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

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.

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).

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.

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.