Basal metabolic rate (BMR), and the closely related resting metabolic rate (RMR), is the amount of energy expended daily by humans and other animals at rest. Rest is defined as existing in a neutrally temperate environment while in the post-absorptive state.
BMR and RMR are measured by gas analysis through either direct or indirect calorimetry, though a rough estimation can be acquired through an equation using age, sex, height, and weight. Studies of energy metabolism using both methods provide convincing evidence for the validity of the respiratory quotient (R.Q.), which measures the inherent composition and utilization of carbohydrates, fats and proteins as they are converted to energy substrate units that can be used by the body as energy.
Basal metabolism is usually by far the largest component of total caloric expenditure. However, the Harris-Benedict equations are only approximate and variation in BMR (reflecting varying body composition), in physical activity levels, and in energy expended in thermogenesis make it difficult to estimate the dietary consumption any particular individual needs in order to maintain body weight.
- Women: BMR = 655 + ( 9,6 x weight (kg)) + ( 1,8 x height (cm)) – ( 4.7 x age (y’s))
- Men: BMR = 66 + ( 13.7 x weight (kg)) + ( 5 x height (cm)) – ( 6.8 x age (y’s) )
Human nutrition needs
Determining what substances must be incorporated in the human diet, and how much of each, is — even after years of research — still under active study. Why the uncertainty?
- Vitamins. Inadequate intake of some vitamins produces easily-recognized deficiency diseases like
- scurvy: lack of ascorbic acid (vitamin C)
- beriberi: lack of thiamine (vitamin B1)
- pellagra: lack of niacin.
However, it is so difficult to exclude some other possible vitamins from the diet that deficiency diseases are hard to demonstrate.
- Minerals. Some minerals are needed is such vanishingly small amounts that it is practically impossible to prepare a diet that does not include them. However, totally synthetic diets are now available for intravenous feeding of people who cannot eat. This so-called total parenteral nutrition has revealed, unexpectedly, some additional trace element needs: chromium and molybdenum.
Despite some uncertainties, the Food and Nutrition Board of the U. S. National Academy of Sciences publishes guidelines. One of the most useful of these is calledrecommended daily allowances or RDAs. These provide the basis for the nutrition labels on food.
Carbohydrates provide the bulk of the calories (4 kcal/gram) in most diets and starches provide the bulk of that. Age, sex, size, health, and the intensity of physical activity strongly affect the daily need for calories. Moderately active females (19–30 years old) need 1500–2500 kcal/day, while males of the same age need 2500–3300 kcal/day.
In some poor countries, too many children do not receive enough calories to grow properly. In order to maintain blood sugar levels, they attack their own protein. This condition of semi-starvation is known as marasmus.
Humans must include adequate amounts of 9 amino acids in their diet. These “essential” amino acids cannot be synthesized from other precursors. However, cysteine can partially meet the need for methionine (they both contain sulfur), and tyrosine can partially substitute for phenylalanine.
The Essential Amino Acids: Histidine, Isoleucine, Leucine, Lysine, Methionine ,(and/or cysteine), Phenylalanine (and/or tyrosine), Threonine, Tryptophan, Valine
Ingested fats provide the precursors from which we synthesize our own fat as well as cholesterol and various phospholipids. Fat provides our most concentrated form of energy. Its energy content (9 kcal/gram) is over twice as great as carbohydrates and proteins (4 kcal/gram).
Humans can synthesize fat from carbohydrates (as most of us know all too well!). However, three essential fatty acids cannot be synthesized this way and must be incorporated in the diet. These are:
- linoleic acid,
- linolenic acid,
- arachidonic acid.
All are unsaturated; that is, have double bonds.
Types of fats
- Saturated. No double bonds between the carbon atoms in the fatty acid chains. Most animal fats (e.g., butter) are highly saturated.
- Monounsaturated. Have a single double bond in the fatty acid chains. Examples are olive, peanut, and rapeseed (canola) oil.
- Polyunsaturated. Have two or more double bonds in their fatty acid chains. Examples: corn, soy bean, cottonseed, sunflower, and safflower oils.
- Trans Fats. Have been partially hydrogenated producing
- fewer double bonds and, of those that remain,
- converting them from a cis to a trans configuration.
- Omega-3 fats. Have at least one double bond three carbon atoms in from the end of the fatty acid molecule. Linolenic acid is an example. Fish oils are a rich source of omega-3 fatty acids.
Many studies have examined the relationship between fat in the diet and cardiovascular disease. There is still no consensus, but the evidence seems to indicate that:
- Mono and polyunsaturated fats are less harmful than saturated ones, except that
- trans unsaturated fats may be worse than saturated fats.
- Ingestion of omega-3 unsaturated fats may be protective. For this reason, 1.1 grams/day for women (1.6 for men) is recommended.