During a long-distance running race, we observed a significant IL-6 plasma concentration increase significantly associated with the running race intensity. IL-6 increase during a long running race occurs during the early stage of the race and remains stable after intensity reduction related to speed race running decrease.
To the best our knowledge, this study is the first to assess the evolution of IL-6 plasma concentration indexed by exercise intensity.
Interleukin-6 (IL-6) is one of the most studied cytokine (15), is an inflammatory cytokine propagating the systemic inflammatory response after released from the skeletal muscle cells into the blood in order to related to meet energy deprivation (12). Previous studies reported that after an early concentration increase (4, 13), IL-6 plasma concentration reaches a peak at the end of the exercise (4) before returning to pre-exercise plasma concentration within the first 24 hours (14). Nevertheless, the relationship between IL-6 plasma concentration and intensity during the exercise intensity was never explored because of methodological approaches - previous studies design assessed pre- and post-race IL-6 plasma concentration (16–19).
Exercise duration (15, 20–22) and intensity (21, 23) are the main factors determining IL-6 plasma concentration. Because during a long-distance running race, the running speed progressively decrease due to tiredness, the exercise intensity also decreases suggesting that the relationship between exercise intensity and IL-6 plasma concentration is not linear. We could suppose that the reduction of the speed race running mainly explain the negative relationship between IL-6 and intensity. Moreover, this assumption is supported by the constant muscle mass involved for running explaining that IL-6 plasma concentration also more influenced by the speed of the race than by the race duration (24–26). Because individual innate and acquired characteristics influence IL-6 plasma concentration at rest as well as in response to exercise (15, 27–30), we choose to assess the variation between section, each volunteer being considered as its own control. Moreover, because the length of the race, IL-6 plasma concentration was indexed on the corresponding speed for each section.
Our results are consistent with previous studies reporting the IL-6 plasma concentration increase with exercise duration (15, 31, 32) and return to baseline after the end of the exercise (13, 14, 33). For running, the ability to maintain speed depends on the muscle mass of the limbs involved (34, 35). The relatively lower values of plasma IL-6 concentration observed in our study compared to previous studies could be explained by the fact that for long distance running, a smaller muscle mass seems to be more efficient, in contrast to short distance running requiring higher explosiveness (24–26).
There are strengths and limitations with the current study. This is a small sample size study. Food and drink intakes were not standardised between all volunteers and may partly contribute to the IL-6 plasma concentration. Only male volunteers were involved in the study; thus, we cannot extrapolate the results to the female gender. We cannot exclude that the participants involved in the current study do not have an “extra-normal” level of the underlying inflammatory state and/or a moderate response (28), related to their frequent practice of long-distance exercises suggesting a potential adaptation of the organism to the intensity of long-term running practice. We cannot exclude that participants had unknown illnesses, or injury influencing baseline of IL-6 plasma concentration and/or modulating the host response. Due to the study design, we are not able to determine whether the participants reached their maximal performances. The methodological design does not allow any causal conclusion between long-distance race exercise and IL-6 increase. This study reports an IL-6 plasma concentration increase without being able to determine its origin(s): blood and/or muscle and/or neuronal and/or adipocyte cells. In addition we defined intensity by speed * race duration; nevertheless, intensity definition is not consensual (20, 21, 36).
Despite these limitations, this study is the first to assess the IL-6 plasma concentration and its variation indexed on intensity during a long-distance race observing a peak on IL-6 increase not reported after intensity indexation. The early IL-6 plasma concentration peak may be related to speed and we can suppose that the intensity decrease reflected by speed race decrease may be related to a decrease in the sympathetic reserve, associated with a progressive heart rate decrease, and a speed decrease, which, in long-distance race, tending towards a limit value, depending on the maximum catabolic rate of the organism (37).