The effect of morning or evening fasted cycling on gastrointestinal function and appetite and metabolic responses in healthy males with overweight

Combining fasting with exercise may influence gastric emptying rate (GER) and provide benefits to weight management and metabolic health. Furthermore, the time of day in which exercise is performed may also influence these variables. The aim was to investigate if fasting or fed exercise at different times of the day would alter GER, appetite and metabolic responses. Twelve males with overweight completed four experimental trials in a randomised crossover fashion involving cycling exercise in the morning fasted (FASTED-AM), evening fasted (FASTED-PM) and after a standardised meal in the morning (FED-AM) and evening (FED-PM). GER of a semi-solid meal was measured using the ¹³C-breath test for 2 h. Appetite hormones, metabolic markers and subjective appetite were measured throughout, with energy intake (EI) monitored for the following 24 h. No difference was observed for GER between trials. No differences were seen between trials for appetite hormone responses except pancreatic polypeptide hormone incremental area under the curve (iAUC) was greater in FED-PM compared to FASTED-AM and FASTED-PM (P < 0.05). Glucose concentrations were greater in the postprandial period of FASTED-PM compared to all trials (P < 0.05). No differences in other metabolic marker responses were seen between trials. GER in individuals with overweight was not sensitive to a diurnal variation following fasted or fed exercise, and an acute bout of fasted exercise did not evoke compensatory effects on appetite responses or 24 h EI. Glucose control may be impaired with FASTED-PM exercise. Future work is required to assess the long-term impact of fasted exercise on gastrointestinal function, appetite regulation and metabolic health.