The Sun-Exposed Mushroom

Nutrition science is a treasure trove of exciting information, and the most exciting piece of nutritional knowledge I’ve learned lately is this: Mushrooms can be a sufficient source of vitamin D2 when exposed to sunlight! 

Before we get into mushrooms, let’s take a quick tour of vitamin D. . . . . 

Vitamin D2 & D3

Vitamin D comes in two major forms; vitamin D2 and vitamin D3. Vitamin D2 primarily comes from plants, algae, and yeast whereas vitamin D3 mainly comes from animal sources and is also synthesized in the skin with the help of sunlight. And get this, vitamin D is actually a HORMONE, not a vitamin or nutrient.

What D Does: Bones!

Vitamin D is popularly known for its role in building and maintaining bone strength. Rickets, a disease caused by a vitamin D deficiency, weakens bones until they become so fragile they bend under the weight of a child’s body. While this disease has mostly been eradicated in the United States, it is still prevalent in other countries. The fortification of milk with vitamin D in the 1930s was the United State’s response to this debilitating disease. 

Risks of D Deficiencies 

While rickets is now less common, vitamin D deficiencies and insufficiencies are on the rise again among both adults and children in the US population. Insufficient intake of vitamin D is associated with osteoporosis and an increased risk of falls and fractures. Recent research is also linking vitamin D deficiencies with a higher risk for ailments like Parkinson’s disease, depression, COVID-19, etc. More research is needed, but that research seems to be coming. * (A large cohort study just recently published on 03 May 2021 in JAMA Network Open concluded that “…SARS-CoV-2 seropositivity was not associated with low levels of vitamin D independently of other risk factors.” It will be incredibly interesting to start seeing more evidence-based information regarding the association, or lack thereof, between vitamin D and the SARS-CoV-2 virus and the disease it causes, COVID-19. The citation for this recently published study is:

[Li, Yonghong, Carmen H. Tong, Lance A. Bare, and James J. Devlin. “Assessment of the Association of Vitamin D Level With SARS-CoV-2 Seropositivity Among Working-Age Adults.” JAMA Network Open 4, no. 5 (May 3, 2021): e2111634. https://doi.org/10.1001/jamanetworkopen.2021.11634.]

How Much Do We Need?

The recommended dietary allowance (RDA) for Vitamin D for adults in the US is 15μg/day (15 micrograms) or 600 IU/day (600 international units) and 20μg/day or 800 IU/day for adults over 70 years of age. Keep in mind, this recommendation doesn’t just include vitamin D from food, but also the vitamin D you access from the sun. Many sources recommend even higher amounts, and since there are so many variables associated with vitamin D intake and synthesis, it’s always best to figure out your own RDA by talking to a registered dietician (RD), an ethical nutritionist, or your primary doctor. 

How We Get Our D: Sunshine, Food, & Supplements

Exposure to the sun synthesizes vitamin D in our skin, so the human body doesn’t need to rely only on food for vitamin D. However the amount of vitamin D your body can synthesize is dependent on variables such as your location, the season, the pigment of your skin, the coverage of your clothing, and your use of sunscreen.

With the exception of the summer months, if you live at latitudes above 37 degrees north of the equator or below 37 degrees south of the equator, your skin will make very little vitamin D.

See the image to the left for reference if you live in the United States. (Image credit: Harvard Health Publishing) (1)

Most vitamin D in the diet comes from foods fortified with Vitamin D, such as fortified milks and cereals. Natural sources of vitamin D3 come from animal sources, such as veal, beef, egg yolks, liver, fatty fish and their oils. And while supplements, fortified foods, and correct sun exposure can supply most people with an adequate amount of vitamin D, there are also many folks who prefer to limit their consumption of UVB sun exposure, processed foods, and supplements. And of course, vegans eschew foods of an animal origin. So here’s where mushrooms enter center stage!!

