Athlete burnout and biomarkers: An exploratory, longitudinal N-of-1 study

Burnout is an increasingly common problem among athletes. In addition to negatively affecting mental health, burnout may also be related to changes in physiological functioning. Research outside of sport suggests that the hypothalamus-pituitary-adrenal (HPA) axis, immune, anabolic, and cardiovascular systems, in particular, may be affected. However, few studies have explored the relationship between burnout and biomarkers of these systems in athletes. Consequently, the aim of the present multidisciplinary study was to explore the relationship between athlete burnout and acute and chronic biomarkers using a longitudinal N-of-1 design. Following a pre-registered protocol with open data, code, and materials, in two athletes, we examined burnout and acute salivary biomarkers (cortisol, testosterone, dehydroepiandrosterone-sulphate [DHEA-S], secretory Immunoglobulin A [sIgA], and C-reactive protein) in 12 samples over six months. In another two athletes, we examined burnout and chronic biomarkers from hair and blood (hair cortisol, glycated haemoglobin [HbA1c], triglycerides, total cholesterol, high-density lipoprotein cholesterol, and DNA methylation in the BDNF, SLC6A4, and NR3C1 genes) in six samples over 12 months. Dynamic regression modelling showed that burnout symptoms predicted decreased testosterone and developed simultaneously with decreases in DHEA-S and sIgA. Visual analyses suggested that burnout symptoms also developed in conjunction with increases in HbA1c and SLC6A4 methylation and preceded increases in hair cortisol and BDNF methylation. Our findings provide a preliminary "physiological fingerprint" that could help explain athlete burnout development and consequences which can be used to guide future theory and research in this area.