Vitamin D for Respiratory Tract Protection

The vitamin D and respiratory infection question has a clean answer once you split the population. In the Martineau individual participant data meta-analysis of 25 randomised trials, vitamin D reduced acute respiratory tract infection by about 12% overall - a small effect - but by considerably more in participants who started with 25(OH)D below 25 nmol/L. In that severely deficient group the number needed to treat fell to around four.

The mirror image is what happened in replete populations. VITAL and D-Health, both large and well conducted, found no reduction in respiratory infection. Regimen mattered too: daily or weekly dosing produced the benefit, while monthly bolus doses did not. So this is a correction of deficiency rather than a general antiviral. If your 25(OH)D is already comfortably above 50 nmol/L, the trials predict no infection benefit from taking more.

Respiratory epithelium expresses both the vitamin D receptor and CYP27B1, the enzyme that converts circulating 25(OH)D into active calcitriol. That means the airway generates its own local active hormone from the circulating substrate, and the amount of substrate available is exactly what supplementation changes.

Locally produced calcitriol induces the antimicrobial peptides cathelicidin (LL-37) and beta-defensin 2 at the mucosal surface, and supports epithelial tight junction integrity. It simultaneously moderates the inflammatory arm of the response, dampening excessive cytokine production. This explains both halves of the trial data: below a substrate threshold, adding vitamin D restores antimicrobial peptide production; above it, more substrate adds nothing because the pathway is already saturated. It also explains why bolus dosing underperforms - large intermittent doses induce 24-hydroxylase and disturb the steady substrate supply the tissue relies on.

Take 1,000 IU/day of vitamin D3 with a fat-containing meal as a starting point, moving to 2,000 to 4,000 IU/day if levels warrant it - 4,000 IU/day is the tolerable upper intake level. Daily or weekly dosing only; the meta-analysis found bolus regimens ineffective. Continue through autumn and winter, or year-round where sun exposure is limited, and test 25(OH)D before committing to high doses long term.

Both linked papers are individual participant data meta-analyses of randomised trials rather than aggregate reviews, which is why the subgroup findings are trustworthy: the effect in profound deficiency was estimated from participant-level baseline 25(OH)D rather than trial-level averages. Their adjusted odds ratio of 0.88 overall and 0.58 in those below 25 nmol/L is the quantitative basis for everything stated above.

No serious adverse events were attributed to vitamin D in the pooled trial data, and doses up to 4,000 IU/day are considered safe for adults without predisposing conditions. The risks appear at sustained high intake: hypercalcaemia, hypercalciuria and kidney stones. People with sarcoidosis, other granulomatous disease, primary hyperparathyroidism or a history of calcium stones need medical advice before supplementing.

Vitamin D interacts through calcium handling and through the drugs that accelerate its own metabolism. Neither is usually a reason to avoid it, but both change the dose that makes sense.