This study aimed to determine the relationship between vitamin D status and upper respiratory tract infection (URTI) of physically active men and women across seasons and then to investigate the effects on URTI and mucosal immunity of achieving vitamin D sufficiency by a unique comparison of safe, simulated sunlight or oral D3 supplementation in winter. In cross-sectional analysis, only 21% of recruits were vitamin D sufficient during winter. Vitamin D–sufficient recruits were 40% less likely to suffer URTI than recruits with deficient vitamin D, an association that remained after accounting for sex and smoking. Each URTI caused, on average, three missed training days. Vitamin D supplementation strategies were similarly effective to achieve vitamin D sufficiency in almost all (≥95%). Compared with placebo, vitamin D supplementation reduced the severity of peak URTI symptoms by 15% and days with URTI by 36%. Vitamin D sufficiency reduced the URTI burden during military training.
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