Outcome
    Strong Evidence

    Vitamin D for Mucosal Immunity

    Individual participant meta-analysis (Martineau 2021) shows vitamin D reduces acute respiratory infections, with the largest protection in people who are deficient.

    Overview

    Vitamin D has a genuine, well-mapped role in mucosal defence. Epithelial cells lining the airway and gut convert circulating 25-hydroxyvitamin D locally and use it to switch on antimicrobial peptide genes, notably cathelicidin and beta-defensin. It also tightens junctional integrity and restrains the inflammatory arm of the mucosal response. The clinical translation is narrower than the biology suggests. Supplementation reduces the rate of acute respiratory infection modestly overall, and the benefit concentrates almost entirely in people who started deficient and who dose daily rather than in large intermittent boluses. Correcting a low level is worthwhile; loading an already-replete person is not.

    Verdict

    Strong yes

    Mechanistic evidence for vitamin D driving mucosal antimicrobial peptide production is strong. Clinical benefit for respiratory infection is real but modest and largely confined to people with low baseline levels on daily dosing.

    How It Works

    Mucosal epithelium expresses both the vitamin D receptor and CYP27B1, the enzyme that converts 25-hydroxyvitamin D to the active 1,25-dihydroxy form. This lets the barrier generate its own active hormone from the circulating pool, which is why serum 25(OH)D — the substrate — tracks with mucosal capacity. Activated receptor signalling induces cathelicidin (LL-37) and human beta-defensin 2, peptides that disrupt bacterial and viral membranes directly. In parallel, vitamin D upregulates tight junction proteins and shifts the local T cell balance toward regulatory phenotypes, reducing collateral inflammatory damage during infection.

    Pathways involved

    Local CYP27B1 conversion in epithelium
    Cathelicidin (LL-37) induction
    Beta-defensin 2 induction
    Tight junction protein expression
    Regulatory T cell promotion
    Dampened NF-kB inflammatory signalling

    Dosing & Protocol

    ScenarioDoseFormTiming
    Maintenance, replete adult800-1000 IU (20-25 mcg)D3Daily with a fat-containing meal
    Low status (25(OH)D 25-50 nmol/L)1000-2000 IU (25-50 mcg)D3Daily
    Documented deficiencyClinician-directed loading, then 1000-2000 IUD3Daily after loading
    Not recommendedMonthly or 3-monthly megadosesBolus D3Associated with no infection benefit
    1. 1

      Test before dosing high· Week 0

      A serum 25(OH)D measurement tells you whether there is anything to correct. Below 50 nmol/L is where benefit concentrates.

    2. 2

      Dose daily, not in boluses· Ongoing

      Daily 1000-2000 IU with food containing fat. Intermittent megadoses have not shown respiratory benefit in pooled analyses.

    3. 3

      Recheck at 3 months· Month 3

      Target the sufficiency range rather than the top of the reference interval. More is not better.

    Daily beats bolus

    The single most consistent finding across respiratory infection meta-analyses is that daily or weekly dosing helps and large intermittent boluses do not. If you take vitamin D for mucosal defence, take it regularly and modestly.

    Evidence

    The mucosal biology is not seriously disputed: epithelial cathelicidin induction by vitamin D has been shown repeatedly in cell and tissue work, and low 25(OH)D tracks with impaired barrier antimicrobial output in observational cohorts. Clinical trial data are more mixed. Pooled individual-participant analyses of acute respiratory infection show a small overall reduction in the proportion of people experiencing at least one infection, with the effect driven by deficient participants on daily regimens. Trials recruiting replete populations, or using annual and monthly boluses, generally find nothing. No individual trials are linked to this pairing in our database yet, so treat the summary above as the state of the wider literature rather than as pair-specific citations.
    Strength of mechanism
    High — receptor, enzyme and peptide pathway all mapped
    Clinical effect size
    Small reduction in respiratory infection risk
    Who benefits
    People with baseline 25(OH)D below about 50 nmol/L
    Certainty of evidence
    Moderate, with meaningful heterogeneity

    Safety

    There is a ceiling

    The tolerable upper intake for adults is 4000 IU (100 mcg) daily. Sustained intake above this risks hypercalcaemia, hypercalciuria and kidney stones. Very high intermittent doses have been linked to increased falls and fractures in older adults.

    Watch for

    Hypercalcaemia at sustained high intake
    Kidney stones in susceptible people
    Sarcoidosis and other granulomatous disease — supplement only under supervision
    Primary hyperparathyroidism

    Interactions & Conflicts

    Interacts withSeverityMechanismAction
    Thiazide diuretics
    moderate
    		Reduced calcium excretion plus vitamin D raises hypercalcaemia riskMonitor serum calcium
    Digoxin
    high
    		Hypercalcaemia potentiates digoxin toxicityClinician supervision and calcium monitoring
    Orlistat and bile acid sequestrants
    moderate
    		Reduced fat-soluble vitamin absorptionSeparate dosing by several hours
    Corticosteroids
    low
    		Impaired vitamin D metabolism and calcium absorptionHigher maintenance intake may be needed

    References

    1. Jolliffe DA et al. Vitamin D supplementation to prevent acute respiratory infections: systematic review and meta-analysis of aggregate data. Lancet Diabetes Endocrinol. 2021
    2. Liu PT et al. Toll-like receptor triggering of a vitamin D-mediated human antimicrobial response. Science. 2006

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