Attenuated Exercise‐heat Tolerance in Type 2 Diabetes and Hypertension

Abstract

Older adults with type 2 diabetes (T2D) or hypertension (HTN) are particularly vulnerable to the adverse health effects of heat stress during strenuous exercise or work. This may be partly due to impairments in cutaneous vasodilation and/or sweating, which can result in augmented thermal and cardiovascular strain during short periods of moderate-to-vigorous exercise in the heat. However, it remains unknown if these alterations reduce tolerance to prolonged exercise-heat stress. The purpose of this study was therefore to assess tolerance to prolonged, moderate-intensity exercise in a cool and hot environment in T2D and HTN. We hypothesized that the reduction in exercise duration in hot compared to cool conditions would be higher in participants with T2D and HTN than healthy controls. Thirty-seven habitually active, older men aged 50-70 years participated (Control: n=14; T2D: n=10; HTN: n=13). Only participants with T2D (HbA1c: 5.5-9.0%) or HTN who were diagnosed ≥5 years prior were included. On separate days, participants walked in a cool (21.9°C) and hot (41.4°C) environment (both 35% relative humidity, <0.3 m/sec air flow) at a metabolic rate of 200 W/m2 (~3.5 METs) while wearing standardized clothing (cotton shirt and pants). Participants were encouraged to walk for 180 min, although trials were terminated prematurely upon volitional fatigue or if rectal temperature exceeded 39.5°C. The heat-induced reduction in exercise tolerance was derived as the percent reduction in exercise duration from the cool to hot environment and compared between Control vs. T2D and Control vs. HTN using Mann-Whitney U tests. Rectal temperature and heart rate, averaged over the final 5 min of exercise, were analyzed using a linear mixed model accounting for each participant's exercise time as a covariate. The heat-induced reduction in exercise tolerance was higher in T2D (median [IQR]; -40% [-49, -5]; p=0.036) and HTN (-33% [-38, -6]; p=0.043) compared to Controls (0% [-27, 0]). However, end-exercise rectal temperature and heart rate did not differ significantly across groups (all p≥0.135), averaging (SD) 37.7°C (0.3) and 104 (17) beats/min in cool conditions, and 38.8°C (0.5) and 140 (18) beats/min in hot conditions, across groups. We show that tolerance to prolonged, moderate-intensity exercise in the heat is attenuated in older men with T2D and HTN, likely due to a lowered tolerance to elevated thermal and/or cardiovascular strain. Although there is a need for larger confirmatory studies, these decrements may represent important considerations for exercise and occupational heat stress guidelines.