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The science of nutrition is dedicated to learning about foods that the human body requires at different stages of life in order to meet the nutritional needs for proper growth, as well as to maintain health and prevent disease. A baby is born with a very high requirement for energy and nutrient intake per unit of body weight to provide for rapid growth. The rate of growth is the highest during the first year and declines slowly after the age of two, with a corresponding decrease in nutrient and energy requirements. During puberty, however, nutritional requirements increase sharply until this period of fast growth is completed. Adulthood begins at about the age of fourteen or fifteen for girls, and eighteen or nineteen for boys.

An adult individual needs to balance energy intake with his or her level of physical activity to avoid storing excess body fat. Dietary practices and food choices are related to wellness and affect health, fitness, weight nutrition: the maintenance of health through proper eating, or the study of same energy: technically, the ability to perform work; the content of a substance that allows it to be useful as a fuel nutrient: dietary substance necessary for health puberty: time of onset of sexual maturity nutritional requirements: the set of substances needed in the diet to maintain health fat: type of food molecule rich in carbon and hydrogen, with high energy content wellness: related to health promotion chronic: over a long period osteoporosis: weakening of the bone structure cardiovascular: related to the heart and circulatory system cancer: uncontrolled cell growth diabetes: inability to regulate level of sugar in the blood physiological: related to the biochemical processes of the body basal metabolic rate: rate of energy consumption by the body during a period of no activity vitamin: necessary complex nutrient used to aid enzymes or other metabolic processes in the cell mineral: an inorganic (non-carbon-containing) element, ion, or compound protein: complex molecule composed of amino acids that performs vital functions in the cell; necessary part of the diet obesity: the condition of being overweight, according to established norms based on sex, age, and height carbohydrate: food molecule made of carbon, hydrogen, and oxygen, including sugars and starches fiber: indigestible plant material that aids digestion by providing bulk diet: the total daily food intake, or the types of foods eaten constipation: difficulty passing feces hemorrhoids: swollen blood vessels in the rectum diverticulosis: presence of abnormal small sacs in the lining of the intestine appendicitis: inflammation of the appendix management, and the prevention of chronic diseases such as osteoporosis, cardiovascular diseases, cancer, and diabetes.

For adults (ages eighteen to forty-five or fifty), weight management is a key factor in achieving health and wellness. In order to remain healthy, adults must be aware of changes in their energy needs, based on their level of physical activity, and balance their energy intake accordingly.

As teenagers reach adulthood, the basal energy needs for maintaining the body's physiological functions (basal metabolic rate, or BMR) stabilize, and so energy requirements also stabilize. BMR is defined as the energy required by the body to keep functioning. These functions include the pumping of blood by the heart, respiration, kidney function, and maintaining muscle tone and a constant body temperature, among others. BMR is directly related to the amount of lean body muscle mass, size, and gender. Physical activity, especially weight-training exercises, help increase and maintain lean body mass.

It is very important to reduce one's energy intake at the onset of adulthood, and to make sure that all of one's nutritional needs are met. This can be accomplished by making sure that an adequate amount of energy is consumed (this will vary by body weight, degree of physical fitness, and muscle vs. body fat), and that this amount of energy is adjusted to one's level of physical activity. Foods that are chosen to provide the energy must be highly nutritious, containing high amounts of essential nutrients such as vitamins, minerals, and essential proteins.

It is usually at this age that young adults start gaining body fat and reducing their physical activity, resulting in an accumulation of fat in the abdominal areas. This is an ever-increasing risk factor in the population of the United States, where obesity is not only a problem in adults, but also in children. It is believed that the high level of obesity in the United States is mostly due to bad dietary practices such as eating a high-fat, low-complex carbohydrate (low fiber) diet, including excessive amounts of meat. The indulgence in fast foods and a lack of regular physical activity are major factors. Obesity is a risk factor for other degenerative diseases, such as type II (adult onset) diabetes, diseases of heart and circulation, and certain cancers. Another nutritional problem related to eating such a diet is constipation, due to low-fiber diets. This may result in hemorrhoids, diverticulosis, appendicitis, and other more serious diseases of the lower intestine. Increasing the number of servings of fruits, vegetables, and whole grains in the diet will prevent these diseases. In the United States, the Dietary Guidelines for Americans (as summarized in the Food Guide Pyramid) provide practical guidelines for healthful eating.

