Vitamin D boosts your immune system

The question has been tested about as thoroughly as a supplement question ever gets, and the answer keeps getting smaller. Pooling 40 randomized trials and 61,589 people, vitamin D supplementation changed the odds of catching an acute respiratory infection by roughly 6 percent, with an interval running from 12 percent fewer infections to no difference at all. Because that interval includes no difference, the result is not statistically significant. Serious side effects were no more common on vitamin D than on placebo.

The answer moved in one direction as the evidence got bigger. The same research group pooled the same question three times: an odds ratio of 0.88 across 25 trials and 11,321 people in 2017, then 0.92 across 43 trials and 48,488 people in 2021, then 0.94 in 2025 after three more trials including one enrolling 15,804. A separate analysis shows where the shrinkage comes from. Respiratory trials enrolling 25 to 248 people pool to 0.69, while those enrolling 248 to 16,000 pool to 0.98, and once you statistically add back the small trials that appear to be missing from the published record, the estimate lands at 0.96 with an interval spanning no effect.

The subgroups that looked strongest early have not held. The 2017 analysis found its largest benefit in people starting below 25 nmol/L, an odds ratio of 0.30, which is where the idea that vitamin D helps the deficient comes from. The 2025 update, with data stratified by starting level, found no evidence the effect varied by baseline vitamin D status, dose size, dosing frequency or age. In young children, where the 2021 analysis saw its clearest signal, a 2026 Cochrane review found a 5 percent reduction in the proportion making a healthcare visit for a respiratory infection, rated low certainty, and no fall in the number of visits per child.

Outside infection the picture has the same shape. In VITAL, 25,871 adults taking 2,000 IU a day had 22 percent less confirmed autoimmune disease over a median 5.3 years, at the edge of statistical significance. Two years after the pills stopped, the seven-year figure was a hazard ratio of 0.98, and fuller case counting moved even the randomized-period estimate to 0.85 with an interval crossing no effect. The biology behind the idea is real and specific: active vitamin D raises a complement receptor in cultured human macrophages, and those cells engulf more bacteria and fungi. But the circulating form that supplements actually raise did not do it alone, only alongside a signal that triggers local conversion. A working pathway exists in a dish. Raising blood levels in people has not reliably produced what it predicts.