Diurnal influences of fasted and non-fasted brisk walking on gastric emptying rate, metabolic responses, and appetite in healthy males

Growing evidence suggests circadian rhythms, nutrition and metabolism are intimately linked. Intermittent fasting (IMF) has become an increasingly popular intervention for metabolic health and combining IMF with exercise may lead to benefits for weight management. However, little is known about the diurnal variation of fasted exercise. This study aimed to investigate the diurnal influences on gastric emptying rate (GER), metabolic responses, and appetite to fasted and non-fasted exercise. Twelve healthy males completed four 45 min walks in a randomised order. Walks were completed in the morning (AM) and evening (PM) and either fasted (FASTED) or after consumption of a standardised meal (FED). GER of a semi-solid lunch was subsequently measured for 2 h using the ¹³C breath test. Blood glucose concentration, substrate utilisation, and ratings of appetite were measured throughout. Energy intake was also assessed for the following 24 h. GER T_lag was slower in PM-FASTED compared to AM-FASTED, AM-FED, and PM-FED (75 ± 18 min vs. 63 ± 14 min, P = 0.001, vs. 65 ± 10 min, P = 0.028 and vs. 67 ± 16 min, P = 0.007). Blood glucose concentration was greater in the FED trials in comparison to the FASTED trials pre-lunch (P < 0.05). Fat oxidation was greater throughout exercise in both FASTED trials compared to FED, and remained higher in FASTED trials than fed trials post-exercise until 30 min post-lunch ingestion (all P < 0.05). No differences were found for appetite post-lunch (P > 0.05) or 24 h post-energy intake (P = 0.476). These findings suggest that evening fasted exercise results in delayed GER, without changes in appetite. No compensatory effects were observed for appetite, and 24 h post-energy intake for both fasted exercise trials, therefore, increased fat oxidation holds positive implications for weight management.