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 ) or SL-TL + heat stress (n = 8, SL ) groups and prescribed identical 2-week cycling training interventions. CON and SL completed all sessions at 20°C, but SL at 35°C. All groups consumed matched CHO intake (6 g·kg ·day ) 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 improved fat oxidation rates at 60% MAP (~66% VO ) at Post+1 compared with CON (p