Does vitamin D help your immune system?

Test if unsure: a 25(OH)D blood test tells you whether you are in the group the benefit lives in.
Dose daily, not bolus: low daily doses through dark months matched the trial signal.
Keep perspective: a 12% relative reduction complements - never replaces - sleep and vaccination.
Get seen if: you supplement high-dose long-term without testing - vitamin D toxicity is real, and levels are cheap to check.

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Yes, modestly - this is one supplement-immunity claim with real trial data behind it. The landmark BMJ individual-participant meta-analysis pooled 25 randomized trials with nearly 11,000 people and found vitamin D supplementation significantly reduced the proportion experiencing at least one acute respiratory infection - adjusted odds ratio 0.88, meaning about 33 people treated to prevent one infection. The effect concentrates where you would expect: the protective signal is strongest in people who start out deficient, and daily or weekly dosing beats big occasional bolus doses. The honest calibration: this is a meaningful nudge, not a shield - a 12% relative reduction leaves most infections untouched, and it does not replace sleep, vaccination, or hand hygiene. Who should act: people with dark winters, indoor lives, darker skin in northern latitudes, or known low levels - a blood test settles it and typical repletion doses are safe and cheap. The move the evidence supports: daily low-dose through the dark months, especially if your level has ever tested low - not mega-doses, which the data does not reward.

Does vitamin D help your immune system?

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