Relationship between Exercise Intensity and IL-6 Increase during an 80 km Long-Distance Running Race

Background: IL-6 plasma concentration (IL-6PC) reflects the systemic inflammation related to exercise intensity level. This study aims to describe the IL-6PC kinetics during a long-distance running race. IL-6PC was measured in 20 male runners before (0 km), at each refreshment point (at 21 and 53 km, i.e., k(21) and k(53), respectively) and at the end of an 80 km long-distance run (k(80)). Methods: IL-6PC variations (absolute and relative values in each of the three sections (S)) were calculated over S1 (0_k(21)), S2 (k(21)_k(53)) and S3 (k(53)_k(80)) and compared with the exercise intensity (duration*race speed) within each section. Results: The mean IL-6PC increased during the run: 2.1 ± 0.6 ng.L(−1) at 0 km, 21.0 ± 11.3 ng.L(−1) at k(21), 38.9 ± 13.0 ng.L(−1) at k(53) and 49.8 ± 11.9 ng.L(−1) at k(80). Exercise intensity increased between S1 (24.2 ± 0.5) and S2 (51.9 ± 3.2) (p = 0.04) but not between S2 and S3 (67.4 ± 4.5) (p = 0.69). IL-6PC variation was associated with exercise intensity within S1 (p = 0.03) and S2 (p = 2 × 10(−3)) and showed at least a trend within S3 (p = 0.06). Conclusions: IL-6PC increases that occur during the early stages of a long-distance run are associated with the running intensity, and then IL-6PC remain stable after the reduction in intensity related to the decrease in running speed.