Vitamin D and respiratory infections
Modest protection in pooled trials, mostly with daily dosing and in people who start low. Large recent trials were null.
Education only, not medical advice. Talk to your clinician before starting a supplement, especially if you are pregnant, take medication or have a condition.
Evidence grade: B (A strong, B good with gaps, C limited or mixed, D early)
The Martineau meta-analysis
Martineau and colleagues pooled individual data from 25 randomized trials with 11,321 participants and found vitamin D supplementation reduced the risk of at least one acute respiratory infection overall (adjusted odds ratio 0.88) [1]. Benefit was seen with daily or weekly dosing, not large single boluses, and was largest in people with very low baseline levels (below 25 nmol/L) [1].
The update and the nulls
A 2021 update with 43 trials found a smaller pooled effect (odds ratio 0.92), again concentrated in daily dosing of 400 to 1,000 IU [2]. The CORONAVIT trial of a test-and-treat approach in 6,200 UK adults found no reduction in respiratory infections or COVID-19 [3]. Honest reading: vitamin D supports normal immune function, and correcting a low level matters more than taking more than you need.*
Adults should stay under the 4,000 IU per day upper limit unless a clinician advises otherwise [4].
*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
Sources
- Martineau et al. 2017, BMJ. doi.org/10.1136/bmj.i6583
- Jolliffe et al. 2021, Lancet Diabetes Endocrinol. doi.org/10.1016/S2213-8587(21)00051-6
- Jolliffe et al. 2022, BMJ (CORONAVIT). doi.org/10.1136/bmj-2022-071230
- NIH ODS, Vitamin D fact sheet. ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
DOIs verified against Crossref and official pages checked on 2026-10-15.
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