The effects of fasting on metabolism and performance

Br J Sports Med. 2010 Jun;44(7):490-4. doi: 10.1136/bjsm.2010.072181. Epub 2010 May 19.


An overnight fast of 8-10 h is normal for most people. Fasting is characterised by a coordinated set of metabolic changes designed to spare carbohydrate and increase reliance on fat as a substrate for energy supply. As well as sparing the limited endogenous carbohydrate, and increased rate of gluconeogenesis from amino acids, glycerol and ketone bodies help maintain the supply of carbohydrate. Many individuals undergo periodic fasts for health, religious or cultural reasons. Ramadan fasting, involving 1 month of abstention from food and fluid intake during daylight hours, is practised by a large part of the world population. This period involves a shift in the pattern of intake from daytime to the hours of darkness. There seems to be little effect on overall daily dietary intake and only small metabolic effects, but there may be implications for both physical and cognitive function. The limited evidence suggests that effects of Ramadan-style fasting on exercise performance are generally small. This needs to be balanced, however, against the observation that small differences in performance are critical in determining the outcomes of sporting events. Studies involving challenging sporting events (prolonged sustained or intermittent high-intensity events, hot and humid environments) are needed. Increases in subjective sensations of fatigue may be the result of loss of sleep or disruption of normal sleep patterns. Modifications to the competition timetable may minimise or even eliminate any effect on performance in sport, but there may be negative effects on performance in some events.

Publication types

  • Review

MeSH terms

  • Athletic Performance / physiology*
  • Basal Metabolism / physiology
  • Dietary Carbohydrates / metabolism
  • Digestion / physiology
  • Eating / physiology*
  • Energy Intake / physiology
  • Energy Metabolism / physiology
  • Exercise / physiology
  • Exercise Tolerance / physiology
  • Fasting / physiology*
  • Humans
  • Intestinal Absorption / physiology
  • Islam*


  • Dietary Carbohydrates