BACKGROUND: Nutrient timing is an effective means of augmenting endurance exercise training and performance. Previous studies report that pre-exercise feeding can influence exercise metabolism up to 4 hours post-prandial. However, this timeline has been confirmed during waking hours only; little is known about how sleep within the post-prandial period may influence metabolism during subsequent exercise. This question is relevant to the endurance competitor, as race start-times often occur in early morning, limiting the opportunity for optimal feeding prior to competition without disrupting sleep or risking gastrointestinal distress. PURPOSE: To investigate the influence of a small, nutrient dense, pre-sleep chocolate milk (CM) beverage on morning metabolism and 10-km running performance in female athletes. METHODS: In a crossover design, twelve competitive female runners (age, 30 ± 7 yrs; VO2peak, 53 ± 4 ml·kg−1·min−1) ingested either pre-sleep CM or a non-nutritive, flavor-matched placebo (PL) ~30 min before sleep and 7-9 hrs before a morning performance running trial. Following initial appetite assessment (visual analogue scales) and resting metabolic rate (RMR), serum glucose (GLU) and lactate (LAC) measurements, the performance trial included a warm-up and three 5-min incremental loads at 55, 65, and 75% VO2peak (to measure respiratory exchange ratio (RER), GLU and LAC), followed by a 10-km treadmill time trial (TT). Paired t-tests, ±90% confidence intervals, and magnitude-based inferences were used to determine differences in means. Significance was accepted at P < 0.05. RESULTS: Relative to PL, CM showed a 'likely small decrease' in perceived hunger (P = 0.041, -22.3% mean effect), and a 'likely small increase' in RMR (P = 0.049, 4.8% mean effect), resting GLU (P = 0. 081, 3.7% mean effect), and RER at 55, 65, and 75% VO2peak (P = 0.115, 0.194, 0.164 and 2.1%, 1.6%, 1.4% mean effect, respectively). Exercise GLU revealed subtle, and unique trends following CM compared to PL. Specifically, GLU was 'possibly increased' during exercise at 65% VO2peak (P = 0.358, 2.0% mean effect) and 'likely trivially decreased' at 75% VO2peak (P = 0.561, -1.0% mean effect) following CM compared to PL. No differences in resting or exercise LAC, or 10-km TT performance (PL: 52.8 ± 8.4 mins versus CM: 52.8 ± 8.0 mins, P = 0.987, -0.1% mean effect, 'most likely trivial' decrement to performance following CM compared to PL) were noted between treatments. CONCLUSIONS: Nighttime supplementation of CM results in acute enhancement to morning metabolism via increased carbohydrate utilization during exercise, but has no apparent effects on 10-km running performance. These results suggest that the nighttime feeding timeline, specifically sleep imposed within the post-prandial period may extend tangible effects to metabolism to over 8 hours out from meal ingestion.
