Outcome
    Strong Evidence

    Vitamin D for Pathogen Resistance

    Vitamin D sufficiency reduces acute respiratory infection risk, with the benefit concentrated in deficient people taking regular doses.

    Overview

    Vitamin D supports the innate immune response to inhaled pathogens by driving production of antimicrobial peptides in respiratory epithelium. The strongest clinical test of this is a large individual participant data meta-analysis of randomised trials, which found that supplementation reduced the risk of at least one acute respiratory infection. The critical detail is who benefits. Protection was concentrated in people who were deficient at baseline and who took regular daily or weekly doses; those already replete gained little, and large intermittent bolus doses did not work. Vitamin D is therefore a way of removing a specific vulnerability rather than a general immune booster.

    Verdict

    Likely effective

    Individual participant data meta-analyses of randomised trials show reduced acute respiratory infection risk with regular dosing, concentrated in deficient participants. Bolus dosing shows no benefit.

    The benefit belongs to the deficient

    If your 25(OH)D is comfortably above 30 ng/mL, trials suggest little further protection from adding more. Testing separates a useful intervention from a pointless one.

    How It Works

    Airway epithelial cells convert circulating 25(OH)D to its active form locally and use it to induce cathelicidin (LL-37) and beta-defensins, peptides that disrupt bacterial and viral membranes at the mucosal surface. This is a first-line barrier function rather than an amplification of inflammation. Vitamin D simultaneously restrains the adaptive response, shifting T-cell differentiation away from excessive Th1 and Th17 activity and supporting regulatory T-cells. The practical consequence is a response that clears pathogens with less collateral tissue damage — one reason the observed benefit is on infection episodes rather than on symptom intensity alone.

    Pathways involved

    Cathelicidin (LL-37) induction
    Beta-defensin expression
    Local epithelial 1-alpha-hydroxylase activity
    Regulatory T-cell support
    Dampened Th17 inflammation
    Mucosal barrier integrity

    Dosing & Protocol

    The trials that showed protection used daily or weekly dosing, most commonly in the region of 400-2,000 IU per day. A practical target is 1,000-2,000 IU daily aiming for a 25(OH)D of 30-50 ng/mL, taken consistently through autumn and winter. Dosing frequency is not a detail. In the pooled analysis, participants on daily or weekly regimens saw the protective effect while those given large one-off boluses did not — so a monthly 50,000 IU capsule is the wrong tool for this particular job.
    ScenarioDoseFormTiming
    General winter prophylaxis1,000-2,000 IU dailyVitamin D3With a meal containing fat
    Known deficiencyClinician-directed repletion then 2,000 IU dailyVitamin D3Daily
    Weekly alternative7,000-14,000 IU weeklyVitamin D3Same day each week
    Not recommended for this purposeMonthly bolus 50,000+ IUVitamin D3No benefit in infection trials
    1. 1

      Check 25(OH)D before winter· Autumn

      Baseline status determines whether you should expect anything at all from this.

    2. 2

      Take 1,000-2,000 IU daily with food· Through winter

      Consistency matters more than the exact number within this range.

    3. 3

      Avoid switching to boluses for convenience· Ongoing

      Intermittent high doses showed no protection against respiratory infection in pooled trial data.

    4. 4

      Retest at three months if deficient at baseline· Month 3

      Confirm you have moved into the 30-50 ng/mL range rather than assuming.

    Regular small doses, not occasional large ones

    This is the clearest practical finding in the vitamin D infection literature: daily and weekly regimens worked, bolus dosing did not.

    Evidence

    The two anchor references are systematic reviews with individual participant data pooling from randomised controlled trials of vitamin D for acute respiratory tract infection. Both report a statistically significant reduction in the proportion of participants experiencing at least one infection, with the effect strongest in those with baseline 25(OH)D below 25 nmol/L. The absolute effect is modest at a population level, and subsequent large trials in mostly replete populations have added little. Read together, the literature supports vitamin D as protective where status is poor and largely inert where it is not — a conclusion that is consistent across the mechanistic and clinical evidence rather than being an artefact of subgroup analysis.

    Linked evidence for this pairing.

    Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data

    Score: 9/10
    2017
    meta_analysis
    n=11321

    Martineau AR, Jolliffe DA, Hooper RL +3 more

    Vitamin D reduced acute respiratory infection risk overall, with the largest benefit in deficient participants

    View source

    Vitamin D supplementation and risk of acute respiratory tract infection: systematic review and meta-analysis

    Score: 9/10
    2017
    meta_analysis
    n=10933

    Martineau, A.R., Jolliffe, D.A., Hooper, R.L.

    Vitamin D supplementation reduced the risk of acute respiratory tract infection among all participants.

    View source
    Best available evidence
    Individual participant data meta-analyses of randomised trials
    Typical effect
    Reduced risk of at least one acute respiratory infection, largest in deficiency
    Studied dose
    Daily or weekly dosing, commonly 400-2,000 IU per day equivalent
    Time to effect
    8-12 weeks of regular dosing
    Certainty of evidence
    Moderate to high, conditional on baseline status and dosing frequency

    Safety

    Doses in the range used for infection prevention are well below the adult tolerable upper intake level of 4,000 IU daily, and adverse events in these trials were no more frequent than with placebo. The risks sit at the extremes and in specific conditions. Sustained intake far above the upper limit causes hypercalcaemia, and people with sarcoidosis, other granulomatous disease, primary hyperparathyroidism or recurrent calcium kidney stones can develop high calcium at ordinary doses and should supplement only with monitoring.

    Reported effects

    Well tolerated at 1,000-2,000 IU
    Upper limit 4,000 IU daily for adults
    Hypercalcaemia at very high sustained doses
    Caution in sarcoidosis
    Caution with calcium kidney stones
    Not a substitute for vaccination or hygiene

    Interactions & Conflicts

    The interactions worth knowing are pharmacokinetic — drugs that speed vitamin D breakdown or block fat absorption — plus the calcium-mediated ones that matter when doses are high. Ordinary prophylactic dosing rarely causes problems, but the combinations below deserve a conversation with a prescriber.
    Interacts withSeverityMechanismAction
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    References

    1. DOI: 10.1136/bmj.i6583
    2. DOI: 10.1136/bmj.i6583

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