That's when they figured out this vitamin D molecule made by your skin in sunshine prevented rickets, a bone disease in kids. Rickets was most commonly found in cloudy Scotland. Of course. And it was not until 2015 that the Mayo clinic came out to say we can take way more than we thought. A nd hey, it's not really that toxic if you over do it. Recommended doses have gone from 200 to 10,000 I.U. over the last 6 years. And this is the year a study finally came out documenting that low vitamin D in the blood increases your chances of dying from a virus. Yup, even THAT virus.
Details... you need a working liver and working kidneys to convert Vitamin D1 to D2 then D3. There are feedback mechanisms in the skin that control how much is made, so no, you can not overdose on the stuff if you get "too much" sun. You will just burn. It is the only vitamin that goes straight into the nucleus of the cell, so it actually acts like a hormone. Maybe that's why it is so great for us hormonal women. The bright mood it gives us is well known. Less known is its support of the immune system, and bone strength. You won't find a calcium supplement without a bit in there to help absorption.
Kids thrive on 500-1,000I.U. daily. I dose adults from 2,500 to 10,000 I.U. especially in the winter, but year round if extra umph is needed. Cancer patients get twice that. Too much makes you feel nauseous. Too much is 50,000I.U. daily for around three months. Ah the wisdom - Thanks Grandma.
Links to cool studies: