The Influence of Pre‐Exercise Snacking on Glycemic Control, Physical Activity, Sleep, and Subjective Mood and Hunger During a 30‐h Fast: A Randomized Crossover Study

Glycemic control is vital to health and disease prevention. A randomized crossover design assessed the effects of fasting alone, fasting with exercise, and fasting with a pre-exercise snack on beta-hydroxybutyrate (BHB) and glucose concentrations, hunger, mood, physical activity, and sleep in 30 participants (16 male). BHB concentrations were elevated in the exercise condition compared to the control at 24 h (p < 0.01), continuing through the end of the intervention (p < 0.01). BHB concentrations were also elevated in the exercise condition compared to the pre-exercise snack condition at 22 h (p < 0.01) and 24 h (p = 0.01) with no differences in BHB between the pre-exercise snack condition and control. There was no main effect of condition on glucose concentrations during the 30-h fast (p = 0.48). However, a condition by time interaction (p < 0.01) revealed elevated glucose levels immediately after exercise (p < 0.01) and lower glucose levels at hour 24 in the exercise-only condition compared to control (p < 0.05). Hunger ratings were lower at 21 h in both exercise conditions immediately following the bout (p < 0.01), but similar at all other time-points. Moderate physical activity was greatest during the pre-exercise snack condition (p = 0.02). Sleep and mood parameters did not differ between conditions. Snacking before exercise produced comparable levels of BHB to a water-only fast. After fasted exercise, BHB decreased, and glucose increased compared to water-only fasting. Hunger was lower following exercise. Snacking before exercise could be a viable alternative for maintaining glycemic control while temporarily reducing subjective hunger compared to a water-only fast.