Vitamin D2 (ergocalciferol) comes almost exclusively from fungi and UV-treated plant sources, making it the go-to form of vitamin D for vegans and anyone avoiding animal products. The richest natural source is mushrooms exposed to ultraviolet light, which can provide several times the daily value in a single serving. You can also get D2 from fortified foods, supplements, and even by treating store-bought mushrooms with sunlight at home.
Mushrooms Are the Best Natural Source
Mushrooms are unique in the food world: they produce vitamin D2 the same way fungi do in nature, by converting a compound called ergosterol into D2 when exposed to UV light. This means any edible mushroom has the potential to be a rich D2 source, but only if it has seen UV light at some point, whether during growing, after harvest, or in your kitchen.
The numbers vary dramatically depending on UV exposure. A half cup of white mushrooms that have been commercially treated with UV light contains about 9.2 mcg (366 IU) of vitamin D, roughly 46% of the daily value. By contrast, the same amount of raw portabella mushrooms without UV treatment has a negligible 0.1 mcg (4 IU). The difference is entirely about light exposure.
Among mushroom varieties, oyster mushrooms respond especially well to UV treatment, generating more than twice the vitamin D2 of shiitake under the same conditions. Shiitake mushrooms concentrate most of their D2 in the gills, which produce about four times as much as the cap. Slicing mushrooms before exposing them to light increases the surface area, which boosts D2 production even further.
How to Boost Vitamin D2 in Store-Bought Mushrooms
You don’t need a UV lamp. Simply placing sliced mushrooms gill-side up in direct sunlight for 15 to 60 minutes can significantly increase their vitamin D2 content. Research shows that UV-B intensity and exposure time are the two biggest factors. Under optimal conditions, mushrooms can generate extremely high concentrations of D2, well beyond what you’d need in a single meal. Even brief pulses of UV light (just a few seconds in commercial settings) are enough to produce meaningful amounts.
The good news for cooking is that vitamin D2 holds up well to heat. Frying and baking retain at least 95% of the vitamin D2 in mushrooms, and both methods preserve significantly more than boiling. So sautéing UV-treated mushrooms in a stir-fry or roasting them in the oven delivers nearly the same D2 as eating them raw.
Fortified Foods and Plant Milks
Many plant-based milks (soy, almond, oat) are fortified with vitamin D, typically providing 2.5 to 3.6 mcg (100 to 144 IU) per cup, or about 13% to 18% of the daily value. However, most fortified cow’s milk in the U.S. actually uses vitamin D3, not D2, so check the label if the form matters to you. The same goes for fortified cereals, which typically offer about 2 mcg (80 IU) per serving but may use either form.
The FDA has also approved UV-treated mushroom powder as a food additive, so you may see it listed as an ingredient in certain packaged foods marketed as vitamin D sources. Products made with yeast exposed to UV light are another D2 source, since yeast produces ergosterol just like mushrooms do.
Vitamin D2 Supplements
Over-the-counter vitamin D2 supplements are widely available and are often labeled as “ergocalciferol.” These are typically derived from UV-irradiated yeast or fungi, making them reliably vegan. You’ll find them in standard daily doses ranging from 400 to 2,000 IU.
There are also high-dose prescription capsules containing 50,000 IU of vitamin D2, used to treat specific conditions like vitamin D-resistant rickets and hypoparathyroidism. These require medical supervision and regular blood calcium monitoring. They’re not appropriate for general supplementation.
How D2 Compares to D3
Vitamin D2 and D3 both raise your blood levels of vitamin D, but they aren’t identical. D2 has a shorter half-life in the body: about 14 days on average, compared to roughly 15 days for D3. That difference sounds small, but it means D2 levels decline faster after a single dose, which is why equal doses of D2 and D3 don’t maintain blood levels equally over time. In one study, the gap was most pronounced in populations with lower baseline vitamin D status.
For practical purposes, this means you may need to take D2 more consistently (daily rather than weekly) to maintain steady levels. If you’re choosing D2 for dietary or ethical reasons, it still works. A meta-analysis of randomized trials confirmed that mushroom-derived vitamin D2 effectively raises blood levels in people who start out deficient or insufficient. The key is regular intake rather than occasional large doses.
How Much You Need
The daily value for vitamin D (both D2 and D3 combined) is 20 mcg, or 800 IU. To hit that with food alone using D2 sources, you’d need roughly two cups of UV-treated white mushrooms, or a combination of fortified plant milk and mushrooms throughout the day. Most people who rely on D2 find that a mix of UV-exposed mushrooms, fortified foods, and a daily supplement is the most reliable strategy.
Getting enough D2 from mushrooms alone is realistic if you make UV treatment a habit. Keep a container of sliced mushrooms on a sunny windowsill for 30 to 60 minutes before storing them in the fridge. The vitamin D2 generated this way remains stable through storage and cooking, so you can prepare a batch and use them over several days.