Here is a simple experiment we did one summer afternoon in Kamilche Point, Washington. We compared several forms of organically grown shiitake mushrooms, which had starting level of 100 IU/100 grams. We compared the vitamin D levels of three sets of mushrooms, all from the same crop. The first was grown and dried indoors. The second set was dried outdoors in the sunlight with their gills facing down. The third set of mushrooms was dried outdoors in the sunlight with their gills facing upwards for full sun exposure. The most vitamin D was found in shiitake dried with gills up that were exposed to sunlight for two days, six hours per day. The vitamin D levels in these mushrooms soared from 100 IU/100 grams to nearly 46,000 IU/100 grams (see chart). Their stems, though, produced very little vitamin D, only about 900 IU. Notably, vitamin D levels dropped on the third day, probably due to over-exposure to UV.

Most interesting to me is that when we tested our mushrooms nearly a year after exposure, they preserved significant amounts of vitamin D2. I suspect values are much higher initially, gradually falling in time, consistent with reports by other researchers. 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.

How much vitamin D can 100 grams of fresh mushrooms make from sunlight? A lot, but a lot more when mushrooms are dried, pulverized, and then exposed to UVB light lamps. Caution: Using UVB light lamps can cause damage to your eyes and skin. Do not attempt this without taking necessary precautions.

read whole experiment and how to diy)

http://www.greenmedinfo.com/blog/place-mushrooms-sunlight-get-your-vitamin-d