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Nutrition (p. 986)

1. Body weight is stable when average daily energy intake and output are equal. Weight appears to have a homeostatic set point determined partly by heredity. About 30% to 50% of the difference in weight between people is hereditary and the rest is due to eating and exercise habits and other environmental variables.

2. Appetite is regulated by a hypothalamic feeding center that produces a sensation of hunger and a satiety center that produces a sensation of satisfaction.

3. Hunger is suppressed by leptin, cholecystokinin, and neurons called glucostats, and is briefly inhibited even by chewing and swallowing. Hunger is stimulated by endocannabinoids, neuropeptide Y, and the hunger contractions of an empty stomach.

4. Dietary Calories (kilocalories) come predominantly from carbohydrates, fats, and proteins. "Empty calories" are calories gained from foods such as sugar and alcohol that provide little or no other nutrition.

5. Nutrients are ingested chemicals that provide material for growth, repair, and maintenance of the body. Dietary substances that never become part of the body's tissues (for example, fiber) are not considered nutrients but are nevertheless important components of a healthy diet. Some nutrients (water, minerals, vitamins) require no digestion and yield no calories.

6. Water, carbohydrates, lipids, and proteins are required in relatively large amounts and are thus called macronutrients. Minerals and vitamins are needed in small amounts and are thus called micronutrients.

7. Essential nutrients must be included in the diet because the body cannot synthesize them from other chemicals.

8. Carbohydrates are used as fuel and as structural components of many biological molecules.

9. In the body, the carbohydrate fuels are blood glucose and liver and muscle glycogen. The balance between glycogen and glucose is regulated by insulin and glucagon.

10. Starch is the most quantitatively significant digestible dietary carbohydrate, but significant quantities of lactose, sucrose, and fructose are ingested, especially in processed foods with added sweeteners.

11. Dietary fiber includes cellulose, pectin, gums, and lignin. Fiber promotes intestinal motility and reduces the risk of colon cancer. Water-soluble fiber (pectin) also lowers the levels of blood cholesterol and harmful low-density lipoproteins (LDLs).

12. Fat contains most of the body's stored energy. Being hydrophobic and less oxidized than carbohydrates, fats contain more than twice as many calories per gram as carbohydrates do.

13. The use of fats for fuel spares glucose and proteins for use by other tissues or for other purposes.

14. Other lipids important in human metabolism and structure include phospholipids, cholesterol, fat-soluble vitamins, prostaglandins, and eicosanoids.

15. Essential fatty acids—linoleic and possibly linolenic and arachidonic acids—must be included in the diet because the body cannot synthesize them.

16. Lipids are transported in the blood as lipoproteins—droplets of cholesterol and triglycerides coated with proteins and phospholipids. The types of lipoproteins are chylomicrons, which are formed in the small intestine and transport dietary lipids throughout the body; very low-density lipoproteins (VLDLs), which transport lipids from the liver to the adipose tissue; low-density lipoproteins (LDLs), which are the remainders of the VLDLs after triglycerides are removed, and which transport cholesterol to cells that need it; and high-density lipoproteins (HDLs), which transport excess cholesterol back to the liver for disposal.

17. A high LDL concentration indicates a heightened risk of cardiovascular disease, while a high HDL concentration is beneficial to cardiovascular health.

18. Protein typically constitutes 12% to 15% of the body mass and performs a wider variety of structural and physiological roles than any other class of biological molecules.

19. The nutritional value of a protein depends on whether it provides the right proportions of the various amino acids, especially the eight essential amino acids. Complete proteins supply all the essential amino acids in the proportions needed for the human body. The body makes more efficient use of animal proteins than of plant proteins.

20. Nitrogen balance is a state in which average daily nitrogen intake and output are equal. A greater intake than output (positive nitrogen balance) is typical of childhood, pregnancy, resistance training, and other states of tissue growth. Greater output than intake (negative nitrogen balance) is typical in stress, muscle atrophy, and malnutrition.

21. Minerals are inorganic elements acquired from the soil by way of plants. Calcium and phosphorus are the body's most abundant minerals; sodium is a close third. Several others are present in relatively small quantities (table 26.3) but are vitally important.

22. Vitamins are small organic molecules that are not used for fuel (caloric content), but are necessary to metabolism. They act as coenzymes, antioxidants, components of visual pigments, and in other roles.

23. Vitamin C and the B vitamins are the water-soluble vitamins; vitamins A,

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1014 Part Four Regulation and Maintenance

D, E, and K are the fat-soluble vitamins.

24. Vitamin deficiencies cause a variety of illnesses, although vitamin excesses (hypervitaminosis) can also be quite harmful.

Carbohydrate Metabolism (p. 996)

1. The complete oxidation of glucose has the equation C6H12O6 + 6 O2 ^ 6 CO2 + 6 H2O.

2. The coenzymes NAD+ and FAD are especially important in transferring electrons from one metabolic pathway to another in this process.

3. Glucose oxidation begins with a pathway called glycolysis, which splits glucose into two pyruvic acid molecules and has a net yield of 2 ATP per glucose.

4. In the absence of oxygen, pyruvic acid is reduced to lactic acid in a one-step reaction called anaerobic fermentation. The primary purpose of this is to regenerate NAD+, which is needed to keep glycolysis running and producing ATP.

5. In the presence of oxygen, pyruvic acid enters a pathway called aerobic respiration, which produces much more ATP and has end products (CO2 and H2O) that are less toxic than lactic acid. Aerobic respiration occurs in the mitochondria.

