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The Carbohydrates: Sugars, Starches, and Fibers
Chapter 4
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Introduction Brain Muscles Sources of carbohydrates
Glucose Muscles Glycogen Fat Sources of carbohydrates “Fattening” – mistaken thinking
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Chemist’s View of Carbohydrates
Carbohydrate family Atoms and chemical bonds Monosaccharides Simple carbohydrates Glucose, fructose, galactose Disaccharides Maltose, sucrose, lactose Polysaccharides Complex carbohydrates
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Atoms and Their Bonds Figure 4-1 Atoms and their bonds
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Monosaccharides Same numbers and kinds of atoms Glucose – blood sugar
Differing sweetness Glucose – blood sugar Essential energy source Part of every disaccharide Fructose Sweetest of the sugars Galactose Only in a few foods
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Chemical Structure of Glucose
On paper, the structure of glucose has to be drawn flat, as it is in the first diagram, but in nature the five carbons and oxygen are roughly in a plane. The atoms attached to the ring carbons extend above and below the plane. In the second diagram, the lines representing some of the bonds and the carbons at the corners are not shown. In the third diagram, the single hydrogens are not shown, but lines still extend upward or downward from the ring to show where they belong. Figure 4-2 The chemical structure of glucose
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Chemical Structure of the Monosaccharides
Figure 4-3 The monosaccharides
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Disaccharides Pairs of three monosaccharides Condensation Hydrolysis
Maltose – two glucose units Sucrose – glucose and fructose Lactose – galactose and glucose Condensation Links two monosaccharides together Hydrolysis Breaks a disaccharide in two
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Hydrolysis of a Disaccharide
Figure 4-5 Hydrolysis of a disaccharide
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Polysaccharides Glycogen Starches Storage form of energy in the body
Glucose molecules linked together in highly branched chains Starches Storage form of energy in in plants Glucose molecules linked together in branched or unbranched chains
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Glycogen and Starch Compared
Figure 4-6 Glycogen and starch compared
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Stepped Art – Glycogen and Starch Compared
Starch (amylose) Starch (amylopectin) A glycogen molecule contains hundreds of glucose units in highly branched chains. Each new glycogen molecule needs a special protein (shown here in red) for the attachment of the first glucose. Figure 4-6 Glycogen and Starch Compared For details of the chemical structures, see Appendix C. A starch molecule contains hundreds of glucose molecules in either occasionally branched chains (amylopectin) or unbranched chains (amylose). Stepped Art – Glycogen and Starch Compared
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Dietary Fibers Structural parts of plants Differ from starches
Soluble fibers – benefits Insoluble fibers – benefits Functional fibers Resistant starches Phytic acid
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The Bonds of Starch and Cellulose Compared
Figure 4-7 The bonds of starch and cellulose compared
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Carbohydrate Digestion
Ultimate goal Glucose for absorption and use Hydrolysis via enzymes Mouth Amylase Stomach Stomach acid and protein-digesting enzymes Role of fiber
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Intestinal Role in Digestion
Small intestine Most carbohydrate digestion occurs here Pancreatic amylase Specific disaccharide enzymes Maltase Sucrase Lactase Large intestine Fibers attract water
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Carbohydrate Digestion in the GI Tract
Figure 4-8 Carbohydrate digestion in the GI tract
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Carbohydrate Absorption
Active transport Glucose Galactose Facilitated diffusion Fructose Liver Conversion of fructose and galactose
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Absorption of Monosaccharides
Figure 4-9 Absorption of monosaccharides
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Stepped Art-Absorption of monosaccharides
Monosaccharides, the end products of carbohydrate digestion, enter the capillaries of the intestinal villi. Glucose is used by most cells in the body. Small intestine In the liver, galactose and fructose share metabolic pathways with glucose. Figure 4-9 Absorption of monosaccharides Monosaccharides travel to the liver via the portal vein. Stepped Art-Absorption of monosaccharides
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Lactose Intolerance Lactase activity Symptoms of intolerance
Highest immediately after birth Declines with age Symptoms of intolerance Causes of intolerance beyond age Prevalence Trait has a genetic component
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Dietary Changes with Lactose Intolerance
Manage dairy consumption rather than restrict GI bacteria Fermented milk products Individualized diets Potential nutrient deficiencies Riboflavin, vitamin D, and calcium
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Lactose in Selected Foods
Lactose (g) Whole-wheat bread, 1 slice 0.5 Dinner roll, 1 Cheese, 1 oz Cheddar or American Parmesan or cream 0.8 Doughnut (cake type), 1 1.2 Chocolate candy, 1 oz 2.3 Sherbet, 1 c 4.0 Cottage cheese (low-fat), 1 c 7.5 Ice cream, 1 c 9.0 Milk, 1 c 12.0 Yogurt (low-fat), 1 c 15.0 Table 4-2 Lactose in selected foods NOTE: Yogurt is often enriched with nonfat milk solids, which increase its lactose content to a level higher than milk's.
