Diet is the food and drink that a person takes regularly day after day. The word diet also refers to the amounts or kinds of food needed under special circumstances, such as losing or gaining weight. Diet needs vary according to age, weight, health, climate, and amount of activity. Dietetics is the science of feeding individuals or groups. The money that is available and health and nutritional needs affect the type of feeding prescribed.

Normal diet, or balanced diet, contains all the food elements needed to keep healthy. A person needs minerals, proteins, vitamins, and certain fats to build and maintain tissues and to regulate body functions. Proteins, fats, and carbohydrates are used to provide energy and heat. Food elements often eaten in smaller than recommended amounts include calcium, iron, and vitamins A, C, and D, and folic acid (a B vitamin also called folate). A diet that lacks any needed food element may cause certain deficiency diseases. For example, lack of vitamin C causes scurvy, and lack of iron, folic acid, or vitamin B12 may cause anemia.