SEX-BASED VARIATIONS IN LOCAL SWEAT RESPONSE TO SUNSCREENS DURING WARM WEATHER EXERCISE

Thermoregulation through sweating is vital during physical activity to maintain thermal homeostasis and prevent hyperthermia. PURPOSE: This study aimed to compare the effects of three sunscreens on localized sweat rate (LSR) during moderate exercise in a warm and humid environment. METHODS: Fifteen physically active subjects (8 males, 7 females) underwent a randomized crossover trial evaluating 4 conditions: control (no sunscreen), sunscreen A (organic), and sunscreens B and C (inorganic). LSR was quantified by sweat patches on the scapular region during 20 min of cycling at 65-75% max HR in 32 °C and 55% RH. RESULTS: The control exhibited significantly higher LSR (182 μL/min·dm2) versus sunscreen A (142 μL/min·dm2), B (158 μL/min·dm2), and C (159 μL/min·dm2) overall (p < 0.05). In males, sunscreen A reduced LSR compared to control, B, and C (p < 0.05). In females, control was superior to all sunscreens for LSR (p < 0.001), with no differences between sunscreen conditions (p > 0.05). Sunscreens inhibited LSR during exercise in warm and humid environment relative to control, especially sunscreen A in males and all sunscreens in females. The organic sunscreen A uniquely contained the silicone dimethicone alongside other potentially occlusive agents absent in sunscreens B and C. CONCLUSION: These results suggest that certain supplementary sunscreen ingredients like silicones may induce a dermal occlusive effect that impairs effective sweating. Although localized sweat rate was similar between sexes without sunscreen, sunscreen use significantly reduced sweating in females more than males. This suggests physiological sex differences modulate the effect of some sunscreens on thermoregulatory sweating during exercise in heat. Applying sunscreen is essential during outdoor exercise, but some products with high concentrations of occlusive ingredients can interfere with the body's ability to cool itself by sweating. More research is needed to understand how different sunscreen formulas affect body temperature regulation when exercising in heat. This will help choose sunscreens that protect skin while allowing sweating and staying cool during outdoor activities in warm conditions.