That's what puzzled me. My D levels came out far below 'deficient' (twice) after a summer that I spent pretty much entirely out doors all day every day.
I keep hearing medics insisting that this far north (the UK being as far north as Canada, with just the gulf stream making us warmer) we probably don't get enough vitamin D. And there seems to be some evidence (contested, of course) that lack of D is a contributory factor to a lot of different health problems, not just rickets.
Then other medics argue with them and talk about the risk of skin cancer from too much sun. But, as skin-cancer rates have increased massively here, while incident sun has not, I'm deeply suspicious that skin-cancer increases are really about people using sun-beds excessively [and flying off to southern countries for holidays and just lying on beaches all day], and have nothing to do with going outdoors here without using sunscreen. This isn't Australia.
So I take vitamin D supplements. I personally don't think others are necessary.