6. The first principal group of reactions in aerobic respiration are the matrix reactions, mainly the citric acid cycle. This cycle breaks pyruvic acid down to CO2, generates 2 ATP per glucose, and most importantly, generates 8 NADH and 2 FADH2.

7. The final reactions of aerobic respiration are the membrane reactions, which occur on the inner mitochondrial membrane. Enzymes and other electron carriers here transport electrons from NADH and FADH2 to oxygen, producing water as an end product. More importantly, the energy from these electron transfers drives proton pumps, which create a steep H+ gradient between the mitochondrial membranes. This gradient drives a chemiosmotic mechanism by which ATP synthase generates ATP.

8. Glycolysis and aerobic respiration collectively produce up to 38 ATP per glucose, with the number varying slightly from one tissue type to another.

9. Glucose in excess of the body's immediate needs can be converted to fat or polymerized and stored as glycogen. Glycogenesis is the synthesis of glycogen. Glycogenolysis is the hydrolysis of glycogen to release glucose. Gluconeogenesis is the synthesis of glucose from glycerol (derived from fats) or amino acids.

10. The liver carries out these processes of carbohydrate metabolism among many other functions (table 26.7).

Lipid and Protein Metabolism (p. 1004)

1. Adipocytes store and release most of the body's fat (triglycerides).

2. Lipogenesis is the synthesis of fats from precursors such as sugars and amino acids.

3. Lipolysis is the breakdown of fats, starting with hydrolysis and continuing with oxidation of the fatty acids and glycerol. Fatty acids are degraded by the process of

P-oxidation. Oxidation of a typical fatty acid can yield 129 ATPs—much more than glucose oxidation.

4. Incomplete fatty acid oxidation produces acidic ketone bodies, a process called ketogenesis. Ketone bodies can be used as fuel but an excess can cause dangerous ketoacidosis, as it does in diabetes mellitus.

5. Proteins turn over at an average rate of about 100 g/day, with especially high turnover in the intestinal mucosa.

6. Free amino acids in the amino acid pool can be used to synthesize new proteins, converted to glucose or fat, or oxidized as fuel.

7. Amino acid catabolism entails deamination, the removal of the amino group. The amino group eventually becomes ammonia (NH3). The liver combines ammonia and CO2 to produce urea, which is a less toxic waste product than ammonia and the most abundant nitrogenous waste in the blood and urine.

Metabolic States and Metabolic

1. The absorptive state lasts about 4 hours after a meal. During this time, nutrients are absorbed from the intestine and may be used immediately. Glucose level is high and excess glucose is stored as glycogen or converted to fat.

2. The absorptive state is regulated mainly by insulin, which promotes glucose uptake and oxidation, glycogenesis, and lipogenesis; promotes protein synthesis; and inhibits gluconeogenesis.

3. The postabsorptive state prevails between meals and overnight, when the stomach is empty and the body uses stored fuels. Glycogenolysis and gluconeogenesis maintain the blood glucose level during this state. Fatty acids derived from lipolysis are used as fuel by many cells.

4. The postabsorptive state is regulated by multiple hormones. Glucagon, epinephrine, and norepinephrine promote lipolysis, glycogenolysis; cortisol promotes fat and protein catabolism; cortisol and glucagon promote gluconeogenesis; and growth hormone raises blood glucose by antagonizing insulin.

5. Metabolic rate is the amount of energy released in the body in a given time, such as kcal/day. It varies according to metabolic state and physical, mental, and hormonal conditions. Basal metabolic rate (BMR) is a standard of reference based on a comfortable, resting, awake, postabsorptive state. Total metabolic rate is a higher nonresting rate that takes muscular activity into account.

6. BMR is about 2,000 kcal/day. A low level of physical activity increases daily energy needs to about 2,500 kcal/day, and hard physical labor can increase them to as much as 5,000 kcal/day. Metabolic rate also varies with age, sex, mental state, stress, and health or illness.

Body Heat and Thermoregulation (p. 1009)

1. Thermoregulation is the homeostatic control of body temperature. Excessively high or low body temperatures (hyperthermia and hypothermia) can be fatal.

2. Core temperature can be estimated from rectal temperature and is usually 37.2° to 37.6°C. Shell temperature, usually estimated from oral temperature, is usually 36.6° to 37.0°C.

3. Body heat is generated mainly by exergonic chemical reactions, especially in the brain, heart, liver, and endocrine glands at rest and in the skeletal muscles during activity. The body loses heat by radiation, conduction, and evaporation.

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Chapter 26 Nutrition and Metabolism 1015

4. The hypothalamic thermostat monitors blood temperature and receives signals from peripheral thermoreceptors in the skin.

5. To rid the body of excess heat, the thermostat sends signals to a hypothalamic heat-losing center, which triggers cutaneous vasodilation and sweating.

6. To generate and retain heat, the thermostat sends signals to a hypothalamic heat-promoting center, which triggers shivering and cutaneous vasoconstriction.

7. Heat can also be produced by nonshivering thermogenesis, in which the metabolic rate is increased and releases more heat from organic fuels.

8. Behavioral thermoregulation includes behaviors that adjust body temperature, such as adding or removing clothes, or getting into the shade or sun.

9. Heat cramps, heat exhaustion, and heatstroke are three effects of hyperthermia. Hyperthermia can be fatal if the body temperature reaches 43°C. Hypothermia may be fatal if the core temperature reaches 32°C or lower.

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Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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