General Nutrition; Calorie Requirements
Recommended energy allowances differ in several ways from allowances for other nutrients. Energy allowances are intended to meet the average needs of each population group and therefore do not include a margin of safety, as dietary energy in excess of individual energy needs will lead to obesity. The recommended energy intakes are not guides for the energy needs of individuals; these are difficult to predict and are affected by such factors as body size, activity level, age, and environmental temperature. There are a number of simple formulas to determine caloric requirement based on body weight. One of these assumes the basal calorie requirement of the average adult to be 10 times the ideal weight in pounds, (e.g., 1270 for 127 lbs), plus 30% for light activity (e.g., 1650 kcal), 50% for moderate activity (1905 kcal), and 100% for heavy activity (2540 kcal). It must be remembered that any estimate of calorie requirements based on such formulas is just that--an estimate. Individual requirements vary widely.
Energy balance implies that, for a particular individual over a finite period of time, the amount of energy taken in (through food) is equal to the amount of energy expended (in heat or work). An individual in positive energy balance takes in more energy than is consumed (the excess energy is stored as fat); this occurs during growth and pregnancy, and any other time that weight is being gained. If, on the other hand, energy expenditure exceeds energy intake, the individual is in negative energy balance, and weight is being lost.
In general, vitamins are organic substances (other than carbohydrate, fat or protein) that are present in minute quantities in food and are necessary for metabolism. RDA tables give recommended daily dietary allowances for vitamins A, D, E, K, C, B-6 and B-12, as well as B-vitamins thiamin, riboflavin, niacin and folate. Recommended daily allowances for biotin and pantothenic acid are given in a separate table (not on-line) because there is less information on which to base allowances. Minute amounts of certain essential minerals must also be contained in the diet. Daily dietary allowances are recommended for calcium, phosphorus, magnesium, iron, zinc, iodine and selenium, as well as sodium, chloride and potassium, and copper, manganese, fluoride, chromium, and molybdenum. Vitamins and minerals will be discussed in more detail in Module 1.3.
Water. Water is the nutrient most critical for life. It is compatible with more substances than any other known solvent, and is an ideal medium for transporting essential nutrients to the cells and for the chemical reactions of cellular metabolism to take place. It also participates actively in hydrolytic and hydration reactions. The human body is approximately 60% water; this is highly variable depending on age, sex and body composition. At birth, water content is about 75% of body weight; this decreases to about 60% by age 10. Women under 40 are approximately 50% water; this decreases to about 45% by 60 years of age. Men under 40 are about 60% water; this decreases to 30% by 70 years of age.
Extracellular fluids comprise about 28% of the total body water, intracellular fluids about 72%.
A suitable allowance of water for adults is about 2.5 liters daily in most instances. The RDA Subcommittee recommends 1 ml/kcal of energy expenditure as the water requirement for adults under average conditions of energy expenditure and environmental exposure. To cover variations in activity level, sweating and solute load, the requirement can be increased to 1.5 ml/kcal.
Fats and oils are predominantly triglycerides, or triesters of fatty acids and glycerol. The term "lipids" includes triglycerides, as well as mono- and diglycerides, phosphatides, cerebrosides, sterols, fat-soluble vitamins and other substances.
Except for the essential fatty acids, the requirement for which can be met by a diet containing 15-25% of appropriate food fats, there is no specific requirement for fat as a nutrient in the diet. Nonetheless, fat contributes 42% of the calories in the national diet at the present time (46% is contributed by carbohydrate). The Food and Nutrition Board of the National Academy of Sciences has recommended that total fat intake not exceed 30% of dietary energy, that less than 10% of total calories be provided by saturated fatty acids and that dietary cholesterol should be less than 300 mg/day. High fat diets have been associated with obesity, heart disease and some types of cancer.
Essential fatty acids have multiple physiological functions. Two groups of fatty acids are essential: the omega-6 series, derived from linoleic acid and the omega-3 series, derived from linolenic acid. A primary essential fatty acid in the human diet is linoleic acid, which is converted in the body to longer-chain highly unsaturated fatty acids which are essential components of membranes. Essential fatty acids are involved in the regulation of cholesterol metabolism, and they are precursors of prostaglandins, thromboxanes and leukotrienes, collectively called eicosanoids. There is abundant experimental evidence that eicosanoids participate in the development and regulation of immunological and inflammatory responses. A deficiency of essential fatty acids has been assumed to be quite rare, virtually limited to infants fed formulas deficient in these nutrients or hospitalized patients being fed exclusively in intravenous fluids not containing fat. Recent studies have indicated that omega-3 fatty acids, found in fish fats and green leafy vegetables, may help prevent hardening of the arteries and heart attacks, as well as maintaining good vision and promoting normal brain development. The substitution of fish oil for other dietary oils was found to lower plasma cholesterol levels by 23%, and plasma triglyceride levels by 43%, probably by lowering blood levels of low-density and very-low-density lipoproteins (LDL and VLDL). LDL and VLDL are carriers of triglyceride and cholesterol, and high blood levels of these lipoproteins are risk factors for atherosclerosis.
