Mushrooms are the only vegetarian food that can make vitamin D. Actually, they contain a “pro-vitamin,” or precursor, called ergosterol that is converted into vitamin D when exposed to the sun’s ultraviolet (UV) radiation—similar to how your skin synthesizes the vitamin in response to sun exposure. However, commercially cultivated mushrooms are almost always grown indoors in the dark, so they usually have negligible amounts of vitamin D. In contrast, wild mushrooms—notably chanterelles, maitake, and morels—are usually rich in D because they get sun exposure.
Since 2009 some commercial growers have produced D-enhanced mushrooms by exposing them briefly to UV lamps, typically after harvesting. They are usually labeled “UV-treated” or “high in vitamin D.” D-enhanced mushrooms from companies such as Monterey Mushrooms contain at least 400 IU of vitamin D per 3-ounce serving and usually cost a little extra. The treatment does not darken the mushrooms or affect other nutrients.
The form of vitamin D produced in mushrooms is D2, unlike the D3 found in the few animal foods that naturally contain the vitamin; most supplements and fortified foods, such as milk, also contain D3. It used to be thought that D2 was less potent and bioavailable in the body than D3. But more recent research—including a study comparing D2 from mushrooms with D3 from supplements, in Dermato-Endocrinology in 2013—has found that D2 is as effective as D3 in boosting the biologically active form of the vitamin in the body. Vegetarians may prefer D2 because it is not derived from animals (D3 in supplements is derived from lanolin in sheep’s wool).
Increase the vitamin D in your mushrooms
Over the past decade, scientists have found that it takes only a modest amount of UV from the sun or special lamps to produce significant levels of vitamin D in mushrooms. Just 15 minutes of direct sunlight can produce 200 to 800 IU in 3 ounces of mushrooms (the daily RDA is 600 to 800 IU), regardless of type or season. At least 90 percent of the vitamin is retained after storage and cooking. Whole button mushrooms synthesize the least D; sliced buttons, however, are proficient D producers. If you do this yourself, place the mushrooms with the “gills” (also called lamellae, under the caps) facing the sun to increase D production. The mushrooms may discolor or dry out a little.
Bottom line: If you like mushrooms, those that have been exposed to UV can help you get more vitamin D, but it’s easier and cheaper to take a vitamin D supplement, if you need one.
Also see our quiz: Vitamin D: How Much Do You Know?