Vitamin D is actually an umbrella term for two forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). They come from different sources, behave differently in your body, and raise your blood levels by different amounts. D3 is the form your skin makes from sunlight and the form found in animal foods. D2 comes from plants and fungi. Both can treat and prevent deficiency, but D3 is measurably more effective at keeping your levels up over time.
Where Each Form Comes From
Vitamin D3 is produced in your skin when UVB rays (in the 280 to 320 nanometer range) hit a cholesterol compound called 7-dehydrocholesterol. Sunlight breaks open part of the molecule, and body heat finishes the conversion. This is the same form found naturally in fatty fish like salmon and trout, egg yolks, and cod liver oil. Three ounces of cooked sockeye salmon delivers about 570 IU, while a single scrambled egg provides roughly 44 IU.
Vitamin D2 is made when UVB light hits ergosterol, a compound found in plants and fungi. The richest natural food source is UV-exposed mushrooms. Half a cup of white mushrooms that have been treated with UV light contains about 366 IU, which is 46% of the daily value. Without UV exposure, those same mushrooms provide almost nothing: a half cup of raw portabella has just 4 IU. Some fortified cereals and plant milks also contain D2, though many manufacturers have shifted to using D3 instead.
How Your Body Processes Them
Both D2 and D3 follow the same basic pathway. Your liver converts them into a circulating form called 25-hydroxyvitamin D, the marker doctors measure in blood tests. Your kidneys then convert that into the active hormone your cells actually use. The machinery is the same, but the efficiency is not. D3 appears to be the preferred substrate for the liver enzyme that performs that first conversion, meaning your body handles it more readily.
D2 also seems to be inactivated faster. The active form made from D2 undergoes an additional breakdown step that the D3-derived form does not, which shortens its useful life. The circulating form of D2 has a half-life of about 14 days, compared to roughly 15 days for D3. That one-day gap sounds small, but it compounds over weeks and months of supplementation, producing meaningfully different blood levels.
D3 Raises Blood Levels More Effectively
A study published in the British Journal of Nutrition compared people taking equal doses of D2 and D3 over the winter months. By the end of the study, participants taking D2 had total blood levels 21 nmol/l lower than those taking D3. That gap represented about 28% of the average baseline level. People in the D3 group maintained their levels through winter with no decline, while the D2 group saw a significant drop. Other research has estimated D3 to be roughly 87% more potent than D2 at raising blood levels when both are given at 50,000 IU doses.
This doesn’t mean D2 is useless. It still raises blood vitamin D and can correct a deficiency. But if you’re choosing a supplement and have no dietary restrictions pushing you toward D2, D3 will do more per unit.
Why D2 Is Still Prescribed
In the United States, the standard high-dose prescription form of vitamin D has historically been a 50,000 IU capsule of D2. For children, a liquid form delivering 8,000 IU per milliliter of D2 is also available. This is largely a legacy of pharmaceutical regulation: D2 was the first form approved and manufactured at prescription strength, and it remains widely used in clinical settings.
The Endocrine Society’s clinical guidelines say either D2 or D3 can be used to treat deficiency in all age groups, from infants through adults. For adults, a typical treatment regimen is 50,000 IU once a week for eight weeks, followed by a daily maintenance dose of 1,500 to 2,000 IU. D3 in over-the-counter supplements at those same doses is now widely available, so the practical advantage D2 once had in high-dose form has largely disappeared.
Which Form Is Better for Vegans
D2 has traditionally been the go-to for people on a vegan diet because it comes from fungi and yeast. D3 supplements, on the other hand, are typically made from lanolin, a waxy substance extracted from sheep’s wool. That makes most D3 products unsuitable for strict vegans.
There is now a workaround: some companies produce D3 from lichen, a plant-like organism that naturally synthesizes cholecalciferol. Lichen-derived D3 is chemically identical to the lanolin-based version and is considered vegan-friendly. It tends to cost a bit more, but it lets vegans access the more potent form. If a vegan-specific label matters to you, check whether the product specifies lichen as the source. The NHS Specialist Pharmacy Service notes that no licensed pharmaceutical vitamin D products are completely free of animal-derived substances, but over-the-counter lichen-based D3 supplements fill that gap.
Food Sources at a Glance
- Highest in D3: Cod liver oil (1,360 IU per tablespoon), farmed rainbow trout (645 IU per 3 oz), sockeye salmon (570 IU per 3 oz), sardines (46 IU per 2 sardines), eggs (44 IU per large egg)
- Highest in D2: UV-treated white mushrooms (366 IU per half cup), fortified cereals (about 80 IU per serving)
- Fortified with either form: Cow’s milk (about 120 IU per cup), fortified plant milks (100 to 144 IU per cup), some orange juices, yogurts, and margarines
Most fortified foods in the U.S. now use D3, though labels don’t always specify. If the distinction matters to you, look for “cholecalciferol” (D3) or “ergocalciferol” (D2) in the ingredients list.
Choosing Between D2 and D3
For most people, D3 is the better choice. It raises blood levels more effectively, maintains them more consistently over months, and is widely available without a prescription at every dose from 400 to 5,000 IU. If your doctor prescribes a high-dose D2 capsule to correct a deficiency quickly, it will still work. But for everyday supplementation, D3 gives you more per dollar and per pill. Vegans who want D3 can look for lichen-sourced products, while those who prefer a more established plant-based option can stick with D2 and simply be aware that maintaining target levels may require slightly higher or more consistent dosing.