Association between haemoglobin A_1c and whole-body heat loss during exercise-heat stress in physically active men with type 2 diabetes

New FindingsWhat is the central question of this study?Is the impairment in heat dissipation during exercise observed in men with type 2 diabetes related to glycaemic control (indexed by glycated haemoglobin; haemoglobin A(1c))?What is the main finding and its importance?No association was found between haemoglobin A(1c) (range: 5.1-9.1%) and whole-body heat loss in men with type 2 diabetes during exercise in the heat. However, individuals with elevated haemoglobin A(1c) exhibited higher body core temperature and heart rate responses. Thus, while haemoglobin A(1c) is not associated with heat loss per se, it may still have important implications for physiological strain during exercise. Type 2 diabetes is associated with a reduced capacity to dissipate heat. It is unknown whether this impairment is related to glycaemic control (indexed by glycated haemoglobin; haemoglobin A(1c)) is unknown. We evaluated the association between haemoglobin A(1c) and whole-body heat loss (via direct calorimetry), body core temperature, and heart rate in 26 physically active men with type 2 diabetes (43-73 years; HbA(1c) 5.1-9.1%) during exercise at increasing rates of metabolic heat production (similar to 150, 200, 250 W m(-2)) in the heat (40 degrees C, similar to 17% relative humidity). Haemoglobin A(1c) was not associated with whole-body heat loss (P = 0.617), nor the increase in core temperature from pre-exercise (P = 0.347). However, absolute core temperature and heart rate were elevated similar to 0.2 degrees C (P = 0.014) and similar to 6 beats min(-1) (P = 0.049), respectively, with every percentage point increase in haemoglobin A(1c). Thus, while haemoglobin A(1c) does not appear to modify diabetes-related reductions in capacity for heat dissipation, it may still have important implications for physiological strain during exercise-heat stress.