Anabolic Requirements

In addition to providing the body with energy, food also supplies the raw materials for synthesis reactions—collectively termed anabolism—that occur constantly within the cells of the body.

Anabolic reactions include those that synthesize DNA and RNA, protein, glycogen, triglycerides, and other polymers. These anabolic reactions must occur constantly to replace those molecules that are hydrolyzed into their component monomers. These hydrolysis reactions, together with the reactions of cell respiration that ultimately break the monomers down to carbon dioxide and water, are collectively termed catabolism.

Acting through changes in hormonal secretion, exercise and fasting increase the catabolism of stored glycogen, fat, and body protein. These molecules are also broken down at a certain rate in a person who is neither exercising nor fasting. Some of the monomers thus formed (amino acids, glucose, and fatty acids) are used immediately to resynthesize body protein, glyco-gen, and fat. However, some of the glucose derived from stored glycogen, for example, or fatty acids derived from stored triglycerides, are used to provide energy in the process of cell respiration. For this reason, new monomers must be obtained from food to prevent a continual decline in the amount of protein, glycogen, and fat in the body.

The turnover rate of a particular molecule is the rate at which it is broken down and resynthesized. For example, the average daily turnover for carbohydrates is 250 g/day. Since some of the glucose in the body is reused to form glycogen, the average daily dietary requirement for carbohydrate is somewhat less than this amount—about 150 g/day. The average daily turnover for protein is 150 g/day, but since many of the amino acids derived from the catabolism of body proteins can be reused in protein synthesis, a person needs only about 35 g/day of protein in the diet. It should be noted that these are average figures and will vary in accordance with individual differences in size, sex, age, genetics, and physical activity. The average daily turnover for fat is about 100 g/day, but very little is required in the diet (other than that which supplies fat-soluble vitamins and essential fatty acids), since fat can be produced from excess carbohydrates.

The minimal amounts of dietary protein and fat required to meet the turnover rate are adequate only if they supply sufficient amounts of the essential amino acids and fatty acids. These molecules are termed essential because they cannot be synthesized by the body and must be obtained in the diet. The nine essential amino acids are lysine, tryptophan, phenylalanine, threonine, valine, methionine, leucine, isoleucine, and histidine. The essential fatty acids are linoleic acid and linolenic acid.

Unsaturated fatty acids—those with double bonds between the carbons—are characterized by the location of the first double bond. Linoleic acid, found in corn oil, contains eighteen carbons and two double bonds. It has its first double bond on the sixth carbon from the methyl (CH3) end, and is therefore desig nated as an n-6 (or omega-6) fatty acid. Linolenic acid, found in canola oil, also has eighteen carbons, but it has three double bonds. More significantly for health, its first double bond is on the third carbon from the methyl end; linolenic acid is an n-3 (also called omega-3) fatty acid. Several studies suggest that n-3 fatty acids may offer protection from cardiovascular disease.

Eskimos who eat a traditional diet of meat and fish have a surprisingly low blood concentration of triglycerides and cholesterol, and a low incidence of ischemic heart disease, despite the high fat and cholesterol content of their food. Several studies suggest that the n-3 fatty acids of the cold-water fish are the source of the apparent protective effect. The n-3 fatty acids of fish include eicosapen-taenoic acid, or EPA (with twenty carbons), and docosahexaenoic acid, or DHA (with twenty-two carbons). The n-3 fatty acids may help to inhibit platelet function in thrombus formation, the progression of atherosclerosis, and/or ventricular arrhythmias. Several studies have confirmed the protective effect of fish and fish oil in the diet, and on the basis of this evidence it seems prudent to eat fish at least once or twice a week on a continuing basis.

Table 19.2 Recommended Dietary Allowances for Vitamins and Minerals1

Fat-Soluble Vitamins

Table 19.2 Recommended Dietary Allowances for Vitamins and Minerals1

Fat-Soluble Vitamins

Age (Years)

Weight2

Height2

Protein

Vitamin A

Vitamin D

Vitamin E

Vitamin K

Category

or Condition

(kg)

(lb)

(cm)

(in)

(g)

(lg RE)3

(l g)4

(mg a-TE)5

(l g)

Infants

0.0-0.05

6

13

60

24

13

375

7.5

3

5

0.5-1

9

20

71

28

14

375

10

4

10

Children

1-3

13

29

90

35

16

400

10

6

15

4-6

20

44

112

44

24

500

10

7

20

7-10

28

62

132

52

28

700

10

7

30

Males

11-14

45

99

157

62

45

1,000

10

10

45

15-18

66

145

176

69

59

1,000

10

10

65

19-24

72

160

177

70

58

1,000

10

10

70

25-50

79

174

176

70

63

1,000

5

10

80

51 +

77

170

173

68

63

1,000

5

10

80

Females

11-14

46

101

157

62

45

800

10

8

45

15-18

55

120

163

64

44

800

10

8

55

19-24

58

128

164

65

46

800

10

8

60

25-50

63

138

163

64

50

800

5

8

65

51 +

65

143

160

63

50

800

5

8

65

Pregnant

60

800

10

10

65

Lactating

1st 6 months

65

1,300

10

12

65

2nd 6 months

62

1,200

10

11

65

Source: Reprinted with permission from Recommended Dietary Allowances, 10th Edition. Copyright 1989 by the National Academy of Sciences. Courtesy of the National Academy Press, Washington, D.C.

1 The allowances, expressed as average daily intakes over time, are intended to provide for individual variations among most normal persons as they live in the United States under usual environmental stresses. Diets should be based on a variety of common foods in order to provide other nutrients for which human requirements have been less well defined.

2 Weights and heights of Reference Adults are actual medians for the U.S. population of the designated age, as reported by NHANES II. The use of these figures does not imply that the height-to-weight ratios are ideal.

3 Retinol equivalents. 1 re = 1 ^g retinol or 6 ^g P-carotene.

4 As cholecalciferol. 10 ^g cholecalciferol = 400 IU of vitamin D.

5 a-tocopherol equivalents. 1 mg d-a-tocopherol = 1 a-te.

Source: Reprinted with permission from Recommended Dietary Allowances, 10th Edition. Copyright 1989 by the National Academy of Sciences. Courtesy of the National Academy Press, Washington, D.C.

1 The allowances, expressed as average daily intakes over time, are intended to provide for individual variations among most normal persons as they live in the United States under usual environmental stresses. Diets should be based on a variety of common foods in order to provide other nutrients for which human requirements have been less well defined.

2 Weights and heights of Reference Adults are actual medians for the U.S. population of the designated age, as reported by NHANES II. The use of these figures does not imply that the height-to-weight ratios are ideal.

3 Retinol equivalents. 1 re = 1 ^g retinol or 6 ^g P-carotene.

4 As cholecalciferol. 10 ^g cholecalciferol = 400 IU of vitamin D.

5 a-tocopherol equivalents. 1 mg d-a-tocopherol = 1 a-te.

Regulation of Metabolism

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  • teodros biniam
    What is anabolic physiology?
    2 years ago

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