The mixture of food and drink consumed by an individual. Variations in illness and death between population groups are believed to be due, in part, to differences in diet. A balanced diet was traditionally viewed as one which provided at least the minimum requirement of energy, protein, vitamins and minerals needed by the body. However, since nutritional deficiencies are no longer a major problem in developed countries, it seems more appropriate to consider a ‘healthy’ diet as being one which provides all essential nutrients in sufficient quantities to prevent deficiencies but which also avoids health problems associated with nutrient excesses.

Major diet-related health problems in prosperous communities tend to be the result of too much food, whereas in underdeveloped, poor communities, it is likely to be too little. Excessive intakes of dietary energy, saturated fats, sugar, salt and alcohol, together with an inadequate intake of dietary fibre, have been linked to the high prevalence of OBESITY, cardiovascular disease, dental CARIES, HYPERTENSION, gall-stones (see GALL-BLADDER, DISEASES OF), non-insulin dependent DIABETES MELLITUS and certain cancers (e.g. breast, endometrium, intestine and stomach – see CANCER). Health-promotion strategies in these countries generally advocate a reduction in the intake of carbohydrate and saturated fat, and salt and the avoidance of excessive intakes of alcohol, increased consumption of starch and fibre; and the avoidance of obesity by taking physical exercise.

In addition to disease prevention, diet has a role in the treatment of certain clinical disorders, for example, obesity, diabetes mellitus, HYPERLIPIDAEMIAS, inborn errors of metabolism, food intolerances and hepatic and renal diseases. Therapeutic diets increase or restrict the amount and/or change the type of fat, carbohydrate, protein, fibre, vitamins, minerals and/or water in the diet according to clinical indications. Additionally, the consistency of the food eaten may need to be altered. A commercially available or ‘homemade’ liquid diet can be used to provide all or some of a patient's nutritional needs if necessary. Although the enteral (by mouth) route is the preferred route for feeding and can be used for most patients, parenteral or intravenous feeding is occasionally required in a minority of patients whose gastrointestinal tract is unavailable or unreliable over a period of time.

A wide variety of weight-reducing diets are well publicised.