The level of vitamin D in most mushrooms bought from a supermarket is typically low. However, putting them in the sun can raise the level 10-fold.
Vitamin D stimulates the absorption of calcium and is important for muscle function. A lack of it is associated with a number of diseases. Evidence suggests getting enough vitamin D plays a role in the prevention cardiovascular disorders, certain cancers, infections of the upper respiratory tract, multiple sclerosis, diabetes mellitus, and inflammatory bowel disease.
However, a study published in 2012 estimates that around half of Switzerland’s population is vitamin D deficient. The lowest levels are detected in spring after the long winter period of low sunlight.
Key sources of the vitamin are sunlight and diet, however nutritional sources are fairly rare. When UVB light in sunshine hits human skin it interacts with a protein called 7-DHC in the skin, converting it into vitamin D.
The recommended regular daily intake of vitamin D for the general population in Switzerland ranges from 400 UI (children under 1) to 800 UI (adults over 60). From the age of 1 to 59 the recommendation is 600 UI a day.
Nutritional sources of vitamin D include sun dried shiitake mushrooms (1600 IU per 100 grams), wild salmon (600 to 1000 IU per 100 grams), cod liver oil (400 to 1000 IU per table spoon), canned sardines (300 to 600 IU per 100 grams), canned mackerel (250 IU per 100 grams), canned tuna (236 IU per 100 grams), farmed salmon (100 to 250 IU per 100 grams) and regular mushrooms (76 IU per 100 grams). Egg yolks, butter and cheese have smaller amounts. In addition, some other foods are fortified with vitamin D.
In the list above, mushrooms stand out as being both low in calories and low in saturated fat, so the risks of adding them to the menu are low. They are also the only food in the list above open to vegans. But not all mushrooms are equal when it comes to vitamin D.
Finnish research published in 1994 discovered that the relatively high level of vitamin D in Finland’s wild funnel chanterelle mushrooms (120 to 1200 IU per 100 grams) compared to commercially produced fresh button mushrooms (less than 40 IU per 100 grams) was due to the extra sunlight the wild ones were exposed to. Many commercial mushrooms are grown in the dark.
However, when commercially produced fresh button mushrooms are deliberately exposed to midday sunlight for 15 – 120 minutes, they generate significant amounts of vitamin D, typically in excess of 400 IU per 100 grams. So 100 grams of regular button mushrooms can be transformed into super mushrooms with enough vitamin D to cover two thirds of the recommended daily intake (600UI for most of the population).
The size of the boost depends on the time of day, season, latitude, weather conditions, and exposure time. Slicing the mushrooms to increase the surface exposed to sunlight makes a difference too. Sliced mushrooms exposed to midday mid-summer sun in Germany for 60 minutes can take the vitamin D content up to 1300 UI per 100g. The boost in winter will be lower.
So next time you head out into the sun to boost your vitamin D level, leave your mushrooms in a sunny place before you leave. Or pick up some sun dried shiitake mushrooms.
NIH research (in English)