Faecal microbiome, gastrointestinal integrity, inflammation and thermoregulation in recent exertional heat illness patients and matched controls.

The gastrointestinal (GI) microbiota and GI barrier integrity are hypothesised to contribute to exertional heat illness (EHI) aetiology. We compared the faecal microbiome, GI barrier integrity, inflammation and thermoregulation of 29 recent (∼4 months) EHI patients (a group with elevated EHI risk) and 29 control individuals without prior EHI history, matched for variables influencing thermoregulation and GI microbiota. Participants completed an exercise heat tolerance assessment (HTA), with faecal microbiome assessed by 16S rRNA gene amplicon sequencing of stool samples and blood biomarkers of GI barrier integrity and inflammation measured pre- and post-HTA. With the exception of the Simpson index (patient = 0.97 ± 0.01 vs. control = 0.98 ± 0.00, P = 0.030), there were no between-groups differences in faecal microbiome composition (α-diversity, β-diversity, relative abundance, differential abundance), GI barrier integrity, inflammation or terminal thermoregulatory indices. Individuals were subsequently classified as heat tolerant (n = 46) or intolerant (n = 12) on the basis of the HTA. Heat intolerant individuals demonstrated lower sudomotor response (intolerant = 0.53 (0.17) vs. tolerant = 0.62 (0.20) L m^-2 h^-1, P = 0.011) despite greater thermoregulatory strain (e.g., terminal T_rec: intolerant = 39.20 ± 0.31 vs. tolerant = 38.80 ± 0.31°C, P < 0.001), lower Firmicutes:Bacteroidota ratio (intolerant = 3.7 (0.6) vs. tolerant = 4.5 (2.0), P = 0.019) and higher plasma [sCD14] (P = 0.014), but other aspects of faecal microbiome, GI integrity or inflammation did not differ from heat tolerant individuals. In conclusion, the faecal microbiome composition and the GI barrier integrity and inflammatory responses to exercise heat-stress showed limited differences between recent EHI patients and matched controls, or between individuals classified as heat intolerant or heat tolerant and are unlikely to explain elevated EHI risk in recent EHI patients, or heat intolerance.