Diets are also observed for a wide range of medical reasons or to accord with dietary laws, or health and ethical positions such as vegetarianism. In the United States and other developed nations where obesity is a common metabolic problem, the promotion of weight-loss programs has become a large enterprise. This interest in dieting is caused in part by a cultural emphasis on slimness, but obesity is a genuine medical hazard that puts people at risk for cardiovascular diseases, diabetes, and other disorders. People often find diets hard to maintain, in part because they may have unrealistic expectations about how quickly they can lose weight. They may blame the diet itself and try another one.
Many fad diets are unhealthy and even dangerous if followed for any length of time.
The only sources of calories are carbohydrates, proteins, fats, and alcohol. If the number of calories taken in greatly exceeds the number used, the excess is stored as fat and obesity results. With proper dieting and exercise the fat stores in the body will supply some of the dieter's energy needs while maintaining good health. The most sensible approach to weight loss is to begin with a medical checkup to make certain that no special health problems exist and to get a diet and exercise prescription from a physician or registered dietician. The diets that are acceptable to the medical establishment are those in which fewer calories are eaten and exercise is increased.