BACKGROUND: Several preliminary studies reported different adverse health conditions in athletes who train heavily for many years, yet no human trial so far evaluated possible connections with exercise-induced mitochondrial dysfunction (MD). METHODS: Cross-sectional study. We evaluated the prevalence of participant-reported indicators of chronic fatigue, and serum levels of fibroblast growth factor 21 (FGF21), a surrogate marker of MD, in 170 professional athletes (141 men and 29 women; age 22.6±3.4 years) who exercised for at least 10 hours per week during the past 5 or more years. RESULTS: Average weekly exercise volume was 23.0±5.0 hours (95% CI: 22.2-23.9 hours), with chronic fatigue appeared in approximately one in four cases (24.1%; 95% CI: 16.8-33.3%). Relative risk for chronic fatigue was 1.69 (95% CI: 0.65-4.41) in athletes who regularly exercise over 20 hours per week, as compared to less active counterparts. A correlation statistically very important has been found for serum FGF21 and weekly training load (r=-0.26; P=0.002). CONCLUSIONS: It appears that heavier training loads were accompanied by higher prevalence of chronic fatigue and lower circulating FGF21 levels, perhaps putting forward this novel biomarker as a proxy for exercise-induced MD.
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