Early Time-Restricted Feeding Reduces Appetite and Increases Fat Oxidation but Does Not Affect Energy Expenditure in Humans

OBJECTIVE: Eating earlier in the daytime to align with circadian rhythms in metabolism enhances weight loss. However, it is unknown whether these benefits are mediated through increased energy expenditure or decreased food intake. We therefore performed the first randomized trial to determine how meal timing affects 24-hour energy metabolism when food intake and meal frequency are matched. METHODS: Eleven overweight adults practiced both early time-restricted feeding (eTRF; eating from 8 am-2 pm) and a control schedule (eating from 8 am-8 pm) each for four days. On the fourth day, 24-hour energy expenditure and substrate oxidation were measured by whole-room calorimetry, in conjunction with appetite and metabolic hormones. RESULTS: eTRF did not affect 24-hour energy expenditure (Δ=10±16 kcal/day; p=0.55). Despite the longer daily fast (intermittent fasting), eTRF decreased mean ghrelin levels by 32±10 pg/ml (p=0.006), made hunger more even-keeled (p=0.006), and tended to increase fullness (p=0.06-0.10) and decrease the desire to eat (p=0.08). eTRF also increased metabolic flexibility (p=0.0006) and decreased the 24-hour non-protein respiratory quotient (Δ=−0.021±0.010; p=0.05). CONCLUSIONS: Meal timing interventions facilitate weight loss primarily by decreasing appetite rather than by increasing energy expenditure. eTRF may also increase fat loss by increasing fat oxidation.