Heat acclimation does not alleviate the reduction in exogenous glucose oxidation that occurs when exercising in hot conditions

Exogenous glucose oxidation (CHOexo) is suppressed during endurance exercise performed in hot conditions, although the underlying mechanisms and effects of heat acclimation are unknown. The present study investigated whether heat acclimation alleviates the heat‐induced suppression of CHOexo and the underlying physiological mechanisms. Seventeen endurance‐trained adults (10 males and seven females) completed 90 min cycling trials at 45% W max with glucose ingestion (90 g·h −1 ) under temperate [20°C; 20% relative humidity (RH); COOL] and hot (40°C; 20% RH; HOT) conditions, and again in the heat (40°C; 20% RH; POST) following a heat acclimation protocol (9 days of isothermic exercise‐heat acclimation across a 10 day period). Dual stable isotope tracers ([U 13 C 6 ] glucose ingestion, [6,6‐ 2 H 2 ]glucose‐infusion), indirect calorimetry and muscle biopsies were used to assess CHOexo, glucose kinetics and muscle glycogen utilisation during exercise. CHOexo (mean [95% confidence limits]) was reduced by ∼27% in HOT compared to COOL (0.37 [0.31, 0.43] g·min −1 vs . 0.51 [0.45, 0.57] g·min −1 ; P < 0.001) and, despite clear thermoregulatory adaptation following heat acclimation, remained suppressed in POST (0.38 [0.31, 0.44] g·min −1 ; P < 0.01 vs . COOL). Rates of plasma glucose appearance, disappearance and exogenous glucose appearance were unaffected by environmental condition or heat acclimation. Glucose metabolic clearance rates and plasma glucose oxidation were reduced in HOT and POST compared to COOL ( P < 0.05). A decreased capacity for plasma glucose disposal and utilisation rather than limitations in splanchnic glucose appearance underpin the reduced exogenous glucose oxidation observed during exercise in the heat, and this is not alleviated by heat acclimation. image Key points Exogenous glucose oxidation is suppressed during endurance‐exercise performed in hot conditions, although the underlying mechanisms and effects of heat acclimation are unknown. A decreased capacity for plasma glucose disposal and utilisation rather than limitations in splanchnic glucose appearance appear to underpin the reduced exogenous glucose oxidation observed during exercise in the heat. Heat acclimation does not alleviate the reduction in exogenous glucose oxidation that occurs when exercising in hot conditions.