digestion

Digestion

• Purpose is to mechanically (with teeth and muscular contractions) and chemically (with enzymes) break down food to allow nutrients to be absorbed into the bloodstream

• Food is broken down into macro- and micronutrients

• Calories (a measure of energy) are obtained that are needed to power all cellular processes

Macronutrients

Classification:

• Carbohydrates (sugars, starches, fiber)

• Fats (saturated and unsaturated (mono- and poly-))

• Proteins

*See handout for more specific information

Micronutrients

• Vitamins - compounds required for life in small amounts and must be obtained via diet (they are involved in many biochemical reactions)

• Minerals - elements that cannot be made by our bodies and are required in many biochemical reactions (they exist as ions in the body: Na+, K+, Ca2+, Mg2+, etc.)

• Phytochemicals - wide variety of plant compounds found in fruits and vegetables that have many beneficial effects on health but are not required nutrients for survival

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Carbohydrates• Function: Used as energy source (glucose) in

cellular respiration and used as a stored energy source (glycogen in liver and skeletal muscle)

• Stimulate secretion of insulin by beta-cells of the pancreas.

• Once liver and muscle glycogen is sufficient, excess carbohydrates are converted to fat and stored in adipose tissues

Hormonal Blood Sugar Regulation

• Insulin and glucagon (both released from pancreas) regulate blood sugar

• Insulin lowers blood sugar if it is too high

• Glucagon raises blood sugar if it is too low

• Carbohydrates stimulate insulin release (fats and proteins have little effect on insulin)



Effect of Carbohydrates on Blood Sugar(Glycemic Index and Glycemic Load)

• All carbohydrates digest into sugars to be released into the blood (except for fiber)

• Glycemic index (GI) measures how fast a carbohydrate is digested and raises blood sugar and insulin levels





Glycemic Index (continued)

• Foods with high GI have very little fiber, protein, and fat

• Examples: Starchy carbohydrates (white bread, white rice, pasta, crackers, potatoes) and sugary drinks (regular sodas, sports drinks, juices)

• Avoid foods where the main ingredients are white flour, high-fructose corn syrup, sugar

Glycemic Load

• Measures the amount of carbohydrate per serving and the impact it has on blood sugar

• Total carbohydrate - fiber = net carbs

GL = Net carbohydrates/serving X GI/100





Glycemic Load (continued)

• Some foods with high GI have low GL and therefore have little effect on blood sugar when eaten in moderation

• Ex. Carrots and watermelon (both have a very high GI, but a very low GL)

Benefits of low GI and GL diet

• Lower insulin levels• Stable blood sugar levels• Improved blood lipid profile (lower LDL, higher HDL,

lower triglycerides) • Less accumulation of body fat and weight gain• Improved satiety (fullness)• Improved gastrointestinal health (due to higher fiber

content)• Reduced risk of cardiovascular disease, obesity and type

II diabetes due to the above factors

Effects of chronically elevated blood sugar levels

• Leads to insulin resistance (cells won’t respond to insulin since pancreas is constantly producing it to deal with the excess sugar)

• Low HDL and high triglycerides (fats) - both contribute to atherosclerosis

• Elevated blood pressure• Higher cancer risk• Higher risk of blood clots• Higher risk of kidney stones



Fats“The idea that all fat is bad for you, with the exclusive focus on adverse effects of fat may have contributed to the obesity epidemic… The emphasis on total fat reduction has been a serious distraction in efforts to control obesity and improve health in general.”

- Walter WillettChair of Department of NutritionHarvard School of Public Health

Fats (Triglycerides)

• Are composed of 3 fatty acids bonded to glycerol











Saturated fatty acid (no double bonds between carbons)

Monounsaturated fatty acid (one double bond)

Polyunsaturated fatty acid (more than one double bond)

Different Types of Fatty Acids

Trans-fats are polyunsatured fats that have been “hydrogenated”(behave more like saturated fats except far worse)

How does the type of fat affect blood cholesterol levels?

Type of fat HDL levels LDL levels

saturated

monounsaturated

polyunsaturated

trans

When substituted for refined carbohydrates and excess saturated fat

Cholesterol

• Used to synthesize hormones, vitamin D, and is a component of cell membranes

• Most (75-80%) is produced by the liver. The remainder comes from diet

• Dietary cholesterol is less of a factor in determining blood cholesterol ratios than dietary fat intake



Lipoproteins and Cholesterol Transport (a review)

• Lipoproteins are molecules that carry cholesterol to and from tissues (are produced in the liver)

• LDL (low density - BAD) particles carry cholesterol to the tissues (excess cholesterol ends up in arterial walls forming plaque)

• HDL (high density - GOOD) particles take cholesterol from tissues and blood vessels and bring it to the liver for disposal


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Carry dietary fats fromsmall intestine

to tissues

Produced in liverLipid transport in the body:

Liver

VLDL Carry triglyceridesto tissues and become

LDL

Brings cholesterol to cells(when cells have enough

cholesterol, LDLdeposits cholesterol in arterial walls - (especially if LDL particle is small))

Plaque formation and beginning stages ofatherosclerosis

HDL

Picks up cholesterol fromtissues and blood

returns cholesterol to liver for

production of bile salts

produces

produces

Can lead to

Soluble fiber, blood sugar and cholesterol

• Soluble fiber prevents bile acids from being reabsorbed into the body (liver must take cholesterol from the blood to make more)

• Soluble fiber also slows digestion and slows the absorption of sugar into the bloodstream, lessening the need for large amounts of insulin at once

• Good sources - oat fiber (oatmeal, oatbran), apples, peas, legumes (beans), citrus fruits, fiber supplements, barley

Digestion

• Mechanical digestion - teeth, churning of stomach, segmentation of small intestine

• Chemical digestion - enzymes break bonds of macronutrient molecules

• Chemical digestion of carbohydrates begins in the mouth, protein digestion begins in the stomach, and fat digestion begins in the small intestine

• All digestion is completed in the small intestine

3 phases of Gastric (Stomach) Secretion

• Cephalic - smell, thought, sight, taste, or texture of food stimulates gastric secretions (HCl (acid) and pepsin (protein digesting enzyme) in the stomach)

• Gastric - Stretching of stomach lining or presence of proteins or caffeine in the stomach increases gastric secretions

• Intestinal - Presence of chyme in the duodenum inhibits gastric secretion (especially if chyme is fatty or acidic (ph below 2))



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Digestive hormones

• Cholestokynin (CCK) - stimulates bile release from gallbladder and lipase release from pancreas (released by cells in the duodenum)

• Secretin - stimulates pancreas to release bicarbonate to neutralize acid in the duodenum (released by cells in the duodenum)

• Gastrin - released by the stomach to increase gastric secretion



Hormonal Appetite Regulation

• Leptin - released by adipose tissue and signals hypothalamus that you are full

• Ghrelin - released by stomach and stimulates appetite by acting on the hypothalamus

• Orexins - released by hypothalamus and stimulate appetite (ghrelin increases orexin levels while leptin decreases them)



Hypothalamic regulation of appetiteby ghrelin and leptin

digestion

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