolerance at breakfast, but the metabolic responses to

subsequent exercise are not affected.

CONCLUSION

Glycemic index manipulation has been evaluated to

improve many aspects of sport nutrition, including first and

second phases of recovery, glycogen load, and modulation of

metabolism during exercise.

Because the fast reaching of high blood glucose concentra-

tions is of value to early recovery, high GI feedings are

supposed to be the most effective choice in this phase. In

addition, blood glucose levels support insulin action in the

second phase of recovery and in glycogen loading, when the

maximum amount of glucose uptake by muscle is a sought-

after effect. Besides its effects on blood glucose levels,

manipulation of GI could influence the second phase of

recovery, glycogen load, and metabolism during exercise

through modulation of postprandial insulin levels.

At the biochemical level, sound evidence suggests that high

GI meals determine faster and higher increases in blood

glucose levels and higher insulin levels. Furthermore, evidence

indicates that even small elevations in insulin levels can inhibit

lipolysis and FFA release by the adipocyte [53], determining

lower plasma FFA levels that, in turn, are associated with

lower rates of lipid oxidation [15,16,21,23,36,37,4245,54]

and higher rates of CHO oxidation [12,19,21,35,4345,53] inthe muscle. Nevertheless, it is not clear whether this translates

into muscle glycogen sparing during exercise following low GI

meals, because evidence in this area is still scarce [15,17,35].

However, even if substantial muscle glycogen sparing is not

achieved, low GI preexercise meals could allow lower rates of

muscle glucose uptake and oxidation [17] and, therefore, more

prolonged availability of the bodys glucose sources.

When the effects of GI manipulation were assessed at a

functional level, though, evidences were at least inconsistent,


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with many studies not favoring low GI interventions [17,2022,24,25,27,35,38]. However, the study protocols differed

greatly, making evaluation of the results difficult. Never-

theless, it is our opinion that even if they are little and are not

easy to demonstrate, the possible advantages of GI manipula-

tion for exercise performance could be relevant at the

professional level, and sport nutrition strategies based on GI

should not be dismissed as useless on the basis of the available

scientific literature. According to the data and hypothesis

hereby presented, the choice of high GI feedings would be

preferable to increase glycogen synthesis, while low GI

feedings would allow sparing of glucose sources and

availability during endurance exercise. The relative value ofhigh GI and low GI feedings in pursuing these effects from the

first phase of recovery to the last meal before exercise is

schematically represented in Fig. 5.

When GI manipulation is used, it seems essential to keep in

mind that most of its effects are mediated by the manipulation

of insulin levels, and that in some cases, GI is actually not

predictive of insulin responses to food ingestion [5557]. In

particular, protein ingestion elicits a blood insulin increase,

and a meal composed of low GI CHO and protein could

actually determine a marked increase in blood insulin levels

[5861]. In manipulation of a preexercise meal for modulation

of exercise metabolism, in addition to the choice of a low GImeal, we believe that it would be preferable to allow the return

of postprandial insulin levels to the basal value before the start

of exercise. Therefore, refinement of CHO stores should be

completed by the last meal before exercise, and no food should

be ingested thereafter if only light warm-up is done before the

start of the race. During intense warm-up, on the contrary,

CHO could be ingested without an increase in insulin level.

Moreover, CHO could be ingested in the very last minute

before the start of the race, in cases where food ingestion

during exercise is difficult. This last CHO intake should be

confined to the very last minute before the start of the race, to

hinder any increase in insulin level. Nevertheless, whenever

feasible, CHO ingestion should be postponed until after the

start of the race, when exercise intensity is not light and the

lack of any insulin response is more certain.

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Received November 13, 2009.



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