Delaying post-exercise carbohydrate intake impairs next-day exercise capacity but not muscle glycogen or molecular responses

Aim: To investigate how delayed post-exercise carbohydrate intake affects muscle glycogen, metabolic- and mitochondrial-related molecular responses, and subsequent high-intensity interval exercise (HIIE) capacity. Methods: In a double-blind cross-over design, nine recreationally active men performed HIIE (10 x 2-min cycling, similar to 94% (W)over dot(peak)) in the fed state, on two occasions. During 0-3 h post-HIIE, participants drank either carbohydrates ("Immediate Carbohydrate" [IC], providing 2.4 g/kg) or water ("Delayed Carbohydrate" [DC]); total carbohydrate intake over 24 h post-HIIE was matched (similar to 7 g/kg/d). Skeletal muscle (sampled pre-HIIE, post-HIIE, +3 h, +8 h, +24 h) was analyzed for whole-muscle glycogen and mRNA content, plus signaling proteins in cytoplasmic- and nuclear-enriched fractions. After 24 h, participants repeated the HIIE protocol until failure, to test subsequent HIIE capacity; blood lactate, heart rate, and ratings of perceived effort (RPE) were measured throughout. Results: Muscle glycogen concentrations, and relative changes, were similar between conditions throughout (p > 0.05). Muscle glycogen was reduced from baseline (mean +/- SD mmol/kg dm; IC: 409 +/- 166; DC: 352 +/- 76) at post-HIIE (IC: 253 +/- 96; DC: 214 +/- 82), +3 h (IC: 276 +/- 62; DC: 269 +/- 116) and + 8 h (IC: 321 +/- 56; DC: 269 +/- 116), returning to near-baseline by +24 h. Several genes (PGC-1 alpha, p53) and proteins (p-ACC(Ser79), p-P38 MAPK(Thr180/Tyr182)) elicited typical exercise-induced changes irrespective of condition. Delaying carbohydrate intake reduced next-day HIIE capacity (5 +/- 3 intervals) and increased RPE (similar to 2 ratings), despite similar physiological responses between conditions. Conclusion:<bold> </bold>Molecular responses to HIIE (performed in the fed state) were not enhanced by delayed post-exercise carbohydrate intake. Our findings support immediate post-exercise refueling if the goal is to maximize next-day HIIE capacity and recovery time is <= 24 h.