As you can see from the RDA tables, nutritional requirements vary throughout the life cycle. A major purpose of this objective is to illustrate how nutritional needs may be affected by certain physiological conditions.
The normal human life cycle has four general phases of growth, and nutritional requirements will vary in these phases:
"Normal growth" as described in this objective will include the first three phases:
Growth, development and maturation are greater during the first year of life than during any other postnatal period. During the first year, the infant grows rapidly with the rate of growth slowing somewhat in the latter half. At six months, birth weight will probably have doubled and by one year it may have tripled. This rapid growth parallels that of fetal development.
Energy requirements are relatively large during this phase as you can see by looking at the RDA Table. In the first six months of life, recommended daily energy allowances are 108 kcal per kilogram body weight. (Please note that this would amount to 7560 kcal/day for the "standard" 70-kilogram adult male) This requirement declines to 98 kcal/kg in the second six months and continues to decline thereafter. Carbohydrate is a major energy source and has an important energy-sparing effect, ensuring that protein necessary for growth will not be diverted for energy needs. However, fat accounts for 50-60% of the energy (caloric) content of human milk, and provides: 1) an efficient source of energy at a very crucial time, 2) an important component of membranes, 3) the source of essential fatty acids needed for organ development, and 4) a carrier of fat-soluble vitamins.
Protein requirements are also relatively higher during this phase than any other period, 2.2 grams per kilogram body weight in the first six months (compared with 0.8 g/kg for an adult) and 1.6 g/kg in the second six months. This declines to about 1.1 g/kg in the next two years of life, and continues to decline thereafter. The very high requirement for protein during early growth and development makes this age group especially vulnerable to protein deprivation.
Water. The infant's need for water is even greater than that of the adult, because: 1) a greater percentage of total body weight is made up of water and 2) a larger proportion of total body water is extracellular and hence more easily lost. Generally, an infant consumes daily an amount of water equivalent to 10-15% of body weight, compared to 2-4% for an adult. The RDA Subcommittee recommends an average water intake of 1.5 ml/kcal energy expenditure for infants.
Minerals. The infant's need for iron is particularly impressive. With a weight about one-tenth that of an adult, the infant in the first six months of life requires 60% as much iron as a full-grown male (6 mg) and in the second six months, exactly as much (10 mg). Calcium is another mineral which is particularly crucial for growth (bone mineralization, tooth development). While the total daily requirement is less than that of the adult, the infant does require more calcium on a body-weight basis (about 67 mg/kg, as compared with about 10 mg/kg for the adult). Zinc and copper are two additional minerals which are significantly associated with the growth process and are crucial during the early years of life.
Vitamins. Vitamin A is essential for bone and tooth development, and for the formation and maturation of epithelial tissue. Requirements (on a body-weight basis) are highest in the first six months of life (five times adult needs) and decline steadily thereafter. Vitamin D is essential for the absorption of calcium and, thus, is crucial to bone development. In the first year of life, twice as much total Vitamin D is required as for the adult. On a body-weight basis, this is approximately twenty-fold. Requirements for other vitamins are generally somewhat less in infancy than adulthood but somewhat higher on a body weight basis: these include vitamin C (required for tissue synthesis), vitamin E (associated with tissue synthesis and muscle metabolism) and B-vitamins, which have many functions associated with growth.
Nutritional problems of particular concern for this age group include increasing prevalence of obesity (often due to consumption of high-fat foods and/or lack of exercise), eating disorders such as anorexia nervosa and bulimia nervosa, low intake of calcium-rich foods contributing to later development of osteoporosis, and iron deficiency (which can be exacerbated by competitive sports). Pregnant adolescents have greater nutritional requirements than any other group. These requirements are increased still more if they breastfeed. The increased nutritional requirements for pregnancy and lactation (discussed below) are superimposed on those for adolescence, making pregnant adolescents a very high-risk population from a nutritional standpoint.
Water. Pregnancy is associated with an increased need for water, on the order of 30 ml/day. A lactating woman will need an increased volume of water to match that secreted in the milk: about 700 ml/day.
