Julien Louis

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, SL Temp ) or SL‐TL + heat stress ( n = 8, SL Heat ) groups and prescribed identical 2‐week cycling training interventions. CON and SL Temp completed all sessions at 20°C, but SL Heat at 35°C. All groups consumed matched CHO intake (6 g·kg −1 ·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°C), and 30‐min performance tests (at 20 and 35°C) were performed Pre‐, Post‐, and 1‐week post‐intervention (Post+1). Results SL Temp improved fat oxidation rates at 60% MAP (~66% VO 2peak ) at Post+1 compared with CON ( p < 0.01). Compared with SL Temp , fat oxidation rates were significantly lower in SL Heat at Post ( p = 0.02) and Post+1 ( p < 0.05). Compared with CON, performance was improved at Post in SL Temp 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.