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Studies monitoring vitamin D status in athletes are seldom conducted for a period of 12 months or longer, thereby lacking insight into seasonal fluctuations. The objective of the current study was to identify seasonal changes in total 25-hydroxyvitamin D (25(OH)D) concentration throughout the year. Fifty-two, mainly Caucasian athletes with a sufficient 25(OH)D concentration (>75 nmol/L) in June were included in this study. Serum 25(OH)D concentration was measured every three months (June, September, December, March, June). In addition, vitamin D intake and sun exposure were assessed by questionnaires at the same time points. Highest total 25(OH)D concentrations were found at the end of summer (113 ± 26 nmol/L), whereas lowest concentrations were observed at the end of winter (78 ± 30 nmol/L). Although all athletes had a sufficient 25(OH)D concentration at the start of the study, nearly 20% of the athletes were deficient (<50 nmol/L) in late winter.

Backx, van der Avoort, Tieland, de Groot, and Mensink are with the Dept. of Human Nutrition, Wageningen University, Wageningen, The Netherlands. Maase is with the Netherlands Olympic Committee Netherlands Sports Confederation (NOC*NSF), Arnhem, The Netherlands. Kies is with the DSM Biotechnology Center, Delft, The Netherlands. van Loon is with the NUTRIM School of Nutrition and Translational Research in Metabolism, Maastricht, The Netherlands.

Address author correspondence to Marco Mensink at marco.mensink@wur.nl.