Background Vitamin D has a role in skeletal muscle function and metabolism, however, its influence on muscle injury risk remains unclear.

Objective To evaluate the association between Vitamin D status and muscle injury risk.

Design Prospective cohort study.

Setting Elite male football team from Spanish LaLiga.

Participants 41 players were prospectively followed from 2016–2017 to 2018–2019.

Assessment of Risk Factors Injuries and exposure time were recorded by the team doctor following the FIFA consensus. Blood analyses were performed in 4 different season periods (July, October, January and May).

Main Outcome Measurements Serum 25-hydroxyvitamin D levels were compared between periods using Student’s T-test. The association of Vitamin D with 56 muscle injuries requiring 4 or more days of absence was investigated using a Cox-frailty model. The influence of days of absence due to all injuries on between-period changes in Vitamin D was assessed using linear mixed models.

Results Vitamin D levels were highest in July (mean±SD; 48.1±9.9 ng/ml, p<0.001 vs. other periods), and lowest in January (27.3±7.9 ng/ml, p<0.001). There were no differences between October (37.3±8.3 ng/ml) and May (34.5±10.7 ng/ml, p=0.89). There was no association between continuous Vitamin D levels and muscle injuries adjusting for season period [hazard ratio=1.01, 95% confidence interval (CI)=0.97–1.05, p=0.66]. Players in the lowest period-specific quartile, i.e. with the lowest vitamin D levels, had a 2.29 times lower risk of injury (95% CI=0.97–5.41, p=0.06, 1.30 muscle injuries/1000 h) compared with players in the middle (3.24/1000 h) and highest (2.87/1000 h) quartiles combined. Days of absence were negatively associated with changes in Vitamin D levels after adjusting for season period (B=-0.06, 95% CI=-0.13–0.01, p=0.06).

Conclusions The association of Vitamin D status with muscle injuries should be further explored as studies are contradictory. Seasonal variations and being injured should be considered when supplementing with Vitamin D.