At the onset of adulthood, energy requirements usually reach a plateau that will last until one's mid-forties, after which they begin to decline, primarily because activity levels and lean muscle mass (amount of muscle vs. body fat), which represents the BMR, decrease. It is believed that the changes in body composition and reduced lean muscle mass occur at a rate of about 5 percent per decade, and energy requirements decrease accordingly. However, these changes in body composition and decreased energy requirements can be prevented by maintaining regular physical activity, including resistance training, which helps maintain lean muscle mass and prevent deposition of excess body fat.

The basal metabolic rate—the number of calories a person's body uses while at rest—generally decreases with age. Good health requires adults to adapt their diets to the body's changing needs by eating low-fat and nutrient-rich foods. [Photograph by Michael Keller. Corbis. Reproduced by permission.]

The basal metabolic rate—the number of calories a person's body uses while at rest—generally decreases with age. Good health requires adults to adapt their diets to the body's changing needs by eating low-fat and nutrient-rich foods. [Photograph by Michael Keller. Corbis. Reproduced by permission.]

By preventing normal age-related decline in lean muscle mass, one can prevent obesity and prolong one's physiological age. The result is that a person is less vulnerable to degenerative diseases, such as cardiovascular diseases, cancer, and diabetes, and can usually perform at a higher level than his or her chronological age would otherwise allow.

Older adults who are not physically active or who have poor nutritional practices will have a decline in BMR, a change in body composition, an increasing percentage of body fat, and a decrease in lean body muscle mass. In addition, they will show the signs of aging and will be more likely to develop degenerative diseases.

hypertension: high blood pressure absorption: uptake by the digestive tract calcium: mineral essential for bones and teeth vitamin D: nutrient needed for calcium uptake and therefore proper bone formation menopause: phase in a woman's life during which ovulation and menstruation ends

Many older adults need to take medications to control the advance of diabetes, hypertension, and cardiovascular disease. Medications can interfere with proper nutrition, however, as they affect appetite, the digestion and absorption of nutrients, and normal function of the digestive system.

As women age, they may develop osteoporosis if they have not built up strong bones by eating foods high in calcium and adequate vitamin D. Women start losing calcium from bones during and after the onset of menopause at the rate of 1 percent per year for about five years, after which the rate of calcium loss is reduced until about age seventy-five or eighty. Therefore, it is important for women to eat foods high in calcium up to the age of thirty-five. The recommended daily intake of calcium is 1,200 milligrams. This requirement can be met by consuming four servings of dairy products and two servings of green vegetables each day. It is well established that calcium from foods is much better absorbed than calcium from supplements. It is beneficial, therefore, to choose foods with a high calcium content, such as low-fat or skim dairy products. This regimen builds a bone density high enough so that, at menopause, losing approximately 5 percent of bone density in five years does not place a woman in the "fracture zone," where bones can break as a result of osteoporosis. see also Aging and Nutrition; Nutrient-Drug Interactions; Osteoporosis.

Simin B. Vaghefi


Poehlman, E. T., and Horton, E. S. (1999). "Energy Needs: Assessment and Requirements in Humans." In Modern Nutrition in Health and Disease, 9th edition, edited by M. E. Shils, J. A. Olson, M. Shike, and A. C. Ross. Baltimore, MD: Williams & Wilkins.

Pi-Suunyer, F. X. (1999). "Obesity." In Modern Nutrition in Health and Disease, 9th edition, edited by M. E. Shils, J. A. Olson, M. Shike, and A. C. Ross. Baltimore, MD: Williams & Wilkins.

Internet Resources

National Institutes of Health (2000). "Osteoporosis Prevention, Diagnosis, and Therapy." NIH Consensus Statement. Available from <>

U.S. Department of Agriculture. "The Food Guide Pyramid." Available from <http://>

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