Nutrition and Metabolism

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Chapter Summary

Why We Eat (page 738)

Nutrients include carbohydrates, lipids, proteins, vitamins, and minerals. The ways nutrients are used to support life processes constitute metabolism. Essential nutrients are needed for health and cannot be synthesized by body cells. Macronutrients include carbohydrates, lipids, and proteins. Micronutrients are vitamins and minerals. Water is also essential. Various factors acting on cells of the hypothalamus control body weight.

Carbohydrates (page 739)

Carbohydrates are organic compounds that are primarily used to supply cellular energy.

1. Carbohydrate sources a. Carbohydrates are ingested in a variety of forms.

b. Polysaccharides, disaccharides, and monosaccharides are carbohydrates.

c. Cellulose is a polysaccharide that human enzymes cannot digest, but it is important in providing bulk that facilitates movement of intestinal contents.

2. Carbohydrate utilization a. Carbohydrates are absorbed as monosaccharides.

b. The liver converts fructose and galactose to glucose.

c. Oxidation releases energy from glucose.

d. Excess glucose is stored as glycogen or converted to fat.

3. Carbohydrate requirements a. Most carbohydrates are used to supply energy, although some are used to produce sugars.

b. Some cells depend on a continuous supply of glucose to survive.

c. If glucose is scarce, amino acids may be converted to glucose.

d. Humans survive with a wide range of carbohydrate intakes.

e. Poor nutritional status is usually related to low intake of nutrients other than carbohydrates.

Lipids (page 740)

Lipids are organic compounds that supply energy and are used to build cell structures. They include fats, phospholipids, and cholesterol.

1. Lipid sources a. Triglycerides are obtained from foods of plant and animal origins.

b. Cholesterol is mostly obtained in foods of animal origin.

2. Lipid utilization a. Before fats can be used as an energy source, they must be broken down into glycerol and fatty acids.

b. Beta oxidation decomposes fatty acids.

(1) Beta oxidation activates fatty acids and breaks them down into segments of two carbon atoms each.

(2) Fatty acid segments are converted into acetyl coenzyme A, which can then be oxidized in the citric acid cycle.

c. The liver and adipose tissue control triglyceride metabolism.

d. The liver can alter the molecular structures of fatty acids.

e. Linoleic acid, linolenic acid, and arachidonic acid are essential fatty acids.

f. The liver regulates the amount of cholesterol by synthesizing or excreting it.

3. Lipid requirements a. Humans survive with a wide range of lipid intakes.

b. The amounts and types of lipids needed for health are unknown.

c. Fat intake must be sufficient to carry fat-soluble vitamins.

Proteins (page 742)

Proteins are organic compounds that serve as structural materials, act as enzymes, and provide energy. Amino acids are incorporated into various structural and functional proteins, including enzymes. During starvation, tissue proteins may be used as energy sources; thus, the tissues waste away.

1. Protein sources a. Proteins are mainly obtained from meats, dairy products, cereals, and legumes.

b. During digestion, proteins are broken down into amino acids.

c. The resulting amino acids can be used as building materials, to form enzymes, or as energy sources.

d. Before amino acids can be used as energy sources, they must be deaminated.

e. The deaminated portions of amino acids can be broken down into carbon dioxide and water or used to produce glucose or fat.

f. Eight amino acids are essential for adults, whereas ten are essential for growing children.

g. All essential amino acids must be present at the same time in order for growth and repair of tissues to take place.

h. Complete proteins contain adequate amounts of all the essential amino acids needed to maintain the tissues and promote growth.

i. Incomplete proteins lack adequate amounts of one or more essential amino acids.

2. Nitrogen balance a. In healthy adults, the gain of protein equals the loss of protein, and a nitrogen balance exists.

b. A starving person has a negative nitrogen balance; a growing child, a pregnant woman, or an athlete in training usually has a positive nitrogen balance.

3. Protein requirements a. Proteins and amino acids are needed to supply essential amino acids and nitrogen for the synthesis of nitrogen-containing molecules.

b. The consequences of protein deficiencies are particularly severe among growing children.

Energy Expenditures (page 745)

Energy is of prime importance to survival and may be obtained from carbohydrates, fats, or proteins.

1. Energy values of foods a. The potential energy values of foods are expressed in calories.

b. When energy losses due to incomplete absorption and incomplete oxidation are taken into account, 1 gram of carbohydrate or 1 gram of protein yields about 4 calories, whereas 1 gram of fat yields about 9 calories.

