Human Nutrition and Food

Diet and Health in the United States

Daphne A. Roe, professor of nutrition, Division of Nutritional Sciences, Cornell University, Ithaca, NY.

Many measures of health status have been used to characterize a population. These measures include objective criteria such as growth, life expectancy, death rates, the prevalence and incidence of specific diseases and the prevalence of disability because of these diseases, the ability of people to perform the activities of daily living, and their ability to perform physical exercise.

They also include subjective criteria, of which the most useful is a self-assessment of health status. Our population's health status is related to its self-perceived condition of wellness and to the prevalence of certain chronic diseases which detract from functional capacity, increase health care costs, and reduce life expectancy.

Perhaps the most important advance in our knowledge of human behavior in the last 25 years has been the realization that, insofar as wellness and the risk of chronic disease are concerned, we are capable of shaping our own destiny. Self-determination of health status, which for any individual is incomplete, is achieved by positive health behaviors associated with disease prevention. Such behaviors are largely determined by an individual's perception of the benefits of such action.

Wellness, Exercise, and Nutritional Needs

The positive behaviors significantly related to health promotion include not only avoidance of social poisons such as cigarettes and alcohol in excess, avoidance of injury from our physical and chemical environment, including sunburn and exposure to chemical toxins in the workplace, but also exercise and a prudent diet. Interest in exercise derives from the association of such activity with desirable health including reduced risk of obesity, hypertension, heart disease, and diabetes. The retention of calcium in bones also is favorably affected by exercise, and exercise may delay the onset, or reduce the severity, of age-related bone loss.

National health organizations that have endorsed the public's enthusiasm for exercise include the American Heart Association, the American Medical Association, the American Dietetic Association, and the American Diabetes Association.

Influence of Exercise on Nutritional Needs

While recognizing the advantages of regular aerobic exercise, it also is necessary to understand that exercise performance influences nutritional needs. Adequate intake of water to offset sweat losses is essential. Then, as the amount of daily exercise increases, so food-energy requirements increase. Protein needs are not much increased by athletic training or by regular exercise, but recent evidence from animal studies has indicated that endurance exercise may have a moderate effect on protein requirements.

Needs for certain B vitamins grow with increases in energy expenditure. In the case of riboflavin, studies in young women have shown that the 1980 RDA is inadequate and that exercise increases requirements, whether or not body weight is constant. There is no evidence that the large supplements of B vitamins advocated for athletes improve athletic performance.

Mineral and trace element needs also increase with exercise, especially for minerals such as sodium and trace elements such as copper which are lost in sweat. Men and women engaging in mild to moderate aerobic exercise under temperate conditions, however, do not require salt pills. Sodium depletion is only a significant risk when heavy exercise is undertaken in hot environmental conditions or when the individual exercising either is on a sodium-restricted diet or is taking diuretics.

While many clinical vitamin and mineral deficiencies are associated with decreased physical performance, nutritionists reassure us that the nutritional needs of most athletic men and women can be met by modifications of the usual diet to meet food-energy and nutrient needs. They are currently more concerned about athletes taking excessive amounts of nutrient supplements or other health food products under the mistaken advice of trainers, friends, or sports magazine writers.

Diet in Preventing Cardiovascular Disease

Coronary Heart Disease. Coronary heart disease is associated with increased blood cholesterol values. Evidence is now substantial, however, that coronary heart disease incidence can be decreased by a reduction of blood cholesterol. While this was demonstrated in the Lipid Research Clinics Primary Prevention Trial, the reduction in cholesterol was because of treatment with cholestyramine, which is a cholesterol-lowering drug.

Epidemiological evidence also indicates that changes in the U.S. life style, with reduction in cholesterol and saturated fat intake as well as cessation of smoking, have lowered vascular death rates. Further, we know that countries such as Japan, which have low intakes of cholesterol and fat, have lower coronary heart disease mortality.

Dietary guidelines aimed at reducing the risk of coronary heart disease emphasize reduction in total fat intake to 30 percent of calories, reduction of saturated fat consumption to account for about 10 percent of total energy intake, and balancing that with 10 percent each of polyunsaturated and monounsaturated fats and reduction in cholesterol consumption to about 300 milligrams per day.

Hypertension. A large number of nutritional factors may influence cardiovascular physiology and may, therefore, be important in the development or prevention of hypertension. Evidence from descriptive epidemiological studies, which has been confirmed by controlled clinical trials, indicates that excessive food-energy intake is the most important nutritional determinant of elevated blood pressure. Other dietary factors which are causal determinants of hypertension include dietary sodium and alcohol excess. While it has been suggested that people vary in their response to increases in dietary sodium, such that those with a family history of hypertension are more susceptible, recent studies refute this hypothesis.

In preventing as well as controlling hypertension, it is important to remember that dietary sodium is largely derived from sodium chloride in foods, drinking water, and also in both prescription and over-the-counter drugs. In addition, other sodium salts are used as intentional food additives including sodium bicarbonate, sodium benzoate, sodium nitrate and nitrite, and monosodium glutamate. The Food and Nutrition Board of the National Academy of Sciences has recommended that intake of salt should be in the range of 3-8 grams a day.

Defective calcium metabolism in the smooth muscle cells has been suggested as a mechanism for the development of hypertension. But these changes may be a consequence rather than a cause of hypertension.

Epidemiological Studies of Diet and Cancer

Several lines of scientific evidence indicate that diet plays an important role in human cancer development. The major and potential roles which have been identified are as follows:

Carcinogens may be present in food, either as natural food toxins, substances formed in food as a result of food processing or food preparation, intentional food additives, or food contaminants;

Carcinogenesis may be produced in the body, for example, in the stomach or intestine from food constituents;

Protein-energy malnutrition or specific vitamin or mineral deficiencies may retard or promote tumor formation;

Dietary excesses of food energy, fat, or protein may promote tumor formation;

Specific nutrients or non-nutrient components of food may provide protection against cancer development.

Examples of carcinogens present in food include aflatoxin, polycyclic hydrocarbons, and nitrosamines. Nitrosamines also may be formed in the body from nitrates ingested in vegetables or in meats preserved with nitrate or nitrite. High fat diets have been linked to several common cancers, including breast and colon cancer. Cancer of the mouth, throat, and esophagus have been linked to alcohol abuse.

However, dietary fiber, vegetables like cabbage and broccoli, vitamin A, beta-carotene, vitamin C, and selenium have been shown to exert a protective effect against cancer, at least in experimental animals.

Interest in the relationship between diet and cancer has been stimulated by international studies showing that large differences exist in cancer incidence between and within countries. Strong correlations have been found between dietary factors and cancer incidence, but to support a causal relationship either between specific dietary factors and cancer development or prevention, it has been recommended that double-blind nutritional intervention trials be carried out. In these trials, it will be necessary to obtain an end-point when it is apparent that the imposed dietary change or nutrient supplement has or has not altered the incidence of particular cancers.