Circadian rhythms in resting metabolic rate account for apparent daily rhythms in thermic effect of food.

Context Daily variation in thermic effect of food (TEF) is commonly reported and proposed as a contributing factor to weight gain with late eating. However underlying circadian variability in resting metabolic rate (RMR) is an overlooked factor when calculating TEF associated with eating at different times of the day. Objective To determine whether methodological approaches to calculating TEF contribute to the reported phenomena of daily variation in TEF.Design, Setting and Participants: Fourteen overweight to obese but otherwise healthy subjects, had their resting and postprandial energy expenditure measured over 15.5 hours at a clinical research unit. TEF was calculated for breakfast, lunch and dinner using standard methods (above a baseline and premeal RMR measure) and compared to a method incorporating a circadian RMR where RMR was derived from a sinusoid curve model and TEF was calculated over and above the continuously changing RMR. Main outcome measures TEF at breakfast, lunch and dinner calculated with different methods. Results Standard methods of calculating TEF above a premeal measured RMR showed that morning TEF [60.8kcal ± 5.6] (mean ± SEM) was 1.6 times greater than TEF at lunch [36.3kcal ± 8.4], and 2.4 times greater than dinner TEF [25.2kcal ± 9.6] (p=0.022). However, adjusting for modelled circadian RMR nullified any differences between breakfast [54.1kcal ± 30.8], lunch [49.5kcal ± 29.4], and dinner [49.1kcal ± 25.7] (p=0.680). Conclusions Differences in TEF between morning and evening can be explained by underlying circadian resting energy expenditure, which is independent of an acute effect of eating.