Differential intestinal injury and unchanged systemic inflammatory responses to leg and whole‐body passive hyperthermia in healthy humans

Hyperthermia can cause intestinal injury, facilitating endotoxin translocation and an inflammatory response that has been associated with heat illness. However, the potential occurrence of these responses has been incompletely reported during passive hyperthermia, and the independent effect of hyperthermia is equivocal. Furthermore, passive hyperthermia is a feature of heat therapy interventions, with mechanistic understanding developing. This experiment quantified the changes in intestinal fatty acid binding protein (iFABP), a marker of intestinal injury, and cytokine, chemokine and growth factor responses during three different prolonged passive hyperthermia protocols. Eight healthy males visited the laboratory on four counterbalanced occasions to undertake 2.5 h of rest (CON), one-leg heating (OLH), two-leg heating (TLH) and whole-body heating (WBH) via a garment circulating water at 50°C. Plasma concentrations of iFABP and 38 cytokines, chemokines and growth factors were quantified periodically, and core temperature (T_core) was measured continuously. The T_core increased from baseline in OLH, TLH and WBH (+0.4°C ± 0.2°C, +0.7°C ± 0.2°C and +2.3°C ± 0.4°C, respectively; P < 0.05) but remained unchanged in CON. iFABP increased from baseline in WBH only (∆587 ± 651 pg ml^-1) and was different from CON and OLH in WBH after 2 h (P < 0.05). Increased iFABP (∆1085 ± 572 pg ml^-1) was observed in 50% of participants at the end of WBH, with the other 50% demonstrating no change (∆89 ± 19 pg ml^-1). All chemokines, cytokines and growth factors were unchanged in all protocols. These data indicate that passive whole-body hyperthermia, but not lower-limb hyperthermia, can cause intestinal injury in some individuals without a systemic inflammatory response.