Influence of Vitamin D Supplementation by Simulated Sunlight or Oral D3 on Respiratory Infection During Military Training

Sophie E. Harrison; Samuel J. Oliver; Daniel S. Kashi; Alexander T. Carswell; Jason P. Edwards; Laurel M. Wentz; Ross Roberts; Jonathan C. Y. Tang; Rachel M. Izard; Sarah Jackson; Donald Allan; Lesley E. Rhodes; William D. Fraser; Julie P. Greeves; Neil P. Walsh


Med Sci Sports Exerc. 2021;53(7):1505-1516. 

In This Article


Study 1

Low Proportion of Wintertime Vitamin D Sufficiency in Healthy Young Men and Women. Baseline serum 25(OH)D concentration was lower in winter than all other seasons (P < 0.01; Figure 2A), when only 21% of participants were vitamin D sufficient (baseline serum 25(OH)D ≥50 nmol·L−1; Figure 2B).

Figure 2.

Seasonal variation in serum 25(OH)D (A), vitamin D sufficiency prevalence (serum 25(OH)D ≥50 nmol·L−1; B), and the URTI prevalence when serum 25(OH)D ≥50 nmol·L−1 or <50 nmol·L−1 (C) in 1644 men and women during 12 wk of military training. a Lower than summer, P < 0.05. b Lower than autumn, P < 0.05. c Lower than spring, P < 0.05. *Lower than participants with serum 25(OH)D <50 nmol·L−1, P < 0.05. Panel A data are presented as mean ± SD. Panels B and C are percentages represented by vertical bars.

Vitamin D Sufficiency Associated With Reduced URTI Prevalence. A total of 110 URTI episodes were recorded with 7% of participants having at least one physician-diagnosed URTI. On average, each URTI resulted in 3.4 ± 3.3 lost training days (4% of total training days). Vitamin D–sufficient participants at baseline were 40% less likely to have a physician-diagnosed URTI, during 12 wk of training, than participants with baseline serum 25(OH)D <50 nmol·L−1 (6% vs 9%, respectively, OR = 0.6, 95% confidence interval = 0.4–0.9, P < 0.05; Figure 2C). Vitamin D–sufficient participants at baseline were half as likely to have a URTI within the first 3 wk of training than participants with a baseline serum 25(OH)D <50 nmol·L−1 (2% vs 5%, OR = 0.5, 95% confidence interval = 0.3–0.8, P < 0.05); approximately half of all URTI episodes occurred during this period of training (47%, 52 URTI episodes). The association between vitamin D status and URTI prevalence remained when controlling for sex and smoking (P < 0.05). URTI prevalence was not different between participants with a baseline serum 25(OH)D ≥75 nmol·L−1 and baseline serum 25(OH)D of <30, ≥50–<75, or <75 nmol·L−1 (P > 0.05).

Study 2

A flow diagram detailing the number of participants assessed, recruited, and excluded from the analysis is provided in Figure 3. There were no differences between treatment or control groups in demographics, anthropometrics, or serum total 25(OH)D at baseline (Table 1 and Figure 4). During the 12-wk intervention, daily sunlight exposure (0.35 ± 0.56 SED·d−1 and dietary vitamin D were not different between groups (153 ± 136 IU·d−1, P > 0.05). Participants were sufficiently blinded to the intervention because only 38.4% correctly guessed their allocated group, 27.3% were incorrect, and 34.3% said they did not know whether they had received an active or placebo intervention.

Figure 3.

Flow diagram of the randomized controlled trial (study 2) investigating the effects of vitamin D supplementation on URTI and mucosal immunity. Flow diagram indicates the number of participants assessed, randomized to solar simulated radiation (SSR) or oral vitamin D3 (ORAL), or a placebo (solar simulated radiation placebo [SSR-P] or oral placebo [ORAL-P]), and statistically analyzed for URTI, salivary SIgA, and cathelicidin.

Figure 4.

