The term “magic mushroom” has just taken on a whole new meaning.
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Vitamin D has long been considered “the sunshine vitamin,” and for good reason. When the sun’s ultraviolet (UV) rays strike our skin, they spark vitamin D synthesis. While it’s found naturally in foods such as fatty fish (salmon, sardines, herring, tuna, cod liver oil), hard cheese, egg yolks, fortified beverages (orange juice, milk, and soymilk), and mushrooms, by far the best source of vitamin D is the sun.

But most people are deficient in this essential nutrient because “we don’t get adequate sun exposure,” says Michael F. Holick, Ph.D., M.D., director of the Vitamin D, Skin, and Bone Research Laboratory at Boston University School of Medicine and author of The Vitamin D Solution.

“Between November and April, you essentially can’t make any vitamin D if you live north of Atlanta or L.A.,” he says. “Even if you lived at the equator, you wouldn’t make enough, unless you were outside daily and having a significant amount of skin exposed to sunlight, like Maasai herders. Most people are working at the time when we make vitamin D—between 10 a.m. and 3 p.m. is really the only time to make it.” Dr. Holick’s research also found that those who remain indoors, due to illness, disability, or lifestyle, significantly lack adequate levels of vitamin D.

The Importance of Vitamin D

Why should we care about this vitamin? It’s necessary for calcium absorption to build strong bones, and it’s responsible for brain development, a healthy immune system, and improved dental and oral health. Some research suggests that vitamin D may reduce the risk of cardiovascular disease and stroke, certain cancers, infectious diseases such respiratory tract infections, musculoskeletal disorders and falls, and Alzheimer's disease and dementia.

Deficiency in vitamin D has been linked to increased risk for osteoporosis, autism, infectious diseases including influenza and tuberculosis, Parkinson’s disease, hypertension, and autoimmune disorders such as diabetes and multiple sclerosis. Low levels have also been linked to schizophrenia and depression. Major disorders aside, vitamin D deficiency can be the root cause of daily aches and pains, as well as general fatigue.

So how do we get adequate amounts of vitamin D? Sensible sunlight exposure is certainly one answer. (Dr. Holick suggests the sunlight-tracking app dminder for guidance.) Supplementation is another. Though the RDA for vitamin D is 600 IU (15 mcg) for adults and 800 IU for those past 70, Dr. Holick recommends that teens and adults of normal weight get a minimum of 1500 to 2000 IU per day, on average. “This is recommended by the Endocrine Society to prevent and treat vitamin D deficiency,” he notes, adding, “I take 5000 IU of vitamin D a day.” Vitamin D is fat soluble, which means it gets diluted in body fat; obese people need 2 to 3 times more vitamin D to satisfy their daily requirement, he says.

Want to know more about vitamins? Read these next:

Health Benefits of Mushrooms

A third answer to the vitamin D question may be lurking in the produce aisle in the form of mushrooms. While mushrooms have long been studied for their effects on a range of illness from cancer to depression, varieties such as maitake, morel, chanterelle, oyster, and shiitake all contain natural levels of vitamin D when they are exposed to sunlight. But Dr. Holick and other researchers have found that we can increase levels of vitamin D even more by exposing mushrooms to prolonged ultraviolet light.

Similar to when humans are exposed to sunlight (or a sunlamp), mushrooms convert a precursor to vitamin D, called ergosterol, into vitamin D₂ when exposed to UV light. “It turns out that just about anything with the precursor to vitamin D, when exposed to sunlight, will develop vitamin D,” says Dr. Holick.

And though this vitamin D is technically different from the vitamin D₃ found in animal products, “that vitamin D is indeed bioavailable,” or absorbable by the body, Dr. Holick confirms. As a vegan source of vitamin D, it is just as effective at elevating and maintaining blood levels of the nutrient, according to research by Dr. Holick and colleagues. It’s also “the only pharmaceutical form of D available in the United States,” he adds. This means that the D you’re getting from mushrooms is exactly what you’d be getting in a pharmaceutical/supplement form. In fact, research has shown that consuming 2000 IU of vitamin D₂, whether from mushrooms or from supplements, leads to the same levels of vitamin D (25-hydroxyvitamin D) in the blood as consuming vitamin D₃.

Some mushrooms end up with more vitamin D after UV exposure. A few wild varieties, such as morels and chanterelles, have more D to begin with since they’re grown outside, where they get more sunlight; as a result, their post-UV-exposure levels soar. Others, such as brown (cremini) and portabella, have naturally higher levels of ergosterol, so their levels of vitamin D are also higher after UV exposure. Indoor-grown varieties do not have any D, unless they are exposed to UV light. Still, “button mushrooms are pretty good,” Dr. Holick notes. “Shiitakes are really good.” And the great news is, you can’t overdo your vitamin D from eating mushrooms alone. “It’s perfectly fine,” he adds.

How to Find Functional Fungi

Some mushroom growers have caught on. While 20 percent of Australian mushrooms are irradiated, according to Dr. Holick, “here in the U.S., it’s a very small number.” But you can do this at home, since sunlight exposure (in the spring, summer, and fall) after harvesting is just as effective at increasing levels of D; Dr. Holick notes that the dminder app can provide guidance on exposure time, similar to the amount of time it takes a person to make an adequate amount of vitamin D. You can also use indoor UV light, which may be even more effective than sunlight due to its higher concentration of UV rays (but follow product directions and take necessary precautions).

For maximum sunlight exposure, slice the mushrooms, which provides more surface area. If you find yourself with too many mushrooms to slice, simply expose them to the sun with their gills up. Paul Stamets, a mycologist, author, and founder of Fungi Perfecti, notes a study he conducted during the summer in Washington state. He found that shiitake mushrooms dried outdoors in 6 hours of sunlight over two days had more vitamin D when they were dried with their gills facing up—they went from 100 IU per 3.5 oz to nearly 46,000 IU! He tested those mushrooms a year later and found that significant amounts of the vitamin had been preserved. Here’s how he did it:

1. Place fresh organic mushrooms evenly on a tray on a summer day in direct sunlight from 10 a.m. to 4 p.m. (If you’re concerned about bugs or dirt, Dr. Holick recommends wrapping them in plastic wrap. While his studies showed that the wrap reduced vitamin D levels by 20%, the mushrooms still had “huge amounts” of the nutrient.) Before dusk, cover them with a layer of cardboard to prevent dew from collecting.

2. Repeat the next day with sun exposure for 6 hours.

3. Remove the mushrooms and finish drying until crispy. (I love using my simple little food dehydrator!) Store in sealed jars or containers. Stamets suggests adding a tablespoon of dry rice to help absorb moisture.

4. Rehydrate mushrooms in hot water for 15 to 20 minutes and cook as desired. And the great thing about this is that “vitamin D is stable up to about 400° to 500°F,” notes Dr. Holick. This means you can still get your vitamin D, even if raw mushrooms aren’t your thing.

Why You Should Eat More Mushrooms

Vitamin D aside, you should include mushrooms as part of your daily diet because they are a powerhouse of nutrients. In addition to B-vitamins, they have fiber, complex carbs, and important minerals such as selenium, copper, and potassium. They’re low in calories and sodium and are free of fat, cholesterol, and gluten.

They’re also cheap, so they make a good addition to ground meat–based dishes or as a stand-in for meat altogether. The Mushroom Council promotes something called “The Blend,” based on a study from the University of California-Davis and the Culinary Institute of America, which is a meat blend of 50 percent chopped mushrooms and 50 percent ground beef (like we mix in our tasty beef-mushroom burger recipe). With more than 2,000 varieties of edible mushrooms to choose from, what are you waiting for?