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Carbohydrate Metabolism
Glucose is key player Storing glucose as glycogen Liver storage Condensation into glycogen Hydrolysis for release of glucose when needed Muscle storage Hoards glycogen for use during exercise
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Using Glucose for Energy
Fuels most of body’s cells Preferred source for brain, nerve cells, and developing red blood cells Cellular breakdown of glucose Making glucose from protein Amino acid conversion Gluconeogenesis
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Ketone Bodies Made from fat fragments
Inadequate supply of carbohydrates Fat metabolism shifts Ketone bodies alternative food source during starvation Ketosis disturbs normal acid-base balance Carbohydrate needs for protein sparing and prevention of ketosis grams/day Using glucose to make fat
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The Constancy of Blood Glucose
Steady supply in blood stream Intestines – food Liver – glycogen Blood glucose homeostasis Insulin Moves glucose from blood into cells Glucagon and epinephrine Brings glucose out from storage
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Maintaining Blood Glucose Homeostasis
Figure 4-10 Maintaining blood glucose homeostasis
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Balancing Within the Normal Range
Balanced meals at regular intervals Diabetes Insulin either inadequate or ineffective Type 1 diabetes Type 2 diabetes Hypoglycemia Prevalence
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Glycemic Response Speed of glucose absorption, level of blood glucose, and return to normal glucose levels Low glycemic response Desired High glycemic response Fast absorption, surge in blood glucose, followed by a plunge below normal glucose levels Glycemic index classifies foods Limited utility
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Health Effects of Sugars
Pleasure in moderate amounts Obesity and chronic disease Nutrient deficiencies Energy with few other nutrients Discretionary kcalories Honey More energy per spoonful
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Dental Caries Bacteria ferment sugars producing acid
Erodes tooth enamel Food factors associated with tooth decay Time of food in mouth Sticky foods Frequency of sugar consumption Food choices Factors associated with tooth decay
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Illustration of Dental Caries
Figure 4-11 Dental caries
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Pulp (blood vessels, nerves)
Enamel Crown Caries Dentin Gum Pulp (blood vessels, nerves) Bone Figure Dental Caries Dental caries begins when acid dissolves the enamel that covers the tooth. If not repaired, the decay may penetrate the dentin and spread into the pulp of the tooth, causing inflammation, abscess, and possible loss of the tooth. Root canal Nerve Blood vessel Stepped Art
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Recommended Intakes of Sugars
Dietary Guidelines Reduce the intake of kcalories from added sugars DRI Added sugars No more than 25% of day’s total energy Impact on other food groups WHO and FAO recommendations Restrict added sugar consumption to less than 10% of total energy
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Alternative Sweeteners
Artificial sweeteners Non-nutritive sweeteners Large doses and adverse effects Stevia – herbal product Generally recognized as safe (GRAS) Sugar alcohols Provide kcalories Benefits and side effects
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Health Effects of Starch and Fibers
Heart disease Whole grains Sources Soluble fibers Improving heart disease risk factors Diet composition for reducing heart disease risk
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Fibers and Other Health Issues
Diabetes High-fiber foods GI health Ample fluids Weight management High-fiber foods and whole grains Feeling of fullness
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Fiber and Cancer High fiber diet protects against colon cancer
Fiber supplements Food sources of dietary fiber Phytochemicals Preventing colon cancer Diluting, binding, and removing Bacterial fermentation
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Harmful Effects of Excessive Fiber Intake
Insufficient energy or nutrients Abdominal discomfort, gas, diarrhea GI obstruction Nutrient absorption Dietary goals Balance, moderation, variety
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Characteristics, Sources, and Health Effects of Fibers
Major Food Sources Types of Fibers Actions in the Body Probable Health Benefits Viscous, Soluble, More Fermentable Barley, oats, oat bran, rye. fruits (apples, citrus), legumes (especially young green peas and black-eyed peas), seaweeds, seeds and husks, many vegetables, fibers used as food additives Gums Pectins Psylliuma Some hemicellulose Lower blood cholesterol by binding bile Slow glucose absorption Slow transit of food through upper Gl tract Hold moisture in stools, softening them Yield small fat molecules after fermentation that the colon can use for energy Increase satiety Lower risk of heart disease Lower risk of diabetes Lower risk of colon and rectal cancer Increased satiety, and may help with weight management Nonviscous, Insoluble, Less Fermentable Brown rice, fruits, legumes, seeds, vegetables (cabbage, carrots, brussels sprouts), wheat bran, whole grains, extracted fibers used as food additives Cellulose Lianins Resistant starch Hemicellulose Increase fecal weight and speed fecal passage through colon Provide bulk and feelings of fullness Alleviate constipation Lower risk of diverticulosis, hemorrhoids, and appendicitis Table 4-9 Characteristics, sources, and health effects of fibers.