2. Energy requirements a. The amount of energy required varies from person to person.

b. Factors that influence energy requirements include basal metabolic rate, muscular activity, body temperature, and nitrogen balance.

3. Energy balance a. Energy balance exists when caloric intake equals caloric output.

b. If energy balance is positive, body weight increases; if energy balance is negative, body weight decreases.

4. Desirable weight a. The most common nutritional disorders involve caloric imbalances.

b. Average weights of persons 25-30 years of age are desirable for older persons as well.

c. Height-weight guidelines are based on longevity.

d. A person who exceeds the desirable weight by 10%-20% is called overweight.

e. A person whose body contains excess fatty tissue is obese.

Vitamins (page 750)

Vitamins are organic compounds (other than carbohydrates, lipids, and proteins) that are essential for normal metabolic processes and cannot be synthesized by body cells in adequate amounts.

1. Fat-soluble vitamins a. General characteristics

(1) Fat-soluble vitamins are carried in lipids and are influenced by the same factors that affect lipid absorption.

(2) They are fairly resistant to the effects of heat; thus, they are not destroyed by cooking or food processing.

b. Vitamin A

(1) Vitamin A occurs in several forms, is synthesized from carotenes, and is stored in the liver.

(2) It is an antioxidant and functions in the production of pigments necessary for vision.

c. Vitamin D

(1) Vitamin D is a group of related steroids.

(2) It is found in certain foods and is produced commercially; it can also be synthesized in the skin.

(3) When needed, vitamin D is converted by the kidneys to an active form that functions as a hormone and promotes the intestine's absorption of calcium and phosphorus.

d. Vitamin E

(1) Vitamin E is also an antioxidant.

(2) It is stored in muscles and adipose tissue.

(3) Its precise functions are unknown, but it seems to prevent breakdown of polyunsaturated fatty acids and to stabilize cell membranes.

e. Vitamin K

(1) Vitamin K1 occurs in foods; vitamin K2 is produced by certain bacteria that normally inhabit the intestinal tract.

(2) It is stored to a limited degree in the liver.

(3) It is used in the production of prothrombin that is needed for normal blood clotting.

2. Water-soluble vitamins a. General characteristics

(1) Water-soluble vitamins include the B vitamins and vitamin C.

(2) B vitamins make up a group called the vitamin B complex and oxidize carbohydrates, lipids, and proteins.

b. Vitamin B complex

(1) Thiamine

(a) Thiamine functions as part of coenzymes that oxidize carbohydrates and synthesize essential sugars.

(b) Small amounts are stored in the tissues; excess is excreted in the urine.

(c) Quantities needed vary with caloric intake.

(2) Riboflavin

(a) Riboflavin functions as part of several enzymes and coenzymes that are essential to the oxidation of glucose and fatty acids.

(b) Its absorption is regulated by an active transport system; excess is excreted in the urine.

(c) Quantities needed vary with caloric intake.

(3) Niacin

(a) Niacin functions as part of coenzymes needed for the oxidation of glucose and for the synthesis of proteins and fats.

(b) It can be synthesized from tryptophan; daily requirement varies with the tryptophan intake.

(4) Pantothenic acid

(a) Pantothenic acid functions as part of coenzyme A; thus, it is essential for energy-releasing mechanisms.

(b) Most diets provide sufficient amounts; deficiencies are rare.

(5) Vitamin B6

(a) Vitamin B6 is a group of compounds that function as coenzymes in metabolic pathways that synthesize proteins, certain amino acids, antibodies, and nucleic acids.

(b) Its requirement varies with protein intake.

(6) Cyanocobalamin

(a) The cyanocobalamin molecule contains cobalt.

(b) Its absorption is regulated by the secretion of intrinsic factor from the gastric glands.

(c) It functions as part of coenzymes needed for the synthesis of nucleic acids and for the metabolism of carbohydrates and fats.

(7) Folacin

(a) Folacin is converted by the liver to physiologically active folinic acid.

(b) It is a coenzyme needed for the metabolism of certain amino acids, the synthesis of DNA, and the normal production of red blood cells.

(8) Biotin

(a) Biotin is a coenzyme needed for the metabolism of amino acids and fatty acids, and for the synthesis of nucleic acids.

