Effect Of Fasting Prior To Evening Exercise On Substrate Oxidation, Performance, And Subsequent Energy Intake

Abstract

Overnight fasted exercise may create a more negative energy balance than the same exercise performed after breakfast and may result in superior metabolic benefits via upregulated fat oxidation. Evening exercise is more common than morning exercise, and several physiological and behavioural processes are subject to circadian variation (e.g., nutrient uptake and metabolism, appetite, and eating behaviour). To extend the benefits of fasted exercise to a wider proportion of the population, methods to upregulate fat oxidation during evening exercise are necessary. PURPOSE: To examine the effects of fasting prior to evening exercise on substrate oxidation, appetite, energy intake, exercise performance, and subjective feelings towards exercise. METHODS: Sixteen healthy males (n = 8) and females (age: 25 ± 3 y; BMI: 23 ± 2 kg/m2; body fat: 20 ± 7 %; VO2peak: 39 ± 6 mL/kg/min) completed two trials in randomised, counterbalanced order. Participants consumed a standardised breakfast (543 ± 86 kcal; 08:00) and lunch (814 ± 129 kcal; 11:30) prior to 30 min cycling (60% VO2peak) and a 15-min performance test (18:30). Two hours before exercise (16:30), participants consumed a snack (543 ± 86 kcal; FED) or remained fasted (FAST). Ad libitum energy intake was assessed 15 min after exercise. Subjective appetite was measured throughout. RESULTS: Total fat oxidation was greater (7.71 ± 2.38 vs. 4.46 ± 2.21 g) and carbohydrate oxidation lower (41.63 ± 17.04 vs. 50.79 ± 18.75 g) during exercise in FAST compared to FED (p < 0.001). Energy intake was 99 ± 162 kcal greater at dinner in FAST (p < 0.05), although energy intake over the day was 458 ± 131 kcal lower in FAST (p < 0.001). Appetite was higher between lunch and dinner in FAST compared to FED (p < 0.05). Total work completed during the performance test was 3.5% lower in FAST (153 ± 57 vs. 159 ± 58 kJ; p < 0.05). In FAST, pre-exercise motivation, energy, readiness, and post-exercise enjoyment were lower than in FED (p < 0.01). CONCLUSION: Fasted evening exercise increases fat oxidation compared to exercise performed 2 h after a snack. However, increases in energy intake at dinner, reductions in voluntary energy expenditure, and negative perceptions of fasted exercise may reduce the effectiveness of this intervention as a long-term weight management strategy.