The objective of a proper diet is to achieve and maintain a desirable body composition and a large capacity for physical and mental work. The daily requirements for essential nutrients depend on a person’s age, sex, height, weight, and metabolic and physical activity.
The U.S. Department of Agriculture originally proposed the basic four food groups (dairy products, meat and protein- rich vegetables, cereals and breads, and fruits and vegetables) as a guide to a balanced diet, but currently it suggests that the food guide pyramid is a better guide. This guide is intended to help people choose a diet that supplies essential nutrients as well as helps reduce the risk of such disorders as cancer, high blood pressure, coronary artery disease, and strokes. In this guide, the number of servings a day for each food group varies, depending on energy needs that range from 1,600 to more than 2,400 calories a day. For example, a person who consumes 1,600 calories a day could eat 6 servings from the bread group and 3 from the vegetable group, whereas a person who consumes 2,400 calories a day could eat 10 servings from the bread group and 5 from the vegetable group. In general, the consumption of fat should be reduced to about 30 percent of calories, and the consumption of fruits, vegetables, and cereals should be increased.
Malnutrition can result from either under nutrition or over nutrition. Both conditions are caused by an imbalance between the body’s need for and the intake of essential nutrients.
Under nutrition, a deficiency of essential nutrients, can result from inadequate intake because of a poor diet or poor absorption from the intestine (malabsorption); abnormally high usage of nutrients by the body; or abnormal loss of nutrients through diarrhea, bleeding (hemorrhage), kidney failure, or excessive sweating. Over nutrition, an excess of essential nutrients, can result from overeating, excessive use of vitamins or other supplements, or under exercising.
Malnutrition develops in stages: First, changes occur in the levels of nutrients in blood and tissues, then changes occur in enzyme levels, next body organs and tissues malfunction, and then symptoms of illness and death occur.
The body needs more nutrients during certain stages of life, particularly in infancy, early childhood, and adolescence; during pregnancy; and while breastfeeding. In old age, nutritional needs are lower, but the ability to absorb nutrients is often reduced. Therefore, the risk of under nutrition is greater at these times of life, and even more so among people who are economically deprived.