Long-term Effects Of Repeated Episodic Heat Exposure On Physical Function In Aged, Obese Mice

Background: Heat therapy (HT) is a practical, non-invasive approach that has been demonstrated to improve cardiovascular and skeletal muscle health in patients with overt cardiovascular disease and endocrine disorders. PURPOSE: The aim of this study was to test the hypothesis that long-term, repeated exposure to heat stress improves body composition, muscle strength, exercise tolerance, and blood pressure in a mouse model of aging and diet-induced obesity. METHODS: Male C57BL/6 J mice (52 weeks of age) were fed a high-fat diet and were randomly allocated to receive a heat treatment (HT; 39 °C; n = 6) or a sham intervention (SHAM; 22 °C; n = 8) using a laboratory incubator. The intervention spanned 26 weeks and comprised an initial program of 30-minute sessions twice a week for 13 consecutive weeks, followed by 20-minute sessions thrice a week for another 13 weeks. Body composition, forelimb grip strength, blood pressure, and treadmill exercise tolerance were assessed at baseline and at the end of the intervention. RESULTS: Repeated episodic heat exposure did not alter body composition or physical function in aged, obese mice. There were no significant differences between the HT and the SHAM groups in the change from baseline to the 26-week assessment in body mass (SHAM:21.9 ± 2.7 vs HT:22.7 ± 1.6 g, p = 0.54), lean mass (SHAM:2.9 ± 2.2 vs HT:1.9 ± 1.8 g, p = 0.41), and fat mass (SHAM:17.0 ± 2.4 vs HT:19.0 ± 3.1 g, p = 0.20). In addition, the changes in absolute forelimb grip strength (SHAM:0.08 ± 0.1 vs HT:0.07 ± 0.1 N, p = 0.86), grip strength normalized to body mass (SHAM:-0.02 ± 0.002 vs HT:-0.02 ± 0.004 N/g, p = 0.62), and maximal distance on the treadmill test (SHAM:-309.7 ± 37.4 vs HT:-278.5 ± 94.9 m, p = 0.40) were comparable between groups. However, the change from baseline in systolic (SHAM:4.36 ± 22.3 vs HT: 23.7 ± 19.6 mmHg, p = 0.004), diastolic (SHAM:-4.2 ± 11.1 vs HT:14.0 ± 6.6 mmHg, p = 0.004), and mean arterial (SHAM:-1.2 ± 12.6 vs HT:17.2 ± 10.5 mmHg, p = 0.01) blood pressures were greater in the HT group when compared to the SHAM group. CONCLUSIONS: Our findings indicate that repeated whole-body HT for 26 weeks does not alter body composition or physical function and may increase blood pressure in aged, obese mice.