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Introduction
Fasting can be defined as the absence of food and fluid intake, but there is no clear definition of the time after the last intake at which fasting might be said to begin. The postingestive period certainly lasts for several hours, but the duration will depend on the amount and type of the food that is ingested. Any consideration of the response to short-term fasting cannot therefore easily be separated from the effects of the last food intake.
While it is clear that prolonged periods without food intake are harmful to both health and performance, it is less clear that shorter periods of total abstinence from food intake, or more prolonged periods of intermittent fasting, are necessarily harmful. The intervals between intake of food or drinks may be short during the day, but most people experience an overnight fast of about 8-12 h on a daily basis. Fasting, often in combination with exercise, has long been used as a tool for the study of regulation of intermediary metabolism, and there is an extensive literature in this area. These studies have focused on total fasts lasting from a few hours to a few days or on energy restricted diets, usually in obese individuals, of longer duration. In some, but not all, of these studies, water intake has been allowed in order to separate the effects of dehydration from the metabolic effects of the absence of exogenous nutrient intake. There have been some studies on intermittent fasting, as practised by Muslims during the period of Ramadan fasting, but in many ways this represents simply a shift in the time base: instead of fasting during the night while asleep, the period of fasting is shifted to the daylight hours. Changes in sleeping patterns and other lifestyle factors complicate the interpretation of any metabolic or performance effects.
More recently, there has been an interest in the performance effects of Ramadan-style intermittent fasting as global participation in international sport has increased. Muslim athletes now represent a significant fraction of the competitors in major sporting events: when these events take place during Ramadan, some athletes will defer fasting on competition days but will continue to train, while others will observe the fast on competition days. The absence of food and...