2.1. Participants
Ninety athletes between 18 and 50 years of age who exercised at least five hours per week were included. All participants lived in Germany (48–55° N) for the entire duration of this study. To avoid endogenous vitamin D synthesis, participants were instructed not to spend time in a region with a UV index greater than 3. Additionally, participants already supplemented with vitamin D2 or D3 were excluded from the trial.All procedures followed in this study involving human participants were in line with the ethical standards of the institutional research committee and with the 1964 Helsinki Declaration and its subsequent amendments. Prior to this study, ethical approval was obtained from the local ethics committee of the German Sport University Cologne (#232/2022) and written consent was obtained from all participants.