Vitamin D: D2, D3, IU and micrograms
Vitamin D is sold in drops, capsules, ampoules, with figures that seem never to speak the same language: here IU, there micrograms, D2 on one side, D3 on the other. Yet three notions are enough to compare everything.
D2 and D3: two forms, one family
Vitamin D comes in two forms. The D3, or cholecalciferol, is the one the skin makes under the effect of the sun and that is found in products of animal origin. The D2, or ergocalciferol, is of plant or fungal origin.
Both belong to the same family and are used in supplements. D3 is the most widespread form and is often preferred to maintain a stable level1,2. Versions of D3 of plant origin, derived from lichen, exist for people who avoid animal products. The form appears on the label, in parentheses.
IU and micrograms: the conversion to know
This is the source of the greatest confusion. Vitamin D is expressed sometimes in international units (IU), sometimes in micrograms (mcg). The conversion is fixed and simple to memorize: 1 microgram equals 40 IU.
Thus, 25 micrograms correspond to 1,000 IU, and 50 micrograms to 2,000 IU. When two products display one in IU and the other in micrograms, it is enough to apply this factor to compare them. Recent labels tend to display micrograms, often with the IU equivalent in parentheses.
Two vitamin D labels that seemed incomparable come down to a single grid: the form, the converted unit, and the timing of the dose.
Daily dose or spaced-out dose
Vitamin D is taken in two ways. As a daily dose, at a modest dose, in the form of drops or capsules; it is the most regular way to maintain an intake. As a spaced-out dose, a stronger dose taken occasionally, often an ampoule, sometimes prescribed by a professional.
Both approaches have their supporters. For an everyday supplement, the regular low-dose intake is the easiest to integrate into a routine. Regularity matters more than the quantity taken at once.
With or without a fatty meal
Vitamin D is fat-soluble : it dissolves in fats. Taken with a meal containing a little fat, it is better absorbed3,4. That is why many forms come already diluted in an oil, as drops or soft capsules.
Concretely, taking it at breakfast or lunch, rather than on an empty stomach, is a useful reflex. This detail also explains why oily forms are common: they deliver the vitamin in its preferred medium.
Reading the label and not overdosing
On the box, spot the form (D2 or D3), the dose per intake, in IU and in micrograms, and the percentage of the NRV. The NRV of vitamin D serves as a reference to situate a daily intake.
Vitamin D is fat-soluble, therefore stored by the body: unlike vitamins eliminated in the urine, a prolonged excess is not harmless. There are safety benchmarks not to be exceeded over time5. Multiplying sources without counting them (a supplement, a fortified product, a prescription) is the real risk: it is better to add up the total and, at the slightest doubt, seek advice.
In summary
Three markers are enough: the form, D3 most often; the unit, with the conversion of 1 microgram for 40 IU; and the timing, preferably with a meal. The rest is only presentation.
With these keys, two vitamin D labels that seemed incomparable come down to the same reading grid. And the principle of adding up one's intakes applies to all fat-soluble vitamins.
References
- Tripkovic L, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. American Journal of Clinical Nutrition, 2012. sciencedirect.com
- Daily vitamin D2 vs D3 supplementation on serum 25-hydroxyvitamin D concentration: a systematic review and meta-analysis. Advances in Nutrition, 2024. ncbi.nlm.nih.gov
- Dietary fat increases vitamin D-3 absorption. Journal of the Academy of Nutrition and Dietetics, 2015. jandonline.org
- Meal conditions affect the absorption of supplemental vitamin D3. Journal of the Academy of Nutrition and Dietetics, 2013. pubmed.ncbi.nlm.nih.gov
- Vitamin D, Health Professional Fact Sheet. Office of Dietary Supplements, National Institutes of Health, accessed 2026. ods.od.nih.gov