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

To examine the effects of a water-resistant sunscreen on body mass loss, skin temperature (T_skin), and core temperature (T_core) during prolonged exercise in the heat.

Randomized, crossover study under controlled conditions.

Fifteen physically active university students completed two 60-minute cycling sessions at 50% VO₂peak in a climate chamber (35.1 ± 0.1 °C dry bulb temperature and 54.7 ± 0.1% relative humidity). Participants applied a water-resistant sun protection factor 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. Body mass loss was calculated from pre-to-post exercise nude body mass.

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). Tskin rose during exercise, reaching approximately ~35.7-35.9°C, with no difference between conditions (all p > 0.15). Body mass loss 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).

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. These findings may inform athlete guidance and heat management strategies.