Does biological sex impact intestinal epithelial injury, small intestine permeability, gastrointestinal symptoms and systemic cytokine profile in response to exertional-heat stress?

This study aimed to determine the influence of biological sex on intestinal injury, permeability, gastrointestinal symptoms, and systemic cytokine profile in response to exertional-heat stress. Male (n= 13) and eumenorrheic female (n= 11) endurance runners completed 2 h running at 60% V̇O _2max in 35°C. Blood samples were collected pre- and post-exercise and during recovery to determine plasma intestinal fatty-acid binding protein (I-FABP) and systemic cytokine profile. Urinary lactulose:L-rhamnose ratio was used to determine small intestine permeability. I-FABP increased 479% pre- to post-exercise (p< 0.001), with no difference between sexes (p= 0.432). No differences between sexes were observed for small intestine permeability (p= 0.808), gut discomfort, total, upper- and lower-gastrointestinal symptoms. However, males reported significantly higher flatulence (p= 0.049) and abdominal stitch (p= 0.025) compared to females. IL-6, IL-8, IL-10 and IL-1ra increased pre- to post-exercise (p< 0.05), with no difference between sexes. However, IL-1β increased post-exercise in males only, and was higher in males compared to females (p= 0.044). Findings suggest that when females are in the follicular phase of the menstrual cycle, biological sex has no effect on intestinal epithelial injury and permeability, and minimal effect on gastrointestinal symptoms and systemic cytokine profile in response to exertional-heat stress.