Exercise and Heat Stress: Inflammation and the Iron Regulatory Response

This study determined the impact of heat stress on postexercise inflammation and hepcidin levels. Twelve moderately trained males completed three, 60-min treadmill running sessions under different conditions: (a) COOL, 18 °C with speed maintained at 80% maximum heart rate; (b) HOT HR , 35 °C with speed maintained at 80% maximum heart rate; and (c) HOT PACE , 35 °C completed at the average running speed from the COOL trial. Venous blood samples were collected pre-, post-, and 3-hr postexercise and analyzed for serum ferritin, interleukin-6 (IL-6), and hepcidin concentrations. Average HR was highest during HOT PACE compared with HOT HR and COOL ( p < .001). Running speed was slowest in HOT HR compared with COOL and HOT PACE ( p < .001). The postexercise increase in IL-6 was greatest during HOT PACE (295%; p = .003). No differences in the IL-6 response immediately postexercise between COOL (115%) and HOT HR (116%) were evident ( p = .992). No differences in hepcidin concentrations between the three trials were evident at 3 hr postexercise ( p = .407). Findings from this study suggest the IL-6 response to exercise is greatest in hot compared with cool conditions when the absolute running speed was matched. No differences in IL-6 between hot and cool conditions were evident when HR was matched, suggesting the increased physiological strain induced from training at higher intensities in hot environments, rather than the heat per se, is likely responsible for this elevated response. Environmental temperature had no impact on hepcidin levels, indicating that exercising in hot conditions is unlikely to further impact transient alterations in iron regulation, beyond that expected in temperate conditions.