Two molecules legally answer to “vitamin D” on labels: D2 (ergocalciferol), produced by UV-irradiating fungi and yeast, and D3 (cholecalciferol), the form human skin synthesises and fish accumulate. For decades they were treated as interchangeable — textbooks said so, fortification policy assumed so. The question was settled the way nutrition questions should be: by pooling every randomised head-to-head comparison. Our earlier news report covered one study’s finding; this is the full evidence picture.
The Tripkovic meta-analysis: design and verdict
Tripkovic and colleagues (American Journal of Clinical Nutrition, 2012) systematically identified randomised controlled trials directly comparing D2 against D3 on the endpoint that defines status — serum 25-hydroxyvitamin D — and meta-analysed ten qualifying RCTs spanning ages, baseline levels, doses and regimens. The pooled result was unambiguous: D3 raised 25(OH)D significantly more than D2, with a weighted mean advantage robust across sensitivity analyses. The subgroup finding became the headline within the headline: with bolus dosing (large intermittent doses), D3’s superiority was dramatic; with daily dosing, the gap narrowed but the direction persisted. Later trials and meta-analyses have reinforced rather than overturned the conclusion, adding a further wrinkle — several studies found D2 administration actively lowers circulating D3, so total status gains understate the compositional shift. As settled questions go in nutrition science, this one is unusually tidy.
Why the molecules behave differently
Three mechanisms, all downstream of a small side-chain difference (D2 carries an extra methyl group and double bond). Binding affinity: 25(OH)D2 binds the circulating vitamin D binding protein (DBP) more weakly than 25(OH)D3 — weaker binding means faster clearance from plasma. Catabolism: the D2 side chain is more susceptible to inactivation, and its catabolites exit the system sooner. Half-life: the combined result is a measurably shorter circulating half-life for 25(OH)D2 — the direct explanation for the bolus-dosing gap, since a slowly-cleared molecule (D3) sustains levels between widely spaced doses while a fast-cleared one (D2) sags. The activation pathway (liver 25-hydroxylation, kidney 1α-hydroxylation) processes both forms; the difference is not whether D2 works — it does — but how much of it remains on duty per microgram swallowed, and for how long.
Which populations show the largest D3 advantage
The trial patterns are consistent with the kinetics: the D3 advantage widens wherever dosing is intermittent (weekly/monthly regimens, common in deficiency correction), in older adults, and over longer maintenance periods where sustained levels matter more than peak response. With conscientious daily dosing at equal microgram amounts, D2 users can approach D3 users’ status — the form difference is partly a forgiveness difference: D3 tolerates the missed days and long intervals of real-world behaviour.
Why D2 persists anyway
Three practical reasons, none scientific superiority. Prescription history: in the United States, the only high-dose (50,000 IU) prescription vitamin D long available is D2 — so clinical correction protocols institutionalised it. Vegan compatibility: classic D3 is lanolin-derived (sheep’s wool), making fungal D2 the traditional plant-based choice — though lichen-derived vegan D3 now removes that constraint at modest cost premium. Fortification economics: D2 is cheap to produce at industrial scale and chemically adequate for staple-food fortification, where daily small doses are exactly the regimen that minimises its disadvantage. Understanding these reasons dissolves the apparent contradiction between the evidence and the marketplace.
Practical implications for supplement choice
Where the choice is free, the evidence points one way: choose D3 — greater and better-sustained 25(OH)D response per microgram, especially outside strict daily-dosing discipline; vegans included, via lichen-source D3. Dose in micrograms/IU with awareness that D2-to-D3 “equivalence” is regimen-dependent (official unit definitions treat them as equal; kinetics do not). Take either form with dietary fat or in an oil-based softgel — the fat-carrier rule applies fully. And anchor decisions to measured 25(OH)D per our testing explainer: the form question matters precisely because the number it moves is checkable.
The clinical bottom line
Ten pooled RCTs, one direction: D3 outperforms D2 at raising and holding 25-hydroxyvitamin D — decisively with bolus regimens, modestly with daily dosing — for coherent kinetic reasons (DBP affinity, catabolism, half-life). D2 survives on prescription tradition, vegan history and fortification economics, not on evidence. For a supplement chosen freely: D3, with fat, dosed to a tested target.
Primary sources
- Tripkovic L, Lambert H, Hart K, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. Am J Clin Nutr. 2012;95(6):1357-64
- NIH ODS: Vitamin D — Health Professional Fact Sheet (forms and equivalence)
- GMJ: D2 supplements lower circulating D3 — study report
- GMJ: Vitamin D deficiency — who is at risk and how to test
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