Mushroom skin, like human skin, produces vitamin D when exposed to sunlight. Mushrooms are naturally rich in ergosterol (provitamin D, or D2) and you can greatly increase the amount they contain by leaving them to soak up the sun.
Shitake mushrooms have the highest natural levels of vitamin D of all mushrooms. For a rough idea of how much you can increase their potency by leaving them to dry in the sun, if the vitamin D starting point is about 100IU/100g before exposure, leaving them in the sun for about 12 hours (6 hours per day for two days when the sun is strongest) will increase the amount to about 46,000IU/100g. Pretty impressive.
For some time, epidemiologists have shown that people living near the equator are less likely to suffer from so many of the diseases that plague the higher latitudes. Living in equatorial regions means you are less likely to suffer from cancer, diabetes, depression, arthritis and heart disease, as well as upper respiratory-tract infections such as flu and tuberculosis.
The sun, and its by-product, vitamin D are as vital to life as a roof over your head, food and water; a big deficiency can have disastrous results.
In the context of Covid-19, I keep hearing people say ‘how can a vitamin D deficiency be significant when sunny countries like Iran, Spain and Italy have been so affected?’
The answer is, even if you enjoy the sun during the winter, in mid-latitude countries (ie between the 35th-50th parallel North or South) where Tehran, Madrid, Bordeaux and Milan are situated, you will make no vitamin D whatsoever from November to February. In the higher latitudes, where London, Berlin, Brussels, Moscow and the Scandinavian countries are situated, you make no vitamin D from the sun between October and March. In either case, unless you supplement, by the end of winter, your levels will be low, and you will be more vulnerable to disease.
Recipe for vitamin D-rich mushrooms
Use fresh organic shitake, maitake, button, oyster or other mushrooms. Slice the mushrooms and place them evenly on a tray, which you should expose to direct sunlight, preferably during the summer months – June, July or August – between 10am and 4pm. Cover the mushrooms before nightfall to protect from dew condensation. Repeat the process the next sunny day. And again, until the mushrooms are totally dried (crispy).
Once thoroughly dry, the mushrooms may be stored in a glass jar, to which you have added a tablespoon of uncooked rice to prevent moisture, for up to a year. To serve, use about 10g person, rehydrate in water for an hour, and cook as desired.