Mushrooms in the Sun

Mushrooms are a substantial source of Vitamin D2 ONLY WHEN EXPOSED TO SUNLIGHT OR ULTRAVIOLET LIGHT! So if you’re wondering why mushrooms aren’t popular on lists of foods that are good sources of vitamin D, this is why. Mushrooms are one of the only non-fortified, plant-based sources of food that supply Vitamin D2 in the diet. But to access that vitamin D, the mushrooms need sunlight; just like we need sunlight to create our own vitamin D.

Ergosterol

Mushrooms contain a substance similar to cholesterol called ergosterol. It’s this substance that is ultimately converted to vitamin D2 when exposed to UV radiation. The way our bodies synthesize vitamin D from sunlight is aided by a precursor that is made from cholesterol. So this link between a cholesterol-like substance in mushrooms and the synthesis of vitamin D isn’t too far-flung from our own physiological processes.

A Scientific Review of the Mushroom Literature

I recently read an article titled “A Review of Mushrooms as a Potential Source of Dietary Vitamin D” (2) published in the October 2018 issue of the journal Nutrients. This study was a review - meaning the authors didn’t conduct their own experiments but reviewed existing literature regarding the vitamin D content of mushrooms. 

Specifically, the authors reviewed different sources of UV radiation (solar radiation, UV fluorescent lamp, and pulsed UV lamp) as a method to enhance vitamin D2 in three of the most common mushrooms; button, oyster, and shiitake.

How Much Sun?

The quantity of vitamin D made available in a mushroom is dependent on the amount of time in the sun/UV light, the season, the location, and the type of mushroom. Just like us, if mushrooms are exposed to inadequate sunlight they won’t synthesize much vitamin D. 

For instance, per the Review:

“When fresh button mushrooms are deliberately exposed to midday sunlight for 15–120 min, they generate significant amounts of vitamin D2, usually in excess of 10 μg/100 g FW, which approaches the daily requirement of vitamin D recommended in many countries."

And

“At midday in mid-summer in Germany, the vitamin D2 content of sliced mushrooms was as high as 17.5 μg/100 g FW after 15 min of sun exposure and reached 32.5 μg/100 g FW after 60 min of sun exposure” 

• Remember, the RDA for vitamin D is between 15 and 20μg/day, so these sun-exposed mushrooms pack a punch.

• FW is “fresh weight” and 100 grams equates to a little less than a cup.

Without being exposed to UV radiation, mushrooms will provide about less than 1μg/per 100 g of vitamin D; not a great source of vitamin D. So get those mushrooms in the sun!

UVB light cannot penetrate window glass, so take your mushrooms outside or pop your mushrooms on a sill next to an OPEN window.

Amp It Up!

Because there is more ergosterol in the gills of mushrooms, than in the stalk or the cap, exposing the gills to sunlight or UV light can generate up to 4 times as much vitamin D. 

Increasing the surface area of the mushroom by slicing prior to UV exposure can also increase the amount of vitamin D created.

Does the Mushroom Matter?

Per the review, whole oyster mushrooms can generate twice as much vitamin D as shiitakes under the same UV conditions. That doesn’t mean you need to ditch the shiitakes tho, as they too will generate sufficient amounts of vitamin D2 when exposed to sunlight/UVB light. But, it’s a good reason to try a recipe featuring oyster mushrooms!

But What About Winter, Spring, and Fall?

But what if you’re at a latitude that reduces your exposure to D-enhancing sunlight? This is where the UV lamps and/or dry storage come in. UV fluorescent lamps and UV pulse lamps both quickly and efficiently aid in vitamin D synthesis in mushrooms with the pulse lamps generating vitamin D2 in 1 to 2 seconds, whereas the UV fluorescent can take several minutes. As it is way more feasible for home cooks to expose mushrooms to sunlight rather than UV lamps, I won’t go much farther into this except to say it’s a method best used by commercial growers.

Storing & Cooking

Let’s say it’s summer, and you expose your mushrooms to sunlight to enhance vitamin D. How long can you store these mushrooms? From all the literature cited in the review, the findings on storage were varied, but the authors agreed on this: 

“Considered together, these studies suggest that UV-exposed fresh mushrooms will retain nutritionally relevant amounts of vitamin D2 when refrigerated for one week or less.” 

