Regular daily or weekly vitamin D slightly reduces the risk of acute respiratory infections, with the largest benefit in people who are deficient
Interpretation
Two large meta-analyses of RCTs agree on direction: 25 trials, 11,321 people, OR 0.88, OR 0.30 in those below 25 nmol/L and no benefit from bolus dosing (Martineau 2017); 46 trials, 75,541 people, OR 0.92, 61.3% vs 62.3%, described as small, with no significant modification by baseline level (Jolliffe 2021). Graded moderate rather than strong because none of the trials were in athletes and the effect size is small and heterogeneous.
Practical implication
A modest daily winter supplement may slightly lower the chance of a cold, but won't prevent illness; avoid large one-off doses. Not tested in triathletes or on training days lost.
Sources
2 sources, independently verified.
25 RCTs, 11,321 participants aged 0-95, not athletes. Vitamin D reduced odds of acute respiratory infection (adjusted OR 0.88); effect largest with baseline 25(OH)D below 25 nmol/L (OR 0.30) versus 25 nmol/L or above (OR 0.75); daily or weekly dosing protective (OR 0.81), bolus doses not (OR 0.97). No difference in serious adverse events.
46 RCTs, 75,541 participants: OR 0.92 (95% CI 0.86-0.99) for at least one acute respiratory infection, 61.3% vs 62.3%; authors describe the reduction as small. Daily 400-1,000 IU OR 0.70. No significant effect modification by baseline 25(OH)D. General population, not athletes.