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

OBJECTIVES

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

DESIGN

Randomized, crossover study under controlled conditions.

METHODS

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.

RESULTS

Tcore 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).

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