Dry Then Expose, or Expose and Then Dry; It’s Up To You!

But what about during the winter months when the amount and angle of sunlight cannot generate significant amounts of vitamin D in mushrooms? You have a couple of options. Per the review, exposing freeze-dried mushrooms to UVB light was the most effective:

“With no detectable vitamin D2 content before irradiation, freeze-dried oyster mushrooms generated 34.6 ug D2/g DM, shiitake 60 ug D2/g DM, and button mushrooms 119 ug D2/g Dm after 30 min of exposure to radiation.”

* DM is dried mass

In addition to exposing dried mushrooms to sunlight or UV lamps, you can go the opposite route and expose your mushrooms to UV light and THEN dry them! The renowned mycologist, Paul Stamets (fungi.com), outlined this method in a two-part article in the Huffington Post (3). They found that even after a year, sun-dried mushrooms offered substantial amounts of vitamin D. Stamets ended the first part of the article with this:

“This means that you can capture vitamin D in mushrooms and have a ready source of this important vitamin - - and delicious mushrooms - - through the fall, winter, and spring.”

The literature review offered similar results:

“Three types of mushroom (button, shiitake, and oyster) exposed to a UV-B lamp and then hot air-dried, had relatively good retention of vitamin D2 up to eight months when stored in dry, dark conditions at 20 °C in closed plastic containers. However, there was a steady loss of vitamin D2 during storage between 8 and 18 months. In the case of hot air-dried button mushrooms, vitamin D2 concentration decreased from 14.3 μg/g DM to 9.3 μg/g DM over eight months, then to 6.9 μg/g DM over the following 10 months.”

So you’ve exposed your mushrooms and then stored them for a few days in the refrigerator or dried them and stored them for several months in a cool, dry, dark spot. Now what? Now it’s time to cook! There hasn’t been a ton of research on the effects of cooking on vitamin D in mushrooms, but the consensus across a couple of studies, as discussed in the review, is that the best way to retain the vitamin D content of mushrooms is pan-frying for 5 minutes without oil.

Phew, that was a Mushroom Mouthful! So Let’s Review

• Expose your mushrooms to summer sunlight gill-side up for anywhere from 20 to 120 minutes. (Too much exposure may degrade vitamin D)

• Store them in the refrigerator for up to 5 days OR dry them and store them for up to 8 months

• OR expose previously dried mushrooms (a variety of freeze-dried mushrooms are sold in local groceries) to sunlight for at least 20 minutes

• Pan-fry for 5 minutes without oil

A lot of research has been done on how our bodies absorb and use vitamin D2 and vitamin D3 differently. For instance, some research has shown that D3 remains in our bloodstream longer than D2. However, this is a topic for an entirely separate post. But for now, know that you CAN efficiently absorb vitamin D2 from mushrooms exposed to UVB light. And the best part is that mushrooms, unlike vitamin D2 supplements, are great sources of vitamin B, potassium, and the antioxidant selenium.

Research continues on the best methods to generate vitamin D2 in mushrooms as well as the best methods to store and cook those mushrooms. So it doesn’t stop here - this is just the beginning! Here’s hoping I not only opened up a new source of vitamin D2 for you but also opened up a world of mushroom possibilities!

(1) Harvard Health. “Time for More Vitamin D,” September 1, 2008. https://www.health.harvard.edu/staying-healthy/time-for-more-vitamin-d.

(2) Cardwell, Glenn, Janet F. Bornman, Anthony P. James, and Lucinda J. Black. “A Review of Mushrooms as a Potential Source of Dietary Vitamin D.” Nutrients 10, no. 10 (October 13, 2018). https://doi.org/10.3390/nu10101498.

(3) HuffPost. “Beyond Sunlight: Try This Trick To Get More D,” July 2, 2012. https://www.huffpost.com/entry/mushrooms-vitamin-d_b_1635941.