Protein in the diet provides amino acids for forming body proteins, including the structural proteins for building and repairing tissues, and the enzymes for carrying out the metabolic processes. In addition, protein may be used as a source of energy when the preferred fat and carbohydrate supply runs low. A body that is in the process of building itself (such as that of a growing child or an adult recovering from illness) will need a greater proportion of protein to weight than one that is fully grown and utilizes protein merely for repair of worn-out tissues. The average adult requires 1 gram of protein per kilogram of body weight per day; children may require two to three times this amount. Human proteins consist of up to 22 different amino acids, of which 9 (called essential amino acids) must be supplied by food protein; the other 13 are synthesized by human cells. Complete protein sources—those foods containing all 22 amino acids—include animal products such as meat, eggs, cheese, and milk. Incomplete protein sources, such as vegetables, beans, and grains, may be combined to create complete proteins.

Carbohydrates (starches and sugars) provide a readily available energy source. Surplus carbohydrates are also converted by the body to glycogen and fat, the storage forms of calories for energy, and to some of the amino acids used in protein synthesis. Most health professionals recommend that carbohydrates comprise 50% to 60% of the dietary calories, of which most (c.80% of all carbohydrates eaten) should be complex carbohydrates, such as cereals and vegetables. Complex carbohydrates are preferred because the fast-acting simple carbohydrates, such as honey and sugar, are difficult for the body (especially the pancreas) to handle in large doses. Simple carbohydrates also lack the vitamins, minerals, proteins, and fiber that generally accompany foods rich in complex carbohydrates. Cereals, fruits, vegetables, legumes, and pasta are good sources of complex carbohydrates.

Fats (see fats and oils) in the diet provide a concentrated source of energy; 1 gram of fat supplies about 9 calories as opposed to only 4 calories per gram of carbohydrates and protein. Fats in the body, in addition to acting as a source of stored energy, supply physical protection and insulation for tissues and form important portions of cell membrane structure. Fats also aid in the absorption of the fat-soluble vitamins (vitamins A, D, E, and K) from the intestine. Milk, butter, meat, and oils are important sources of fat.

To keep the body functioning properly it is necessary to have, in addition to the basic foods, a sufficient intake of accessory substances such as vitamins, minerals (see mineral, dietary), and enough water to carry nutrients to the tissues and waste products away from them. A minimum of about 2 liters of liquid per day are recommended for the average adult. Vitamins function as coenzymes in important body processes, with the exception of vitamin D, which is synthesized upon exposure to sunlight. A large variety of minerals are required, some in trace amounts and others, such as calcium and iron, in relatively large amounts. Milk, cheese, and dark, leafy green vegetables are excellent sources of calcium; liver, meat, and egg yolks are good sources of iron. Minerals are vital to the development of teeth and bones (calcium, phosphorus, and fluoride) and to the functioning of a number of the body's metabolic systems. Iron is a necessary part of hemoglobin in the blood; various metals are required in many enzymes; sodium and potassium are essential to maintenance of fluid balance and functioning of the nervous system; magnesium is needed for the normal functioning of nerves and muscles; and iodine is required for thyroid hormone. The usefulness of vitamin and mineral supplements for a person of good health who eats a well-balanced diet continues to provoke debate among health experts.

Good nutrition is reflected not only in the growth and function of the body but also in its appearance. The eyes, skin, hair, and teeth indicate whether body nourishment is good or poor. A poorly nourished child will fail to grow properly; a poorly nourished adult will have a decreased resistance to infection and disease. A diet deficient in proteins causes a disease called kwashiorkor in children; a diet deficient in both protein and calories results in marasmus, with lethargy, abdominal enlargement, and wasting—the classical malnutrition syndrome. Poor nutrition may result from excesses in the diet as well as deficiencies; excess of certain vitamins or minerals can produce potentially lethal disease states, and excess of carbohydrates or fat can result in obesity.

The U.S. Dept. of Agriculture (USDA) illustrates a well-balanced diet with the Food Guide Pyramid (1992), which emphasizes the need to eat less fat and proportionally more complex carbohydrates. At the base of the pyramid is the grains group, which should be eaten in the largest quantity (6–11 servings per day). Just above are the two groups fruits (2–4 servings) and vegetables (3–5 servings). Near the top are the meat products (2–3 servings) and dairy products (2–3 servings). At the apex are the fats, oils, and sweets, which are not considered a food group and should be consumed sparingly. See also food pyramid.

Specialized diets are useful in the treatment of certain disease states; the most common is a low-calorie diet to produce weight loss in obese persons. A diet low in phenylalanine is used to treat phenylketonuria. A diet low in cholesterol and saturated fats seems to be useful in the treatment and prevention of heart disease. Elimination of certain foods from the diet may be necessary to control allergies in some individuals. In all cases, however, specialized diets must provide all classes of essential nutrients in adequate amounts to maintain health in adults and support growth in children.