Environmental heat stress offsets adaptation associated with carbohydrate periodization in trained male triathletes

Purpose: Carbohydrate (CHO) intake periodization via the sleep low train low (SL-TL) diet-exercise model increases fat oxidation during exercise and may enhance endurance-training adaptation and performance. Conversely, training under environmental heat stress increases CHO oxidation, but the potential of combined SL-TL and heat stress to enhance metabolic and performance outcomes is unknown. Methods: Twenty-three endurance-trained males were randomly assigned to either control (n = 7, CON), SL-TL (n = 8, SLTemp) or SL-TL + heat stress (n = 8, SLHeat) groups and prescribed identical 2-week cycling training interventions. CON and SLTemp completed all sessions at 20 degrees C, but SLHeat at 35 degrees C. All groups consumed matched CHO intake (6 g center dot kg(-1)center dot day(-1)) but timed differently to promote low CHO availability overnight and during morning exercise in both SL groups. Submaximal substrate utilization was assessed (at 20 degrees C), and 30-min performance tests (at 20 and 35 degrees C) were performed Pre-, Post-, and 1-week post-intervention (Post+1). Results: SLTemp improved fat oxidation rates at 60% MAP (similar to 66% VO2peak) at Post+1 compared with CON (p < 0.01). Compared with SLTemp, fat oxidation rates were significantly lower in SLHeat at Post (p = 0.02) and Post+1 (p < 0.05). Compared with CON, performance was improved at Post in SLTemp in temperate conditions. Performance was not different between any groups or time points in hot conditions. Conclusion: SL-TL enhanced metabolic adaptation and performance compared with CON and combined SL-TL and heat stress. Additional environmental heat stress may impair positive adaptations associated with SL-TL.