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Recommended Intakes of Starch and Fibers
DRI for carbohydrates 45 to 65 percent of energy requirement RDA for carbohydrates 130 grams per day Fiber DV: 11.5 grams per 1000 kcalories DRI: 14 grams per 1000 kcalories No UL
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Fiber in Selected Foods
Grains Whole-grain products provide about 1 to 2 g (or more) of fiber per serving: 1 slice of whole-wheat, pumpernickel, rye bread 1 oz read-to-eat cereal (100% bran cereals contain 10 g or more) ½ cooked barley, bulgur, grits, oatmeal Tips to Increase Fiber Intake Eat whole-grain breads that contain ≥3 g of fiber per serving. Eat whole-grain cereals that contain ≥5 g of fiber per serving. Vegetables Most vegetables contain about 2 to 3 g of fiber per serving: 1 c raw bean sprouts ½ c cooked broccoli, brussels sprouts, cabbage, carrots, cauliflower, collards, corn, eggplant, green beans, green peas, kale, mushrooms, okra, parsnips, potatoes, pumpkin, spinach, sweet potatoes, swiss chard, winter squash ½ c chopped raw carrots, peppers Eat raw vegetables. Eat vegetables (such as potatoes and zucchini) with their skins. Fruits Fresh, frozen, and dried fruits have about 2 g of fiber per serving: 1 medium apple, banana, kiwi, nectarine, orange, pear ½ c applesauce, blackberries, blueberries, raspberries, strawberries Fruit juices contain very little fiber Eat fresh and dried fruit for snacks. Eat fruits (such as apples and pears) with their skins. Legumes Many legumes provide about 6 to 8 g of fiber per serving: ½ c cooked baked beans, black beans, black-eyed peas, kidney beans, navy beans, pinto beans Some legumes provide about 5 g of fiber per serving: ½ c cooked garbanzo beans, great northern beans, lentils, lima beans, split peas Add legumes to soups, salads, and casseroles. Table 4-10 Fiber in selected foods NOTE: Appendix H provides fiber grams for more than 2000 foods.
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From Guidelines to Groceries
Grains 1 ounce provides about 15g of carbohydrate “Three are key” message Vegetables Corn, peas, and potatoes have higher starch content Fruits Milk and milk products Protein foods
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Read Food Labels Food labels list Total carbohydrate Sugars
Starch, fibers, sugars Sugars Added and natural sugars
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Bread Labels Compared Figure 4-13 Bread labels compared
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Carbs, kCalories, and Controversies
Highlight 4
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Carbohydrates’ kCalorie Contributions
Obesity and the link to carbohydrates Total daily energy intakes have increased Activity levels have declined Increase in body weight Epidemiological studies Inverse relationship between carbohydrates and weight Weight loss kCalorie intake
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Energy Nutrients over Time
Figure H4-1 Energy nutrients over time
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Daily Energy Intake over Time
Figure H4-2 Daily energy intake over time
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Increases in Adult Body Weight over Time
Figure H4-3 Increases in adult body weight over time
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Sugars’ Share in the Problem
Increase in consumption of added sugars High-fructose corn syrup Body fat stores Carbohydrate cravings Self-imposed labeling of foods as good or bad Carbohydrate “addiction” Not physiological or pharmacological
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Appetite Control Fructose and insulin Food form – liquid or solid
Flaws in plausibility Food form – liquid or solid Beverages can influence weight gains by providing energy but not satisfying hunger
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Insulin’s Response Surge of insulin levels Glycemic effect
How high the surge influences whether body stores or uses its glucose Glycemic effect Factors impacting glycemic effect Glycemic index and body weight Insulin resistance Most often caused by being obese Body’s insulin response
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