Energy. About 2500 kcal daily will meet the needs of most pregnant women. The average weight gain is 25-30 pounds, but there is wide variation.
Protein. Large amounts of nitrogen are used by both mother and fetus during pregnancy, resulting in significantly greater demands for protein (the recommended allowance increases from 50 g/day to 60 g/day). During the last half of gestation, the amount of stored fetal nitrogen increases from 0.9 to 55.9 grams. More protein is essential to meet the demands imposed by a rapid fetal growth, enlargement of maternal tissue (uterus, mammary glands, placenta), increased maternal blood volume and formation of amniotic fluid.
Two minerals have traditionally been given particular emphasis during pregnancy: Calcium requirements increase by 50%--this is the essential element for construction and maintenance of bones and teeth. During the second half of pregnancy, fetal calcium stores increase from 1 gram to 23 grams. Low calcium intake during pregnancy is associated with a high incidence of eclampsia (pregnancy-induced hypertension), and calcium supplementation has been shown to reduce the risk of this disorder. An adequate supply of iron is needed to maintain maternal hemoglobin in a blood volume which increases 20-40%, as well as for fetal development. A 3 to 4 months' supply of iron is stored in the fetal liver to supply the infant's needs after birth. Because it is difficult if not impossible to meet the increase in RDA (from 15 mg to 30 mg) through the diet, iron supplementation has usually been recommended during pregnancy. However, more recently, some question has been raised as to the wisdom of prescribing iron supplementation for all pregnant women. Some of the disadvantages of such a policy include: possible detraction from the emphasis on a good diet; gastrointestinal upset caused by iron in 20% of women; relatively poor patient compliance; development of macrocytosis in a few women; an increased susceptibility to fungal and viral infection in the hyperferremic individual, and the lack of apparent benefit to most pregnant women. It has been well established that zinc deficiency in pregnant mammals can result in a wide variety of congenital malformations and may also increase fetal susceptibility to a variety of teratogens. Zinc deficiency need only be transient to have these effects, and there do not seem to be available maternal "stores" of zinc that can be released to meet fetal requirements. There is, at this point, less information pertaining to zinc deficiency in human pregnancy, but there is increasing evidence that the risks are similar to those documented in other mammals; low birth-weight (LBW) is particularly associated with low serum zinc levels late in pregnancy. There is, on the other hand, evidence that the decrease in zinc concentrations found in pregnant women is a normal physiological adjustment to pregnancy.
During pregnancy, there is need for increased amounts of B Vitamins (for increased metabolic activity), Vitamin C (for formation of intercellular cement substance, as well as to enhance iron absorption) and Vitamin D (to promote absorption of increased calcium and phosphorus). The RDA for folate is more than doubled during pregnancy; supplementation is often provided. The increases in recommended daily allowances of these vitamins may be seen in the Table.
Lactation. A lactating woman requires about 3000 calories (an increase of 500) to meet the demands of milk production as well as to replace the calories lost in the secreted milk. Protein requirement is increased by 15 grams; calcium and iron are required in the same amounts as during pregnancy, the recommended allowance for Vitamin C is 25 mg more than in pregnancy and the Vitamin A requirement is increased over that of pregnancy by an additional 500 µg (Vitamin A is an important constituent of milk). Fluid intake should also be increased.
Energy and Protein. Because lean body mass, BMR and physical activity ordinarily decline with age (i.e., after 65-70 years), the energy (caloric) intake of the elderly must be decreased as compensation. Although the 1989 edition of Recommended Dietary Allowances includes all adults over 50 in a single category with regard to energy needs, the Subcommittee notes that energy requirements are probably less for those over 75. Requirements for essential nutrients, however, do not decline. Good dietary habits, then, are extremely important for the elderly--they can't afford to waste calories.
An expert committee on energy and protein requirements has suggested that energy requirements for the moderately active person be decreased by 5% for each decade between 40 and 59 years, by 10% between 60 and 69, and by an additional 10% for age beyond 70. However, Dr. R. N. Butler, Director of the National Institute on Aging, notes that overall mortality appears lower among elderly persons who are mildly or even moderately obese.
Many high-protein foods, such as meat, are excellent sources of trace minerals and iron--they are particularly important sources when caloric intake is reduced. The National Research Council has stated that protein intakes in excess of the RDA are therefore desirable. However, renal function deteriorates with age, and the work of the kidney is increased when protein intake is high due to the need to eliminate a large amount of nitrogenous end products. Thus, a balance must be struck--a solution is to keep protein content of the diet close to 12% of the caloric total (slightly above the RDA but not excessive).
