sIL-6R is Related to Weekly Training Mileage and Psychological Wellbeing in Athletes

Introduction: IL-6 has been ascribed both positive and negative roles in the context of exercise and training. The dichotomous nature of IL-6 signalling appears to be determined by the respective concentration of its receptors (both membrane-bound (IL-6R) and soluble (sIL-6R) forms). The purpose of the present study was to investigate the response of sIL-6R to long-term training, and to investigate the relationship between sIL-6R, self-reported measures of wellbeing, and upper respiratory illness symptoms (URS) in highly-trained endurance athletes. Methods: Twenty-nine athletes provided resting blood samples, and completed wellbeing and illness monitoring questionnaires, on a weekly basis for a period of 18 weeks during a winter training block. Results: URS were not correlated to concentrations of sIL-6R or cortisol, but there was a non-significant trend (P=0.08) for the most illness-prone athletes (as defined by self-reported illness questionnaire data) to exhibit higher average sIL-6R concentrations compared to the least ill (23.7±4.3 Vs 20.1±3.8 ng/ml). Concentrations of sIL-6R were positively correlated to subjective measures of stress (r=0.64, P=0.004) and mood (r=0.49, P=0.02), but were negatively correlated to sleep quality (r=-0.43, P=0.05) and cortisol concentration (r=-0.17, P=0.04). In a sub-group of 10 athletes, weekly training distance was quantified by coaching staff, and this negatively correlated with sIL-6R in the following week (r=-0.74, P<0.005). Conclusion: The findings of the current study suggest that sIL-6R is responsive to prolonged periods of exercise training, with sIL-6R levels varying related to the volume of training performed in the preceding week. Importantly, our data indicate that changes in sIL-6R levels could be linked to common symptoms of overreaching such as high levels of stress, and/or depressed mood.