Comparison between exercise and hot water immersion on skin microvascular responses in older adults with overweight and obesity.

PURPOSE: Skin microvascular function plays a critical role in thermoregulation and vascular health, but due to technical limitations, responses to passive heat stress remain poorly quantified, particularly in overweight and obese older adults. This study aimed to compare the acute effects of hot water immersion (HWI) and moderate-intensity exercise (EXE) on skin microvascular function using optical coherence tomography (OCT), and to explore potential sex differences. METHODS: Twenty-five overweight older adults (10♂ and 15♀; age 59 ± 7, BMI 29.4 ± 3.4 kg/m2) underwent two experimental sessions in randomized crossover design: 30 min of aerobic exercise (EXE) on a cycle ergometer at 60-70%HRmax and seated hot water immersion (HWI) at 40 °C, immersion to the xiphoid process and arms submerged. Forearm skin microvascular responses were assessed pre- and post-intervention utilising OCT. RESULTS: HWI elicited significantly greater increases in all OCT metrics than EXE (all P < 0.001): diameter (+ 5.2 ± 4.3 vs. - 1.1 ± 3.7 μm), velocity (+ 22.8 ± 11.7 vs. - 1.2 ± 4.5 μm·s⁻¹), flow (+ 68.9 ± 29.8 vs. - 8.3 ± 13.8 pL·s⁻¹), and vessel density (14.1 ± 7.8 vs. - 3.1 ± 4.8%). These vascular changes were accompanied by a higher Tc increase (+ 0.9 ± 0.3 °C vs. +0.7 ± 0.3 °C) and larger reductions in systolic and diastolic BP following HWI (P < 0.01). Qualitatively, laser Doppler flowmetry paralleled the OCT findings, showing enhanced skin blood flow after HWI (P < 0.01). CONCLUSIONS: When time-matched, an acute bout of HWI induces changes in skin microvascular perfusion that exceed those observed in response to EXE, likely reflecting the combined influence of direct local skin heating and greater systemic thermal strain, supporting its potential as a passive vascular stimulus in populations with limited exercise tolerance.