Vitamins and Minerals. Although the RDA Table shows no increase in calcium RDA for persons over 50 (as compared with those younger), there is increasing evidence that older individuals need more calcium than the recommended 800 mg/day. Postmenopausal women, in particular, may have an increased dietary requirement for calcium related to loss of estrogen and its calcium-conserving effects. There is also evidence that calcium is less efficiently absorbed in the elderly, but the results of many studies on calcium requirements during aging are inconclusive. Certainly, calcium is lost from the skeleton during aging and the average elderly person is in negative calcium balance, but it is not entirely clear that increasing calcium intake above the RDA can ameliorate the situation. Other dietary factors may be involved as causative agents in the progressive loss of bone substance in aging: e.g., Vitamin D is essential for calcium absorption, and it may be decreased in the elderly due to decreased exposure to sunlight and/or decreased dietary intake.
Anemia is common in the elderly, but it is uncertain whether this results from a higher need for iron, folic acid, or Vitamin B-12 or (more likely) some combination of them. It has been suggested that the folate deficiency so common in the elderly is caused by impaired ability to obtain folate from ingested foods.
The data from a large number of dietary surveys show conclusively that many elderly persons are at great risk for vitamin deficiencies. Age itself has been shown to be associated with striking decreases in blood levels of B-complex vitamins and Vitamin C. Many elderly persons are too poor to buy food which would prevent vitamin deficiencies. Also, their diets may be nutritionally inadequate for such reasons as disinterest in cooking, social isolation or inability to chew certain vitamin-rich foods. There are numerous studies describing the beneficial effects of vitamin supplements in the elderly.
Whether or not the many claims for the disease-preventive action of fiber are true, it would seem advisable for elderly persons to include in their diets foods which will provide a moderate amount of fiber to improve intestinal function and alleviate constipation.
Coronary heart disease is responsible for more than 550,000 deaths in the United States each year, more than all forms of cancer combined. There are more than 5.4 million Americans with symptomatic CHD and undoubtedly, a large number of others not yet diagnosed. The estimated cost (direct and indirect) in this country is $60 billion a year. The most clearly established risk factors are cigarette smoking, high blood pressure and high blood cholesterol levels. Risk is greater in men, increases with age and has a strong genetic component. Other risk factors include obesity, diabetes mellitus, physical activity and behavior patterns. Obviously, dietary factors could be involved in several of the risk factors listed (e.g., blood pressure, cholesterol levels, obesity, and diabetes).
The earliest suggestions that dietary factors played a role in the incidence of CHD came from epidemiological studies. When different populations with widely different prevalences of CHD were compared, it was found that differences in fat consumption were roughly parallel to prevalence of CHD. However, this relationship is found not only with fat intake but with any measure of affluence. In 1957 John Yudkin found the best correlation with increased CHD incidence in Britain (better than that with any dietary constituent) to be the increase in the number of people having radios and television. Vegetarians have a lower mortality from heart disease, but the underlying cause of this phenomenon is not certain; type of dietary protein, amount of fiber, or proportion and type of fat have been suggested.
Because CHD is apparently caused not just by one factor but by several, it is really not possible to isolate any one factor by comparing populations which differ in several ways and in varying degrees. For example, a population in which many people smoke cigarettes but are also extremely active might have no higher incidence of CHD than a sedentary but nonsmoking population.
To the extent that there is a good correlation between diet and CHD between populations in different parts of the world, the dietary practice that can be most definitely incriminated is the intake of saturated fatty acids. However, there are other dietary constituents which have been implicated in the etiology of CHD: refined sugar, high sodium (particularly related to increased blood pressure), heavy alcohol consumption, lack of dietary fiber, total calories, low potassium, low calcium, low magnesium, low intake of omega-3 fatty acids, low intake of antioxidants, and softened water. The most conservative approach to preventing CHD in a susceptible subject would be to reduce saturated fat intake (by increasing the proportion of polyunsaturated fat in the diet), as well as consumption of refined sugar, salt, and alcohol. Additionally, total caloric consumption would be somewhat limited, fiber-rich foods would be eaten and soft water would be avoided.
As a result of the concern over CHD, clearly a major health problem in this country, efforts have been undertaken to modify the eating habits of all Americans, not just those recognizably at risk. So, recommendations for reduction in saturated fats, total calories, sodium, refined sugar and alcohol, as well as for increases in dietary fiber, calcium, and fish oils are aimed at the general population. This, despite the fact that (to use sodium as an example) 82% of the population is estimated to be sodium-resistant and therefore able to maintain normal blood pressure at even the highest habitual levels of sodium intake. The underlying assumption is that the "heart-healthy" diet is good for overall health, as well.
