Interpreting intermittent fasting physiology in the context of behaviour

Intermittent fasting (IF) is a popular diet strategy involving extended periods of fasting or marked energy restriction, interrupted by periods of unrestricted intake. Several protocols exist under the IF umbrella, differing in timing, duration, and whether they require complete fasting or permit limited energy intake during restriction periods. These factors affect physiological and behavioural responses to IF. This review considers evidence that common IF approaches produce distinct physiological effects, and how different temporal structures alter behaviours affecting energy balance. The clearest evidence for distinct physiological effects is from early time-restricted eating. By shifting food intake early in the day, well-controlled studies have noted improved glucose control independent of weight change. In contrast, the physiological response to modified fasting, permitting ∼400-600 kcal/day during restriction periods, does not appear to differ meaningfully from continuous energy restriction, while some complete fasting protocols report greater fat-free mass loss. The clinical effectiveness of IF may depend more on how effectively the energy deficit is retained across repeated restriction and feeding cycles. Several IF studies report reduced physical activity and increased energy intake outside of restricted periods, attenuating the intended energy deficit. Appetite control, food reward, eating traits, and routines, may influence compensation, adherence, and individual variability in response. In summary, some IF approaches can achieve distinct physiological effects, but behavioural changes often mitigate these under free-living conditions. Future research should characterise achieved fasting and energy restriction, identify predictors of behavioural compensation, and explore how adherence can be maximised whilst achieving a fasting-like metabolic response.