(b) It is stored in metabolically active organs.

c. Ascorbic acid (vitamin C)

(1) Vitamin C is closely related chemically to monosaccharides.

(2) It is needed for production of collagen, the metabolism of certain amino acids, and iron absorption.

(3) It is not stored in large amounts; excess is excreted in the urine.

Minerals (page 75 7)

1. Characteristics of minerals a. Minerals are responsible for about 4% of body weight.

b. About 75% by weight of the minerals are found in bones and teeth as calcium and phosphorus.

c. Minerals are usually incorporated into organic molecules, although some occur in inorganic compounds or as free ions.

d. They comprise structural materials, function in enzymes, and play vital roles in various metabolic processes.

e. Mineral concentrations are generally regulated by homeostatic mechanisms.

f. The physiologically active form of minerals is the ionized form.

2. Major minerals a. Calcium

(1) Calcium is essential for the formation of bones and teeth, the conduction of nerve impulses, the contraction of muscle fibers, the coagulation of blood, and the activation of various enzymes.

(2) Its absorption is affected by existing calcium concentration, vitamin D, protein intake, and motility of the digestive tract.

b. Phosphorus

(1) Phosphorus is incorporated for the most part into the salts of bones and teeth.

(2) It plays roles in nearly all metabolic reactions as a constituent of nucleic acids, proteins, enzymes, and some vitamins.

(3) It also occurs in the phospholipids of cell membranes, in ATP, and in phosphates of body fluids.

c. Potassium

(1) Potassium tends to be concentrated inside cells.

(2) It functions in maintenance of osmotic pressure, regulation of pH, metabolism of carbohydrates and proteins, conduction of nerve impulses, and contraction of muscle fibers.

d. Sulfur

(1) Sulfur is incorporated for the most part into the molecular structures of certain amino acids.

(2) It is also included in thiamine, insulin, biotin, and mucopolysaccharides.

e. Sodium

(1) Most sodium occurs in extracellular fluids or is bound to the inorganic salts of bone.

(2) The blood concentration of sodium is regulated by the kidneys under the influence of aldosterone.

(3) Sodium helps maintain solute concentration and regulates water balance.

(4) It is essential for the conduction of nerve impulses, the contraction of muscle fibers, and the movement of substances through cell membranes.

f. Chlorine

(1) Chlorine is closely associated with sodium in the form of chloride ions.

(2) It acts with sodium to help maintain osmotic pressure, regulate pH, and maintain electrolyte balance.

(3) It is essential for the formation of hydrochloric acid and for the transport of carbon dioxide by red blood cells.

g. Magnesium

(1) Magnesium is particularly abundant in the bones as phosphates and carbonates.

(2) It functions in the production of ATP and in the breakdown of ATP to ADP.

(3) A reserve supply of magnesium is stored in the bones; excesses are excreted in the urine.

3. Trace elements a. Iron

(1) Iron primarily occurs in hemoglobin of red blood cells and in myoglobin of muscles.

(2) A reserve supply of iron is stored in the liver, spleen, and bone marrow.

(3) It is needed to catalyze the formation of vitamin A; it is also incorporated into various enzymes and the cytochrome molecules.

b. Manganese

(1) Most manganese is concentrated in the liver, kidneys, and pancreas.

(2) It is necessary for normal growth and development of skeletal structures and other connective tissues; it is essential for the synthesis of fatty acids, cholesterol, and urea.

c. Copper

(1) Most copper is concentrated in the liver, heart, and brain.

(2) It is needed for synthesis of hemoglobin, development of bones, production of melanin, and formation of myelin.

d. Iodine

(1) Iodine is most highly concentrated in the thyroid gland.

(2) It provides an essential component for the synthesis of thyroid hormones.

(3) It is often added to foods in the form of iodized table salt.

e. Cobalt

(1) Cobalt is widely distributed throughout the body.

(2) It is an essential part of cyanocobalamin and is probably needed for the synthesis of several important enzymes.

f. Zinc

(1) Zinc is most concentrated in the liver, kidneys, and brain.

(2) It is a constituent of several enzymes involved with digestion, respiration, bone metabolism, and liver metabolism.

g. Fluorine

(1) The teeth concentrate fluorine.

(2) It is incorporated into enamel and prevents dental caries.

h. Selenium

(1) The liver and kidneys store selenium.

(2) It is a constituent of certain enzymes.

i. Chromium

(1) Chromium is widely distributed throughout the body.

