vitamin · for frequent colds
Vitamin D
Switches on the front line of your immune defences, and keeps the response from overshooting.
Say that in clinical terms
Vitamin D activates innate immune defenses by inducing cathelicidin and defensin antimicrobial peptides. It also enhances macrophage pathogen killing, modulates T-cell response, and reduces excessive inflammatory cytokine production.
How strong is the research?
2 papers read, 1 supporting.
Tested as: vitamin D supplementation — a mix of D2 and D3 preparations across the 25 pooled trials; D3-specific effect not confirmed as the primary finding
Who was studied: Pooled 25 trials and 10,933 people aged 0 to 95 across 14 countries — broad, and the benefit was concentrated in those who were deficient.
How much to take
daily with a fat-containing meal
2000–4000 IU
Test 25(OH)D levels. Those with levels <25 ng/mL benefit most from supplementation for immune support.
Before you take it
- NIH ODS Tolerable Upper Intake Level (UL) for adults aged 19+ is 4,000 IU/day
- Drug interaction: thiazide diuretics combined with vitamin D3 increase risk of hypercalcemia by reducing urinary calcium excretion
- Drug interaction: anticonvulsants (phenobarbital, phenytoin) induce CYP enzymes that accelerate vitamin D catabolism, reducing circulating levels and therapeutic efficacy
- Drug interaction: bile acid sequestrants (cholestyramine, colestipol) impair absorption of fat-soluble vitamins including vitamin D3; doses should be separated
- Drug interaction: orlistat inhibits dietary fat absorption and reduces vitamin D3 absorption; monitoring of vitamin D status is recommended with concurrent use
- Drug interaction: corticosteroids (e.g., prednisone, dexamethasone) impair vitamin D metabolism and reduce intestinal calcium absorption, potentially antagonizing vitamin D effects
- Drug interaction: vitamin D3 at high doses may alter CYP3A4 activity and potentially affect pharmacokinetics of statins metabolized by that pathway (atorvastatin, lovastatin, simvastatin); clinical significance is modest
- Contraindication: pre-existing hypercalcemia or hypercalciuria — supplementation without medical supervision is contraindicated
- Caution: granulomatous diseases (sarcoidosis, tuberculosis, histoplasmosis, some lymphomas) produce unregulated endogenous calcitriol independent of serum 25(OH)D; exogenous vitamin D supplementation can precipitate severe hypercalcemia — requires endocrinology oversight
- Caution: impaired renal function reduces conversion of vitamin D to active calcitriol and impairs urinary calcium excretion, increasing toxicity risk; monitoring is required
- Caution: primary hyperparathyroidism — calcium-vitamin D axis is dysregulated; supplementation requires specialist oversight
- Toxicity signs at sustained supra-UL doses: hypercalcemia, hypercalciuria, nausea, polyuria, weakness, nephrolithiasis, soft-tissue calcification, and irreversible renal damage
When to take it
Best taken: morning
What we make of it
The best evidence we have — and the benefit is concentrated in people who were deficient.
The detail
This pools individual data from 25 trials and 10,933 people, which is the strongest design available. Overall the effect is modest — about one fewer infection for every 33 people treated — but it is much larger in those who were severely deficient to begin with.