Here's a fun fact for you - exposing your mushrooms to direct sunlight for two days causes them to act like tiny edible solar panels, soaking up much higher levels of vitamin D.
Mushrooms are the best vegan source of vitamin D on the planet. Ingesting a handful of vitamin D-enriched mushrooms four times a week can bring serum levels to a healthy baseline. We evolved living in more sunlight than today. We make our own vitamin D when sunlight hits our skin cells. Even in sunny Australia, however, many people suffer from lower levels of vitamin D.
Fortunately, you can make your own supply of vitamin D-enriched mushrooms by simply exposing them to sunlight. You can sun-dry or UV-zap store-bought or homegrown shiitake, maitake, button, and many other mushroom species. The high vitamin D levels generated will last for more than a year. Surprisingly, even sliced and dried mushrooms will soar in vitamin D when placed outdoors under the sun.
Many doctors view low levels of vitamin D as a serious medical pre-condition. In extreme cases, deficiencies can cause diseases such as rickets. Insidious diseases, such as cancer, are immune-mitigated, and without adequate vitamin D levels, your immune system is impaired. This is why cancer patients are often prescribed high daily doses of vitamin D, up to 4,000-10,000 International Units (IU), compared to the FDA recommended Daily Value (DV) dose of 400 IU for adults over 50 years in age. Please consult your doctor before taking such large doses (40 IU is equal to 1 microgram).