There are a lot of reasons to be discontented with winter. It can be miserably cold, dark, wet and messy, and may keep you from your favorite outdoor activities. But even more troubling than any of those reasons, is the unsettling fact that winter robs you of vitamin D.
During the winter, most areas of the United States see more clouds than sunlight. In addition, bundling on layers of clothing leaves little surface skin to receive the ultraviolet B ray exposure needed for the human body to naturally produce vitamin D.
In northern cities, like Buffalo, New York, nearly 50 percent of residents have insufficient amounts of vitamin D and 25 percent are considered deficient; this according to nutrition researcher Peter Horvath of the University of Buffalo School of Public Health and Health Professions. 
Before you roll you eyes at yet another article citing the importance of vitamin D, just take a look at the serious health problems that develop from a lack of vitamin D:
- Lower bone density.
- Weakened immune system.
- Increased risk for type 2 diabetes.
- Higher susceptibility to some cancers.
- Increased risk of death from cardiovascular disease.
- Cognitive impairment in older adults.
“Every cell in your body is responsive to vitamin D,” says Horvath. “If you’re deficient, you won’t see the health effects for years and it could take months to get your levels back up.”
You can’t control the weather, but you can get your vitamin D levels where they need to be through diet and supplementation. Professor Horvath recommends vitamin D supplementation of between 1,000 to 2,000 international units (IU) daily and consumption of foods rich in vitamin D, including:
- Wild-raised salmon and oily fish.
- Breakfast cereals.
- Enriched milk.
- Cod liver oil.
Irradiated mushrooms are another great source of vitamin D, and are currently a focus of Professor Horvath’s research. He says they are mushrooms that “basically go through a little tanning bed” to produce a mega-dose of vitamin D.
It’s easy to “irradiate” mushrooms yourself and if you don’t own a miniature tanning bed, all you need is sunlight. Paul Stamets is the founder of Fungi Perfecti and an advisor for the Program of Integrative Medicine at the University of Arizona Medical School in Tucson. Using the following process detailed below, he demonstrates how you can produce 46,000 IUs of vitamin D by irradiating just 3.5 ounces (100 grams) of shiitake mushrooms with natural sunlight.
Because ample sunlight is needed, it’s best to irradiate your mushrooms during summer months. The process results in dried mushrooms that can be stored for use all winter long to naturally boost your vitamin D when you need it most.
Irradiated Mushrooms 
- Obtain fresh organic shiitake, maitake, button, oyster, shimeji or other mushrooms.
- On a sunny day in June, July or August, slice the fresh mushrooms. Place them evenly on a tray exposed directly to the sun from 10 a.m. to 4 p.m.
- Before nightfall, cover the mushrooms with a layer of cardboard to block moisture from dewfall.
- The next clear day repeat exposure to the sun from 10 a.m. to 4 p.m.
- Remove the mushrooms and finish drying (if necessary in a food dehydrator until they are crispy).
- When thoroughly dry, store in a glass jar or sealed container. Adding a tablespoon of uncooked rice as a moisture absorber will help keep the mushrooms dry. The mushrooms should be good for a year or more, depending upon conditions.
- Take 10 grams daily per person, about a small handful. Rehydrate in water for one hour. The mushrooms will swell. Then cook as desired.