LAUSR

Hyperthermia and exertional heat illness increase gastrointestinal (GI) permeability, although whether the latter is only via hyperthermia is unclear. The aim of this pilot study was to determine whether different changes in GI permeability, characterized by an increased plasma lactulose:rhamnose concentration ratio ([L:R]), occurred in exercise hyperthermia in comparison to equivalent passive hyperthermia. Six healthy adult male participants (age 25 ± 5 years, mass 77.0 ± 6.7 kg, height 181 ± 6 cm, peak oxygen uptake [ V·O2peak ] 48 ± 8 ml.kg−1.min−1) underwent exercise under hot conditions (Ex‐Heat) and passive heating during hot water immersion (HWI). Heart rate (HR), rectal temperature (TCORE), rating of perceived exertion (RPE), and whole‐body sweat loss (WBSL) were recorded throughout the trials. The L:R ratio, peak HR, change in HR, and change in RPE were higher in Ex‐Heat than HWI, despite no differences in trial duration, peak core temperature or WBSL. L:R was strongly correlated (p < 0.05) with HR peak (r = 0.626) and change in HR (r = 0.615) but no other variable. The greater L:R in Ex‐Heat, despite equal TCORE responses to HWI, indicates that increased cardiovascular strain occurred during exercise, and exacerbates hyperthermia‐induced GI permeability at the same absolute temperature.