ObjectiveTo examine the effects of a water-resistant sunscreen on body mass loss (BML), skin temperature (Tskin), and core temperature (Tcore) during prolonged exercise in the heat. DesignRandomized, crossover study under controlled conditions. MethodsFifteen physically active university students completed two 60-minute cycling sessions at 50% VOpeak in a climate chamber (35.1 0.1 C dry bulb temperature and 54.7 0.1% RH).Participants applied a waterresistant 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. ResultsTcore was higher with sunscreen at 50 minutes (38.57vs 38.38 C; = +0.19C; p = 0.012) and at 60 minutes (38.69 vs 38.48 C; = +0.21 0.07 C; p = 0.006).Tskin rose during exercise, reaching approximately ~35.7-35.9C, 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). ConclusionsDuring 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.
Garzón-Mosquera et al. (Wed,) studied this question.