(2) It regulates glucose utilization.

Healthy Eating—The Food Pyramid and Reading Labels (page 763)

1. An adequate diet provides sufficient energy and essential nutrients to support optimal growth, as well as maintenance and repair, of tissues.

2. Individual needs vary so greatly that it is not possible to design a diet that is adequate for everyone.

3. Devices to help consumers make healthy food choices include Recommended Daily Allowances, Recommended Dietary Allowances, food group plans, the food pyramid, and food labels.

4. Malnutrition a. Poor nutrition is due to lack of foods or failure to make the best use of available foods.

b. Primary malnutrition is due to poor diet.

c. Secondary malnutrition is due to an individual characteristic that makes a normal diet inadequate.

5. Starvation a. A person can survive fifty to seventy days without food.

Critical Thinking Questions

1. For each of the following diets, indicate how the diet is nutritionally unsound (if it is) and why it would be easy or difficult to follow.

a. The bikini diet consists of 500 calories per day, including 45% to 55% protein, 40% to 50% carbohydrate, and 4% to 6% fat.

b. The Cambridge diet is a powder mixed with water and drunk three times a day. One day's intake equals

33 grams of protein, 40 grams of carbohydrate, and 3 grams of fat.

c. For the first 10 days of the Beverly Hills diet, nothing but fruit is eaten. On day 10, you can eat a bagel and butter, and then it is back to only fruit until day 19, when you can eat steak or lobster. The cycle repeats, adding more meat. This diet is based on "conscious combining"—the idea that eating certain combinations of foods leads to weight loss.

d. The Weight Loss Clinic diet consists of 800 calories per day, with 46.1% protein, 35.2% carbohydrate, and 18.7% fat.

e. The macrobiotic diet includes 10% to 20% protein, 70% carbohydrate, and 10% fat, with a half hour of walking each day. Most familiar foods are forbidden, but you can eat many unusual foods—such as rice cakes, seaweed, barley stew, pumpkin soup, rice gruel, kasha and onions, millet balls, wheat berries, and parsnip chips.

f. The No Aging diet maintains that eating foods rich in nucleic acids (RNA and DNA) can prolong life, since these are the genetic materials. Recommended foods include sardines, salmon, calves' liver, lentils, and beets.

2. Why does the blood sugar concentration of a person whose diet is low in carbohydrates remain stable?

b. A starving body digests itself, starting with carbohydrates, then proteins, then fats.

c. Symptoms include low blood pressure, slow pulse, chills, dry skin, hair loss, and poor immunity. Finally, vital organs cease to function.

d. Marasmus is lack of all nutrients.

e. Kwashiorkor is protein starvation.

f. Anorexia nervosa is a self-starvation eating disorder.

g. Bulimia is an eating disorder characterized by bingeing and purging.

Life-Span Changes (page 7 70)

1. Changing nutrition with age reflects medical conditions and social and economic circumstances.

2. Basal metabolic rate rises in early childhood, declines, then peaks again in adolescence with decreasing activity during adulthood.

3. Weight gain, at any age, occurs when energy in exceeds energy out and weight loss occurs when energy out exceeds energy in.

3. A young man takes several vitamin supplements each day, claiming that they give him energy. Is he correct? Why or why not?

4. A soccer coach advises his players to eat a hamburger and french fried potatoes about 2 hours before a game. Suggest a more sensible pregame meal.

5. Anorexia nervosa is a form of starvation. If it is a nutritional problem, then why should treatment include psychotherapy?

6. Why do starving children often die of infections that are usually mild in well-nourished children?

7. With the aid of nutrient tables available in reference books, calculate the carbohydrate, lipid, and protein content of your diet in grams for a 24-hour period. Also calculate the total calories represented by these foods. Assuming that this 24-hour sample is representative of your normal eating habits, what improvements could be made in its composition?

8. Examine the label information on the packages of a variety of dry breakfast cereals. Which types of cereals provide the best sources of vitamins and minerals? Which major nutrients are lacking in these cereals?

9. If a person decided to avoid eating meat and other animal products, such as milk, cheese, and eggs, what foods might be included in the diet to provide essential amino acids?

10. How might a diet be modified in order to limit the intake of cholesterol?

11. How do you think the nutritional requirements of a healthy 12-year-old boy, a 24-year-old pregnant woman, and a healthy 60-year-old man differ?

Review Exercises i. 2.

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