6 weeks of fartlek-style endurance training does not alter physiological responses to mild cold exposure but attenuates the cold-induced plasma noradrenaline response: A randomized, matched control trial.

Background and aims: Rodent studies suggest that endurance training may promote the browning of white adipose tissue and enhances adipose tissue thermogenesis during cold exposure. However, it remains unclear whether endurance training leads to similar adaptations in humans. The aim of this study was therefore to find out whether six weeks of intensive, fartlek-style cycle endurance training alters responses to cold exposure, specifically cold-induced changes of oxygen uptake, plasma noradrenaline concentrations and the effect of conditioned serum on markers of adipocyte browning in culture. Methods and results: Twenty-four healthy, untrained participants completed a six-week fartlek-style cycling intervention (3 sessions/week) or control period. We estimated cold-induced adaptive thermogenesis before and after the intervention by indirect calorimetry during 60 min of mild cold exposure at 18 °C. Endurance training increased on average the V˙O₂max by 5 ml/kg/min (95% CI 1 to 8; Conclusion: Six weeks of fartlek-style endurance training increased cardiorespiratory and metabolic fitness but had little effect on the response to cold exposure with no evidence for adipose tissue browning based on the conducted serum incubation experiment.