Can the supplementation of vitamin D, sun exposure, and isolation during the COVID-19 pandemic affect the seasonal concentration of 25(OH)D and selected blood parameters among young soccer players in a one-year training season?

Joanna Jastrzębskaa Diabetology and Endocrinology, Department of Pediatrics, Gdansk Medical University, Department of Pediatrics, Gdansk, PolandView further author information

Maria Skalskaa Diabetology and Endocrinology, Department of Pediatrics, Gdansk Medical University, Department of Pediatrics, Gdansk, PolandView further author information

Łukasz Radzimińskib Department of Health and Natural Sciences, Gdansk University of Physical Education and Sport, Department of Health and Natural Sciences, Gdansk, PolandView further author information

Guillermo F. López Sánchezc Division of Preventive Medicine and Public Health, Department of Public Health Sciences, School of Medicine, University of Murcia, Division of Preventive Medicine and Public Health, Department of Public Health Sciences, School of Medicine, Murcia, SpainView further author information

Lee Hilld Department of Pediatrics, Division of Gastroenterology and Nutrition, McMaster University, Department of Pediatrics, Division of Gastroenterology and Nutrition, Hamilton, ON, CanadaView further author information

Katja Weisse Institute of Primary Care, University of Zurich, Institute of Primary Care, Zurich, SwitzerlandView further author information

Beat Knechtlee Institute of Primary Care, University of Zurich, Institute of Primary Care, Zurich, Switzerland;f Medbase St. Gallen Am Vadianplatz, St. Gallen, SwitzerlandCorrespondence[email protected]