Water-resistant sunscreen use may increase core body temperature during prolonged exercise in the heat

Objective To examine the effects of a water-resistant sunscreen on body mass loss (BML), skin temperature (T skin ), and core temperature (T core ) during prolonged exercise in the heat. Design Randomized, crossover study under controlled conditions. Methods Fifteen physically active university students completed two 60-min cycling sessions at 50% VO₂peak in a climate chamber (35.1 ± 0.1 °C dry bulb temperature and 54.7 ± 0.1% RH). Participants applied a water-resistant SPF 50 or no sunscreen. Core temperature was monitored continuously via an ingestible capsule; skin temperature, heart rate, and perceptions of heat and thirst were compared between the two conditions via linear mixed models. BML was calculated from pre-to-post exercise nude body mass. Results T core was higher with sunscreen at 50 min (38.57 vs 38.38 °C; Δ = +0.19 °C; p = 0.012) and at 60 min (38.69 vs 38.48 °C; Δ = +0.21 ± 0.07 °C; p = 0.006). T skin rose during exercise, reaching approximately ~35.7–35.9 °C, with no difference between conditions (all p > 0.15). BML did not differ between conditions (1.07 ± 0.32 vs. 1.08 ± 0.33 kg; p = 0.818). Thirst was higher with sunscreen (mean difference = 1.20 points; p = 0.017). Conclusions During 60 min of cycling, water-resistant sunscreen slightly increased core temperature (~0.2 °C) and thirst, with no changes in body mass loss or skin temperature. Although evaporative cooling efficiency was not directly measured, these results suggest that water-resistant formulations may reduce evaporative cooling efficiency and increase thermal strain in hot environments.