An additional risk factor for cardiovascular disease is elevated blood concentrations of homocysteine (an intermediate in the interconversion of methionine and cysteine), and one cause of hyperhomocysteinemia is inadequate intake of vitamins involved in homocysteine metabolism: vitamins B6 and B12, and folacin.
The following guidelines have been recommended by the American Heart Association for all healthy adult Americans.
In recent years, there has been a renewal of interest in dietary fiber as it relates to human health. High fiber intake is now recommended by the American Heart Association, American Cancer Society and American Diabetic Association, among other medical groups. It now seems as if dietary fiber, which for many years has been associated with bowel "regularity," may also play a key role in the prevention of several diseases. It is not unreasonable to speculate that there may be an RDA for dietary fiber at some future time, and it is because of the potential importance of this nutrient that this goal is included.
Dietary fiber may be defined as the ingested plant material that is not digested by enzymes in the gastrointestinal tract. This includes carbohydrates--hemicelluloses A and B, pectic substances--and the noncarbohydrate lignin. Hemicellulose A is a polymer consisting of xylose, galactose, glucose, mannose and arabinose units. Hemicellulose B contains all these sugars, as well as uronic acid derivatives. Lignin is made up of phenylpropane units; the content of this substance increases as the plant cell ages.
The above definition of fiber is not completely satisfactory; however, the degree of fiber degradation depends on the type of fiber and the individual digestive system and is therefore widely variable. Some nutritionists prefer to define fiber as nonstarch polysaccharide (NSP), or NSP + lignin.
Crude fiber is the residue of plant material following treatment with sulfuric acid, sodium hydroxide, water, alcohol and ether. It consists primarily of cellulose and lignin. Food composition tables contain crude fiber values, but since 50% of dietary fiber is lost in the treatment described above, it is very difficult to calculate true dietary fiber intakes. In the case of wheat bran, the most common source of fiber in the human diet, the true fiber value is about 4 times that of the crude fiber value. Since crude fiber values are erratic and poorly related to the true fiber content of food, there is an urgent need for reanalysis of all foods by appropriate methods and the replacement of standard tables of food composition.
The major source of dietary fiber is whole grain, such as wheat. Wheat grain consists of bran (the outer coat--25%); endosperm (72%) and germ (embryo--3%). Milling removes most of the fiber, so that whole wheat bread has 8 times as much crude fiber as white bread. Raw fruits and vegetables contain fiber which is largely removed by processing. Nuts and legumes are also good sources of fiber.
Food consumption patterns have changed over the course of this century with decreased consumption of cereal grains and raw fruits and vegetables and increased consumption of processed and refined foods. It has been estimated that the dietary fiber intake has dropped by about five to one since 1900.
The amount of fiber in the diet affects gut motility. Increasing dietary fiber will increase fecal output and decrease transit time. Also, bran inhibits the action of bacteria on bile salts and, thus, may prevent the formation of carcinogens. All these factors may act to prevent colon cancer.
Fiber appears to have important nutritional implications in addition to its well-known gastroenterological effects. Fiber apparently affects the rate and route of absorption and metabolism of dietary fat, carbohydrate and protein, as well as altering sterol metabolism and mineral balance. Foods rich in fiber have been shown to lower serum cholesterol and triglycerides, as well as lowering blood pressure.
The work of several researchers has suggested that people who eat a fiber-rich unrefined diet may greatly reduce their chances of developing diseases of overnutrition (obesity, diabetes, gallstones, coronary heart disease), the diseases of an underworked mouth (caries and periodontal disease) and the diseases of an underfed large bowel (constipation, hemorrhoids, spastic colon, diverticular disease, appendicitis and colon cancer). Somewhat more speculative is the evidence linking a low-fiber diet with Crohn's disease, hiatus hernia, varicose veins and deep vein thrombosis.
Whereas a low-fiber or "bland" diet has long been prescribed for patients suffering from peptic ulcer, more recent studies indicate that a high fiber diet is probably more beneficial, due to its greater buffering capacity and increased intestinal transit time. High-fiber intake also provides well established benefits for diabetics: lowers insulin requirements, provides better control of blood glucose and reduces serum lipids. Preliminary studies have indicated that dietary fiber, by inhibiting generation of ammonia by colonic bacteria and increasing fecal nitrogen excretion, can decrease hepatic urea synthesis, thereby reducing plasma urea in patients with chronic renal failure.
This is well described in the referenced article (Essential Resource #2).