“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 concentration throughout the year.”

“52 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).”

“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.”

Evelien Backx, et al
Seasonal Variation in Vitamin D Status in Elite Athletes: A Longitudinal Study
International Journal of Sport Nutrition and Exercise Metabolism — Volume 27 #1 — February 2017 — page 6