Does vitamin D supplementation prevent respiratory infections?

Does vitamin D actually prevent respiratory infections? The latest evidence says the effect is small at best.

For years, vitamin D was promoted as a simple way to boost immunity and ward off colds and flu. But the most recent and largest analyses tell a more cautious story. A 2025 meta-analysis—the gold standard for combining evidence—pooled data from 40 randomized controlled trials with over 61,000 participants and found that vitamin D supplementation did not statistically significantly reduce the risk of acute respiratory infections (ARIs) overall [4]. The odds ratio was 0.94, meaning a 6% relative risk reduction, but the confidence interval crossed 1.00, meaning the result could be due to chance [4]. This updates a 2021 analysis that had shown a small but statistically significant benefit [3], suggesting that as more high-quality trials were added, the apparent benefit shrank.

A separate 2022 meta-analysis of 30 trials (over 30,000 participants) came to a similar conclusion: no significant effect overall [2]. When the authors looked only at high-quality studies, any hint of benefit disappeared entirely [2]. So for the average healthy adult, taking vitamin D supplements is unlikely to meaningfully lower your chance of catching a respiratory infection.

Who might still benefit? Daily dosing, children, and people with low vitamin D levels.

While the overall effect is weak, certain groups and dosing strategies show more promise. The 2021 meta-analysis found that daily doses of 400–1000 IU reduced ARI risk by about 30% (odds ratio 0.70) in children aged 1–16 years [3]. A 2024 cross-sectional study in children and adolescents reported that those taking more than 800 IU per week had an infection rate of 16.7%, compared to 60% in those taking less than 400 IU per week [5]. The protective effect was strongest in children under 6 years old [5].

People who start with low vitamin D levels may also get more benefit. A 2024 trial in young men found that those with insufficient vitamin D (below 50 nmol/L) had twice the risk of ARI compared to those with sufficient levels, regardless of whether they took supplements [1]. Similarly, a 2020 trial in patients with inflammatory bowel disease found that vitamin D (500 IU/day) reduced upper respiratory infections by 41%, and the effect was even stronger in those with low baseline vitamin D [13]. A 2022 study in US veterans linked vitamin D supplementation to a 20–28% lower risk of COVID-19 infection, with the largest reductions in those with blood levels below 19 ng/mL [8].

Timing and dose frequency matter too. Daily supplementation appears more effective than weekly or monthly boluses [3][11]. A 2024 systematic review concluded that daily doses of 400–1200 IU, taken during fall, winter, or spring, may offer slight protection [11]. However, weekly high-dose regimens (e.g., 5600 or 11,200 IU per week) in preschoolers showed no benefit in a 2024 trial [7].

What are the caveats? The evidence is mixed, and not everyone agrees.

Despite some positive signals, the evidence is far from uniform. A 2024 meta-analysis focused on older adults (over 41,000 participants) found that vitamin D supplementation probably does not reduce ARI incidence at all (relative risk 0.99) [6]. Even in the subgroup with low baseline vitamin D, the benefit was not statistically significant [6]. Maternal supplementation during pregnancy also showed no effect on respiratory infections in children, based on three trials with over 3,200 mother-child pairs [10].

Some of the positive findings come from observational studies or trials with limitations. For example, a 2025 review of 12 studies in children found that while lower vitamin D levels were linked to more infections, only five of nine supplementation trials showed a benefit, and three found no effect [12]. High doses (e.g., 2000 IU/day) did not outperform lower doses (400 IU/day) in one study [12]. Additionally, a 2023 expert consensus from Russia recommended vitamin D for ARI prevention [9], but such consensus statements are weaker than large randomized trials.

Importantly, the 2025 meta-analysis that found no overall benefit included a large new trial of 15,804 participants, which likely diluted the earlier positive signal [4]. This highlights a common pattern in nutrition research: early promising findings often shrink or disappear when larger, more rigorous studies are conducted.

Sources used in this answer

Related research questions