All you have to do is leave them in the sun for about 60 min. before cooking - scientists have found that even if heat treated, this will not lower the levels of vitamin D.
The researchers who reached this bizarre conclusion recommend taking the mushrooms out of the package and leaving them out in the sun, in the backyard for example.
An hour is best but even 30 min. may be enough. The prime hours for leaving them out in the UV light are between 10 am and 3 pm. Scientists recommend this time frame especially during the spring and summer seasons, when the sun shines brightest.
Afterwards, you can cook the mushrooms as you normally would. The Daily Mail newspaper, where the entire study is published, adds that humans transform UV light from the sun's rays into vitamin D in a way similar to mushrooms.
30 adults, who were divided into 2 different groups, participated in the study. One group was given powdered mushrooms, which had been put out in the sun for one hour. The other group took vitamin D in capsule form - 1 tablet a day. The study continued for 3 months.
The results were looked at at the end of the 3rd month and it was found that there wasn't a significant difference in the vitamin D levels in participants from both groups. This means that the mushrooms that were exposed to sunlight for 60 min. had the same amount of vitamin D as the capsules that the volunteers took.
The study and its results were presented at the annual convention of The American Society for Biochemistry and Molecular Biology in Boston. Mushroom growers in Australia and the US do actually keep their mushrooms out in the sun unlike other countries, where this practice is quite uncommon at this time.