The book advises putting mushrooms in the sun before using them to increase their vitamin D content. This may sound daft but will it work if they are on a window sill and still in the plastic wrapper? Not all solar energy penetrates glass.

It doesn’t sound daft at all. The advice I have seen is so vague as to be useless.

The research and patents refer to exposure to both UVA and UVB light causing a substantial increase in vitamin *D2* in the mushrooms. As you say UVA rays penetrate glass but UVB rays do not. In addition the strength of UVB rays varies widely with the time of day and time of year.

Humans need vitamin *D3*, which we get primarily from oily fish (mackerel, sardines/ pilchards, herring, salmon trout), fortified foods (varies massively by country), and D3 supplements for ‘at risk’ groups (vary by country).

Converting D2 to D3 naturally in/ on human skin requires exposure to UVB light. For this the latitude, time of year, time of day and our skin colour are all relevant. Here in the UK we cannot rely on sunlight for vitamin *D3* production between October and April. So D2-enriched mushrooms are not much use.

All in all I would suggest sticking to official recommendations for your own country regarding UV exposure, intake of vitamin D rich foods and supplementing for ‘at risk’ groups.

HTH!