Are Type 2 Diabetes-Related Impairments in Heat Dissipation Heat Load Dependent?

Abstract

Older adults with type 2 diabetes (T2D) have an attenuated physiological ability to dissipate heat during moderate intensity exercise in the heat. However, it is unclear if T2D-related impairments in heat dissipation only occur above a certain exercise-induced heat load and therefore level of heat stress. PURPOSE: To examine whether T2D-related impairments in whole-body heat loss, as assessed by direct calorimetry, occur above a certain heat load threshold. METHODS: Twelve older (60 ± 7 years) habitually active males with (n=6, hemoglobin A1c: 6.8 ± 0.6 %, duration of diabetes: 9 ± 5 years) and without (n=6) T2D (CON) matched for age, body surface area, and fitness (VO2peak) completed three successive 30-min bouts of semi-recumbent cycling performed at fixed incremental rates of metabolic heat production of 300 (Ex1), 400 (Ex2) and 500 (Ex3) W in the heat (40°C). This was equivalent to 37, 50 and 62% of their pre-determined VO2peak. A 15-min recovery period followed each exercise bout. Whole-body heat loss (evaporative and dry heat exchange) was measured using direct calorimetry. The simultaneous measurement of metabolic heat production via indirect calorimetry was used to calculate the change in body heat storage. RESULTS: Whole-body heat loss was reduced in the T2D group relative to CON at the end of Ex1 (T2D: 235 ± 23 W; CON: 261 ± 34 W, p=0.004), Ex2 (T2D: 294 ± 24 W; CON: 335 ± 44 W, p=0.030), and Ex3 (T2D: 330 ± 59 W; CON: 390 ± 39 W, p=0.02). Given that dry heat gain was similar between groups at the end of all exercise bouts (all p>0.05), differences in whole-body heat loss were only due to differences in evaporative heat loss. The relative difference in the maximal level of whole-body heat loss achieved between groups became greater with increasing exercise-induced heat loads (Differences: Ex1=10.5%, Ex2=14.0% and Ex3=20.5%). As such, the individuals with T2D stored more heat during all exercise bouts (Ex1: 197 ± 39 kJ; Ex2: 236 ± 67 kJ; Ex3; 334 ± 78 kJ, p<0.05) compared to the control group (Ex1: 150 ± 61 kJ; Ex2: 160 ± 74 kJ; Ex3; 248 ± 80 kJ). CONCLUSION: Our preliminary findings show that type 2 diabetes impairs the body’s ability to dissipate heat during exercise in the heat and that these differences are first evident at a moderate heat load of 300 W (equivalent to 37% of VO2peak). Supported by the Canadian Institutes of Health Research