Mushrooms & Vitamin D
By Glenn Walker
Interest in vitamin D has increased in recent years and this has resulted in an upward trend in sales that is predicted by many to continue. Interesting for a vitamin that is possibly the most easily obtained without even a dietary requirement. Most people are aware now that this vitamin is formed in the skin of humans upon exposure to the proper wavelengths of ultraviolet light, UV-B, and that only a short duration of exposure to legs and forearms is needed to form adequate daily vitamin D requirement, depending on the season, latitude, and skin pigmentation of the person. Even so, it is still reported that most people today are deficient in this important vitamin.
It turns out that mushrooms, being more related to animals than plants, also form vitamin D upon exposure to sunlight. The fungal form of vitamin D is slightly different however. In animals and humans, a form of cholesterol is converted by the UV-B wavelengths of light into vitamin D3. Fungi have their own form of cholesterol, called ergosterol, that is converted into a different form of vitamin D, vitamin D2, by UV-B light. Animals and humans convert vitamin D2 into vitamin D3 so it has been used as a dietary supplement for people needing vitamin D. Fungal vitamin D2 is commercially produced by exposing yeasts to UV-B. Vitamin D3 is produced by exposing sheep wool to UV-B. Mushrooms can also be a good source of vitamin D2, if they are exposed to UV-B light.
Most cultivated mushrooms today are grown indoors without exposure to any outside sunlight. Indoor UV-B lighting is rare and expensive as the trend recently has been to recommend avoiding UV exposure to limit risk of skin cancer. Some mushroom growers have created vitamin D enriched mushrooms by irradiating them after harvest with an extremely intense unnatural pulse of UV-B light of very short duration, similar to irradiation used to decontaminate packaged chicken meat from Salmonella on meat packing lines.
While working to cultivate mushrooms at Gourmet Mushrooms, I became interested in this topic and the relevance to the health of the mushrooms themselves. The understanding of the biological activity of vitamin D2 in mushrooms is still vague and the actions and importance unknown. It is even believed that vitamin D2 production is purely a relic chemical reaction and that the reaction and its products have no biological importance or role to the mushroom biology. Since humans seem to need very little exposure to maintain adequate vitamin D, and mushrooms are not usually thought of as basking in the sun, I became curious and decided to bring some mushrooms outside with me for my lunch break and expose them to some indirect outside light. I really felt like the odd professor bringing mushrooms out to lunch with me every day!! I had initial ambitions of testing the vitamin D content, but it turns out that testing for vitamin D is expensive. However, it wasn’t long before I began to notice a difference in appearance between the mushrooms that had been outside and the ones left inside (admittedly, in one of the darker spots in the growing room).
The mushrooms with some indirect light exposure looked healthier to me, with more well developed caps and darker color. Since this time, we have added harvest room lighting to make the appearance of our mushrooms more consistent and have done further research with exposure to UV-B light and vitamin D production, but we still lack the UV-B wavelengths to create vitamin D in the mushrooms in our grow rooms. However, hope is not lost! It turns out that the ergosterol to vitamin D2 conversion happens even after the mushroom is harvested, even as the mushrooms are dehydrating. So, to enrich the vitamin D content of any mushroom, you can simply expose it to some UV-B light, perhaps just by putting them out in the sun. There is not much data out there on the minimum time needed and what levels of vitamin D will be produced, but Paul Stamets and his crew at Fungi Perfecti has done some good basic work and some guidance can be found here.
To further ensure your adequate vitamin D status, sit out in the sun with your mushrooms!! Pay close attention to avoid sunburn or excessive exposure – you don’t need much – 10 minutes is a good place to start.