Serum 25(OH)D in men completing military training while receiving 12 wk of vitamin D supplementation (solar simulated radiation [SSR] or oral vitamin D3 [ORAL]) or a placebo (solar simulated radiation placebo [SSR-P] or oral placebo [ORAL-P]). Combined vitamin D interventions (SSR and ORAL) vs combined placebo (SSR-P and ORAL-P; A), ORAL vs ORAL-P (B), and SSR vs SSR-P (C). *Greater than placebo, P < 0.05. †Greater than baseline, P < 0.05. ‡Greater than week 5, P < 0.05. Data are presented as mean ± SD.

Winter Simulated Sunlight and Oral Vitamin D3 Increased Vitamin D Sufficiency. At baseline, before wintertime vitamin D supplementation began, only one-quarter (27%) of participants were vitamin D sufficient. Both SSR and ORAL supplementation strategies were successful in achieving vitamin D sufficiency in almost all by week 5 (≥95%). Week 5 and week 12 serum 25(OH)D concentrations in the SSR and ORAL groups were higher than those in the respective placebo groups (P < 0.001, Figure 4).

Winter Vitamin D Supplementation Reduced URTI Burden. A total of 93 Jackson-defined URTI episodes were recorded with 69% of participants having at least one self-reported URTI. The URTI prevalence was similar in vitamin D and placebo supplementation groups for the restoration (weeks 1–4), maintenance (weeks 5–12), and entire 12 wk period of training (ORAL and SSR vs ORAL-P and SSR-P 57% vs 63%, 29% vs 32%, and 71% vs 68%, respectively, P > 0.05). The URTI average duration was also similar in vitamin D and placebo supplementation groups (Figure 5A, P > 0.05). Winter vitamin D supplementation reduced URTI burden compared with placebo, whereby participants had 15% lower peak URTI severity (P < 0.05; Figure 5B), and 36% fewer total days with a URTI (P < 0.05; Figure 5C). Participants beginning vitamin D supplementation with serum 25(OH)D <50 nmol·L−1 had 33% shorter average URTI duration (P = 0.05; Figure 5D), 21% lower peak URTI severity (P < 0.05; Figure 5E), and 43% fewer total days with URTI (P < 0.05; Figure 5F) when receiving vitamin D rather than placebo supplementation. There was no difference in URTI prevalence, duration, severity, or total days with URTI between vitamin D supplementation strategies or between the different placebo groups (P > 0.05). Specifically, the effect of ORAL and SSR vitamin D supplementation strategies on URTI burden was similar (ORAL vs SSR, URTI prevalence 70% vs 72%, total days with URTI 9.2 ± 8.4 vs 8.4 ± 6.7 d, URTI average duration 6.9 ± 5.0 vs 6.5 ± 5.7 d, peak URTI severity 10.8 ± 3.0 vs 12.3 ± 3.8 AU, all P > 0.05). A physician-diagnosed URTI was recorded for 8% of recruits, which was comparable with 8% prevalence in the same seasonal period in study 1, and resulted in 3.3 ± 1.3 training days lost.

Figure 5.

URTI average duration (A and D), peak URTI severity (B and E), and total days with URTI during military training (C and F), in the vitamin D supplementation (SSR and ORAL) vs placebo supplementation groups (SSR-P and ORAL-P) in all participants (left-hand column) and participants with a baseline 25(OH)D <50 nmol·L−1 (n = 62; right-hand column). * and # lower than placebo, P < 0.05 and P = 0.05, respectively. Data are presented as mean ± SD. d = Cohen's d effect size. a Maximum possible peak severity (24 AU). b Total number of days for military training (84 d).

Vitamin D Supplementation and Mucosal Immunity. Vitamin D supplementation and placebo groups did not differ at baseline, and weeks 5 and 12, for saliva flow rate, SIgA concentration, SIgA secretion rate, cathelicidin concentration, and cathelicidin secretion rate (P > 0.05; Table 2).