The word diet comes from the Greek diaita, meaning "manner of living." Dietetics applies nutritional knowledge to the feeding of humans.

Nutritional Requirements

Foods from the food guide pyramid may be part of any meal. A grilled cheese sandwich or a bowl of whole-grain cereal is just as nutritious in the morning as it is at noon. A good breakfast consists of any foods that supply about one fourth of the necessary nutrients for the day.

The United States government publishes guidelines for appropriate nutrient intakes. These are known as the Recommended Dietary Allowances. They are updated regularly on the basis of new evidence from the science of nutrition. Just as wise shoppers get the most for their money, so wise eaters select foods that give the most nutrition for the lowest number of calories. For example, snacks can furnish about one fourth of the calorie requirements among teenagers. Those snacks should also furnish that much of the day's allowances for protein, minerals, and vitamins. Sandwiches, fruit, and milk make good snacks for active teenagers.

Fast Foods and Convenience Foods

Fast foods and convenience foods are major parts of many people's diets. Convenience foods, such as TV dinners and cake mixes, are those prepared at home from foods already cooked or otherwise processed before reaching the retail store. Fast foods are prepared in quick-order restaurants (see Fast Food).

A meal consisting of a cheeseburger with lettuce, tomato, and onion, french fries, and a milk shake, though high in fat and sodium, does include foods from each of the groups in the food guide pyramid. So does a taco with its cereal shell, ground beef, shredded cheese, lettuce, olives, and tomato.

Frequent fast-food meals, though, require some thought about nutrients and calories. High in protein, iron, and B vitamins, fast-food meals are sometimes low in calcium and vitamins A and C and often extremely high in sodium. A milk shake provides calcium, but it can also be high in fat and contain more than 800 calories.

Depending on the items ordered, the calorie content of a typical fast-food meal ranges from 900 to 1,800. A burger with all the trimmings has about 600 calories, but a plain cheeseburger has only about half as many, and a plain hamburger even less. A serving of pizza can have from 300 to 600 or more calories.

Special Diets

Special diets for people with health problems should be prescribed by a physician. Many doctors refer patients to a dietitian, who draws up an individual diet plan.

The typical diet in the United States contains much more salt than the body needs. Persons with high blood pressure must avoid as much salt as possible. Diabetes mellitus is another disease that requires careful meal planning. In diabetes mellitus the body does not make normal use of glucose, or blood sugar. In addition to a special diet, diabetic children and some adults need medication. If diabetes develops later in life, it often can be controlled with diet alone.

Diets high in fat and cholesterol have been linked with blockage of the arteries, a common factor in heart disease and stroke. Studies show that this condition can begin in adolescence or earlier.

Some people are vegetarians; that is, they eat no meat and sometimes no eggs, cheese, or milk. Those who avoid all animal products are called vegans. Such people must plan their diets carefully to get balanced nutrition.


Malnutrition is the imbalance between the body's demand for nutrients and the available supply of nutrients. Malnutrition can result from an unsatisfactory diet or from a disorder that interferes with the body's use of food.

Obesity, or the state of being excessively fat, is a form of malnutrition that contributes to many health problems. It may be defined as body weight more than 20 percent above a person's ideal weight.

Anorexia nervosa is a condition characterized by extreme weight loss. Nervosa means "of the nerves," or neurotic. This life-threatening disease usually occurs in young women. It requires professional treatment.

Protein-calorie malnutrition, known as kwashiorkor, is common among children in unindustrialized nations. A lack of protein results in failure of the body to grow and often damages digestive organs. A severe calorie deficiency results from starvation.

Mineral and vitamin deficiencies are responsible for various disorders. Insufficient iron can cause iron-deficiency anemia. Lack of iodine can cause goiter, an enlargement of the thyroid gland.

Vitamin A deficiency causes loss of vision in dim light. Vitamin D deficiency leads to a faulty deposit of calcium in bones and teeth, resulting in rickets. A child with rickets may have bowed legs and a prominent sternum (breastbone). Vitamin C deficiency is known as scurvy. It causes infected and bleeding gums and painful joints.

Vitamin B1 deficiency, or beriberi, damages the nerves, heart, and circulation. Whole-grain flour and enriched flour that is processed in the United States contain vitamin B1.

Vitamin B12 deficiency causes blood disorders and affects the nervous system. It rarely results from an inappropriate diet but rather from a defect of absorption in the digestive tract.