Vitamin D supplements are not all the same, even when the label says “vitamin D.” The two forms, D3 and D2, differ in origin, bioavailability, and how effectively they raise your blood levels. The difference matters, and it is consistently underappreciated by people choosing supplements.
What Are D3 and D2?
Vitamin D3 (cholecalciferol) is the form humans synthesize naturally when UVB radiation from sunlight hits the skin. It is also found in animal-based foods: fatty fish (salmon, sardines, mackerel), cod liver oil, egg yolks, and liver. It is the form the human body evolved to use.
Vitamin D2 (ergocalciferol) is derived from plant sources, primarily from UV-irradiated ergosterol found in fungi and yeast. Mushrooms exposed to UV light naturally produce some D2. In the supplement industry, D2 is synthesized by irradiating ergosterol from yeast.
Historically, D2 was used to fortify foods and as a prescription supplement. Many doctors prescribe high-dose D2 (50,000 IU weekly) for deficiency correction. D3 is now the dominant form in the supplement market and increasingly the preferred form in clinical practice.
Bioavailability: The 87% Difference
The most cited comparison study: Armas et al. (2004) in the American Journal of Clinical Nutrition. This study gave subjects equivalent doses of D3 and D2 and measured the change in 25-hydroxyvitamin D (25(OH)D), the blood marker used to assess vitamin D status.
Finding: D3 was approximately 87% more potent than D2 in raising 25(OH)D levels. Both forms initially raised levels, but D3 maintained elevated levels significantly longer.
A 2012 meta-analysis confirmed this advantage across multiple studies. D3 supplementation consistently raises and maintains 25(OH)D levels more effectively than equivalent D2 doses.
Why D3 Is More Effective
The difference comes down to metabolism. Both D3 and D2 require two hydroxylation steps to become the active hormone form (1,25-dihydroxyvitamin D or calcitriol):
- First hydroxylation in the liver produces 25(OH)D (the storage form measured in blood tests)
- Second hydroxylation in the kidneys produces 1,25(OH)2D (calcitriol, the active hormone)
D3 is hydroxylated in the liver more efficiently and produces a more stable 25(OH)D3 form. D2’s equivalent (25(OH)D2) is metabolized and cleared faster, which is why D2 produces a shorter-lived increase in blood vitamin D levels. D3 also binds more effectively to vitamin D-binding protein in the bloodstream, extending its circulation time.
The net result: at the same dose, D3 delivers more sustained vitamin D activity in the body.
Why Doctors Now Prefer D3
Through most of the 20th century, D2 was the standard supplemental form partly for historical reasons (it was the first form successfully synthesized) and partly because it was assumed D2 and D3 were equivalent. The research accumulated over the past 20 years has shifted clinical practice.
Most vitamin D researchers now prefer D3. The Endocrine Society, Vitamin D Council, and most major academic medical centers recommend D3 supplementation. The main remaining clinical use for D2 is the high-dose weekly prescription format for severe deficiency correction, where the convenience of once-weekly dosing outweighs the bioavailability advantage of D3.
For daily supplementation, there is no good reason to choose D2 over D3.
Does D2 Work at All?
Yes, it does raise vitamin D levels. It is not inert. The clinical concern is not that D2 fails entirely; it is that it requires a higher dose to achieve the same blood level as D3, has shorter duration of action, and may produce less stable vitamin D activity over time.
Vegans sometimes prefer D2 because D3 is typically derived from animal sources (lanolin from sheep’s wool is the most common source of supplement D3). Lichen-derived D3 is now available and provides the D3 form from a vegan-compatible source. For vegans who want D3’s superior bioavailability without animal products, lichen D3 is the answer.
How to Check Your Supplement
The form will be on the ingredient list, not just the “vitamin D” entry on the nutrition panel. Look for:
- D3: “cholecalciferol” or “vitamin D3”
- D2: “ergocalciferol” or “vitamin D2”
If it just says “vitamin D” without specifying the form, contact the manufacturer or assume it may be D2. Quality supplement companies label this clearly.
For a product recommendation that uses D3 in a comprehensive formula, see our guide to the best vitamin D3 supplements.
Bottom Line
D3 is superior to D2 for supplementation: more potent, more bioavailable, longer-lasting, and better aligned with how human physiology actually uses vitamin D. When choosing a supplement, D3 (cholecalciferol) is the form to select. The 87% bioavailability advantage is not a small difference; at typical supplement doses, it translates to meaningfully different blood levels and thus different actual benefits.