As it stands medical opinion at the moment is that the benefits of fasting are unproven and until there are more human studies it's better to eat at around 2000 calories a day.
If you really want to fast then you should do it in a proper clinic or under medical supervision, because there are many people, such as pregnant women or diabetics on medication, for whom it could be dangerous.
The British Dietetic Association says that rapid weight loss occurs when fasting or severely restricting dietary intake, but this weight loss is mainly water, glycogen (the body's carbohydrate stores) and muscle, rather than body fat.
"Routine fasting is practiced successfully by many cultures for religious purposes and may have some health benefits, says BDA spokesperson Rick Miller, "However, the clinical evidence for fasting as a treatment in healthcare is not clear.
"Fasting could potentially be unsafe in some individuals without medical supervision or lead to the development of poor eating patterns. Dietitians would always recommend a well-planned, healthy diet in the first instance for health and longevity."
Dietitian Sian Porter says: "A lot of people have a five days and two days eating pattern.
"Make sure you eat foods that make you feel full and keep your fluid intake up."