Why do mushrooms make vitamin D?

SUNBATHE YOUR MUSHROOMS - BUT WATCH OUT FOR SUNBURN

On the way back from grandparents day at our grandsons' school today we heard one of those little programs on the ABC about this and that. And this little bit was about mushrooms and vitamin D. I think I vaguely knew they were rich in Vitamin D but what I didn't know and which I found really interesting was that the Vitamin D is not an intrinsic part of the mushroom as it were - it gets made by the mushroom when it is exposed to the sun or to ultra-violet light - either in the farm where they are cultivated or in the supermarket. And it doesn't take long. Put your mushrooms in the sun for a couple of hours and you will have your day's supply of Vitamin D, even if you only eat two or three. The danger is that the longer they are left in the sun the browner they become, and so maybe do not look so tempting? And actually you don't really need to put them in the sun, because just being on the supermarket shelves is enough.

And nobody seems to know why they do it. What do they need the Vitamin D for? Again - they don't know. We all know that we need Vitamin D for healthy bones and a whole lot of other things, but what do mushrooms need them for? And as the interviewer in the ABC program, rather facetiously said - do they get melanomas? Well they wouldn't live long enough is my guess - but if they go brown in the sun, (or ultra-violet light) it sounds a bit like sunburn.

Anyway all good stuff - all hail the mushroom. The next best thing being oily fish and meat. So if you are a vegetarian it's all good news. And before I go on, let me say that lots of learned and authoritative people are saying these things.

But there's always a dissenting voice isn't there? It seems it's not as simple as that. Wrong kind of Vitamin D according to The Healthy Home Economist. (Sarah - whoever she is.)

"Vitamin D2 and Vitamin D3 are Not the Same Thing (but you can make a lot of money if you are a Vitamin D researcher and pretend they are)" The Healthy Home Economist

Actually my brief perusal of the 'learned' literature did make a distinction between D2 and D3, but it didn't seem to be quite as negative. So I think I will go with the professors. Mushrooms D2, oily fish and meat D3. And it's a massive source of umami too - so you get a double good hit - of Vitamin D and super taste.

The other thing the man on the ABC mentioned (and I did know this) is that, like icebergs, most of the mushroom is underground. It has long white thread-like things that spread out over a very wide area - tapping into trees as well I think. Which is often why they are found under trees - witness truffles and oak trees.