Does wearing a breathable T-shirt improve the microclimate and thermoregulatory responses during a hot outdoor running?

Clothing hinders heat and moisture transfer from the skin to the environment, potentially disrupting thermoregulation during exercise in hot conditions. We therefore sought to determine whether wearing a newly developed T-shirt, designed to enhance breathability when dampened with sweat, would improve the clothing microclimate and thermoregulatory responses during a hot outdoor running. Eleven male distance runners were evaluated over two separate days, each involving two consecutive 30-min outdoor running sessions at a target heart rate of 120–130 bpm. A single-blind randomized crossover design was employed wherein participants wore a conventional polyester T-shirt (CON) during the first session on both days but changed into either the new fabric T-shirt (Dry Aeroflow [DAF]) or another CON for the second session. To account for uncontrolled outdoor conditions, responses during the first sessions of both days were compared, confirming no significant day-to-day differences in most variables. In the second session, wearing DAF reduced the intra-clothing humidity on the chest (p = 0.004) and back (p = 0.048) by 4.3% and decreased the upper arm skin temperature by 0.3 °C (p = 0.033) compared to wearing the CON. However, no differences in gastrointestinal temperature, mean skin temperature, or whole-body sweat rate were observed between the T-shirts (all p ≥ 0.178). These findings suggest that although the functional T-shirt improved the clothing microclimate and decreased the local skin temperature, these effects were likely marginal and did not significantly affect systemic thermoregulatory responses (e.g., core temperature, sweat rate) during outdoor running in a hot environment.