People who do not live in sunny places may not make enough vitamin D. Skin that is exposed to sunshine indoors through a window will not produce vitamin D. Cloudy days, shade, and having dark skin also cut down on the amount of vitamin D your skin makes. If you don’t get any sun exposure, you need to take a supplement to maintain healthy levels of vitamin D in your blood.
Plants and most animals do not provide a source of vitamin D to humans, with the exception of some fish that live at high latitudes. This exception demonstrates that natural environments support their inhabitants: in this case, eating fish provides the preformed hormone vitamin D to animals such as polar bears who live with very little sunlight almost all year long.
Mushrooms also provide vitamin D. Eating mushrooms packed with vitamin D2 provides many other nutrients, including beta glucans for immune enhancement, ergothioneines for antioxidative potentiation, nerve growth stimulators for helping brain function, and antimicrobial compounds for limiting viruses.
Organically-grown shiitake mushrooms have 100 IU of vitamin D2 per 100 grams. When dried outdoors in the sunlight with their gills facing upward for full sun exposure for two days, six hours per day, the vitamin D levels in these mushrooms soars to nearly 46,000 IU per 100 grams. (Their stems produce relatively little vitamin D: about 900 IU.) Mushrooms dried this way preserve significant amounts of vitamin D2 for nearly a year after exposure. 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.