Late meal timing has been linked to increased body mass, although the physiological mechanisms are unknown. While previous research has suggested that identical meals consumed in the morning produce greater thermogenic responses than evening meals, it is unclear whether this is due to endogenous circadian control or due to behavioral and environmental factors. Therefore, this mechanistic randomized trial tested whether the endogenous circadian system, thus independently of sleep/wake, rest/activity, body posture, dark/light, and fasting/eating cycles, modulates diet-induced thermogenesis (DIT). To unmask endogenous circadian effects from environmental and behavioral confounds, 16 healthy adults with overweight or obesity (12 males; mean ± SD age, 36 ± 11 years; BMI, 28.8 ± 2.4 kg·m; HbA1c, 5.4 ± 0.3%) completed a gold-standard Constant Routine (CR) protocol, consisting of 36 h of continuous wakefulness, rest, semi-recumbent posture, dim light, and identical test meals every 6 h. The data demonstrated a significant endogenous circadian rhythm in DIT, with a peak-to-trough amplitude of 44% (~10 kcal/4 h), a peak during the biological morning and a trough in the evening (p = 0.005; equivalent to ~8 am/~8 pm, respectively). After adjusting for the fasting energy expenditure's endogenous circadian rhythm, the circadian rhythm in DIT remained significant with similar circadian timing, although with slightly reduced amplitude (~7 am/~7 pm; ~29%; ~7 kcal/4 h; p = 0.026). In exploratory analyses, data revealed that this endogenous circadian rhythm in DIT was not affected by prior eating schedules. The endogenous circadian rhythm in DIT, with a peak in the biological morning, may be one of the mechanisms contributing to the reported link between late meal timing and increased body mass. TRIAL REGISTRATION: ClinicalTrials.gov; NCT02298790; clinicaltrials.gov; registered on November 20, 2014. -2
