Fasting is deliberate abstinence from food over a period of time. It is a medical requirement for some gastrointestinal examinations, for example, fluoroscopy and X-ray studies, blood tests, for example, a glucose tolerance test, and before a general anesthetic. The fast required is generally 8 to 12 hours.
Fasting is a requirement in some religious rites; it is also used as a means of political or social protest. Some people fast as a means of loosing weight rapidly, but this method is not recommended because it can result in various health disorders.
In the absence of food, the body's energy requirements are supplied first by the body's sugar, then by reserves of fat. Pangs of hunger occur during the first few days of a fast, but then become less noticeable.
As for precautions for the person who wants to fast, if a person intends to fast for more than two days it is better to seek the advice of a physician. Vitamin supplements are recommended.
When a fast is broken, a normal diet should be resumed gradually, beginning with light, easily digested foods.