Human Biochemistry, Volume 2 - Murray R. 1993
Special Topics
Nutrition, Digestion, and Absorption
Energy Requirements
Energy Supply
The mammalian Organism must ingest a sufficient amount of nutrients so that their Free energy can cover the daily requirement for high-energy phosphates (primarily ATP) and reducing equivalents (2H) necessary to sustain all body Functions (see Fig. 16.1). Nutrients are supplied in the form of CARBOHYDRATES, fats, and Proteins in ratios that vary widely across different population groups. Alcohol can also serve as an energy source. A constant body weight under conditions of unchanged energy demand indicates an adequate dietary energy intake.
The amount of metabolizable energy contained in the major nutrients is listed in Table 53.2. It should be noted that 1 g of fat yields significantly more energy than equivalent amounts of proteins and carbohydrates; alcohol is also high in energy. Table 53.3 provides data on the recommended levels of energy intake for humans.
Class="center">Table 53.1. Requirements for essential dietary components
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Humans |
Other species |
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|
Histidine1), isoleucine, Lysine, Methionine (Cysteine3)), phenylalanine (Tyrosine3)), Threonine, Tryptophan, valine |
Growing rats require Arginine2). Glycine is required for chicks, and taurine for cats. Ruminants can dispense with Most amino acids; the requirements of other herbivores are met by intestinal microorganisms |
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|
Linoleic acid (arachidonic acid3)), a-linolenic acid4) |
A specific requirement for arachidonic acid exists in cats |
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Water-soluble |
Ascorbic acid (C), biotin3), cobalamin (B12), folic acid, niacin, pantothenic acid, pyridoxine (B6), riboflavin (B2), thiamine (B1) |
Most mammals can synthesize ascorbic acid, but it must be present in the diet of primates, guinea pigs, and fruit bats. Water-Soluble Vitamins are not essential for ruminants; the requirements of other herbivores are met by intestinal microorganisms |
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Fat-soluble |
Vitamins A, D6), E, K5) |
Most species can utilize ß-carotene as a source of vitamin A (retinol); cats must receive it in the form of retinol |
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Minerals |
Calcium, chloride, magnesium, phosphorus, potassium, sodium |
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|
Chromium, copper, iodine, iron, manganese, molybdenum, selenium, zinc |
Silicon, vanadium, nickel, arsenic, fluoride, and tin are important for various species and may be required by humans. Cobalt is essential for the synthesis of cobalamin by rumen microorganisms |
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Fiber |
Required for health |
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Water |
The most essential dietary component |
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Energy |
Utilization of carbohydrates, fats, and proteins in various proportions |
1) Required for infants and presumably for children and adults.
2) May be partially indispensable for infants.
3) Cysteine, tyrosine, and arachidonic acid reduce the requirements for methionine, phenylalanine, and linoleic acid, respectively.
4) There is some controversy regarding the necessity of a-linolenic acid in the human diet.
5) Synthesized by intestinal microorganisms, therefore the requirement is uncertain.
6) Exposure of the Skin to sunlight reduces the dietary requirement for the vitamin.
Table 53.2. Heat of Combustion and metabolizable energy from major dietary sources. (Adapted from Davidson S. et al. Human Nutrition and Dietetics, 7th ed. Churchill Livingstone, 1979.)
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Energy, kcal/g (kJ/g) |
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heat of combustion (bomb calorimeter) |
oxidation in humans |
standard conversion factors 1) |
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|
Proteins |
5.4 (22.6) |
4.1 (17.2)2) |
4 (17) |
|
Fats |
9.3 (38.9) |
9.3 (38.9) |
9 (38) |
|
Carbohydrates |
4.1 (17.2) |
4.1 (17.2) |
4 (17) |
|
Ethanol |
7.1 (29.7) |
7.1 (29.7) |
7 (29) |
1) Conversion factors are derived by rounding data on the heat of combustion and adjusting them for absorption efficiency.
2) Protein oxidation adjusted for the loss of amino groups excreted in the urine.
Last update: 06/08/2026
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