The effects of fasting on metabolism and performance

Abstract

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.

Effect of 24-h severe energy restriction on appetite regulation and ad libitum energy intake in lean men and women

“…This has been reported previously (37)(38)(39) and is indicative of altered nutrient supply and/or endogenous stores. Carbohydrate provision in the current study may have been insufficient to meet obligate glucose requirements (40), resulting in an increase in lipolysis to provide NEFA for energy metabolism to preserve endogenous glycogen (40).…”

Electrolyte supplementation during severe energy restriction increases exercise capacity in the heat

“…The majority of previous studies have examined the effects of complete ER, whilst in the present study, subjects consumed 3693 (219) kJ•d -1 and 129 (13) g•d -1 carbohydrate. During the 48 h restriction period, subjects performed no exercise and thus carbohydrate consumed was likely sufficient to meet subjects' obligate requirement (Maughan et al 2010). Therefore, the ER imposed in the present study was unlikely to influence muscle glycogen (Dohm et al 1986;Maughan and Williams 1981) and the provision of some carbohydrate during ER would have reduced the impact on liver glycogen compared to complete ER (Nilsson and Hultman 1973).…”

Effect of fasting versus feeding on the bone metabolic response to running

“…To promote the same response, endurance athletes exercise in the fasted state [1], which increases lipolysis in adipose tissue and subsequent fat oxidation [8], and promotes an adaptation to fat metabolism. The potential for increased fat oxidation, might also make this practice worthy of consideration for the purposes of weight management [9].…”