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Mushrooms naturally produce vitamin D when they see sunlight (or another source of UV light). Through the action of sunlight, they convert their abundant ergosterol to ergocalciferol (vitamin D2). Wild mushrooms in Europe commonly have 2-40 mcg vitamin D/100g (Mattila 1994; Mattila 2002; Teichman 2007).
Farmers generally don’t subject their mushrooms to light other than during growing operations and harvesting,  however mushrooms from retail stores can have 1-5 mcg vitamin D per 100g, possibly due to UV light exposure in-store (University of Sydney analysis 2013, unpublished). An analysis in 2015 of mushrooms from five cities undertaken by the National Measurement Institute confirmed an average 2.3 mcg vitamin D per 100g, which is 23% of daily needs.

Store-bought mushrooms are able to generate over 20 mcg per serve after being placed in sunlight for a couple of hours in the sun for about an hour produced about 10 mcg of vitamin D in a 100g serve. That is a significant amount of vitamin D when you consider our daily requirements for vitamin D are 5-15 mcg, the higher level for older people over 70 years.

Unfortunately, mushrooms begin to shrivel and brown if they are left for extended periods in the sun so mushroom farmers have found a way to generate vitamin D conveniently just after they have harvested the mushrooms.
If the mushrooms are exposed to a short burst of ultraviolet light after harvesting they quickly generate vitamin D, while retaining the good looks and nutrition of the mushroom. Once consumed, the vitamin D2 in mushrooms is converted to 1, 25 (OH) ergocalciferol. Both vitamin D2 and D3 act in the same way in the body.

Proof of the effectiveness of light on vitamin D levels in mushrooms came from a study by the University of Western Sydney (Koyyalamudi 2009). This break-through research reveals that 1-2 seconds of a pulsed UV light source can stimulate mushrooms to naturally produce enough vitamin D for the daily needs of adults.
Vitamin D Mushrooms are in supermarkets in major Australian cities. A single 100g serve of vitamin D mushrooms provides at least the daily need for vitamin D. That’s about three button mushrooms. Vitamin D mushrooms are the only food to provide a day’s needs of vitamin D in a single serve. In fact, most people get barely 10% of their vitamin D from food, relying on sun exposure and supplements to make up the difference (Nowson 2012).

The vitamin D in mushrooms is easy to absorb and effective in improving vitamin D status (Urbain 2011; Koyyalamudi 2009; Jasinghe 2005; Outila 1999). It also supports bone growth (Calvo 2012).
There is at least an 85% retention of vitamin D in wild mushrooms after frying for five minutes (Mattila 1999), so even after cooking most of the vitamin D remains in the mushroom. Furthermore, there is very little loss of vitamin D2 when mushrooms are refrigerated for eight days (Koyyalamudi 2009) or even three months (Mattila 1999).
Vitamin D mushrooms can be a simple and delicious way for Australians to get 100% of their daily vitamin D needs, especially if they are unable to get adequate sun exposure.