What would happen if our milk-full breasts were in the sun?? Did you know they used to expose cow's milk to UV light in order to raise the vitamin D levels in the milk? Of course breastmilk is not naturally vitamin D deficient, but many of us adult women are deficient and so our milk can be too. Babies are intended to get a little bit of sun themselves, I'm sure.
Liver is another high vitamin D food that's gone by the wayside, as well as the skins of meats. Liver has vitamin C too. I have to imagine that in a day when chickens had time outdooors, they had more vitamin D in their eggs (and when chickens were allowed to eat bugs, naturally, they had more omega-3s in their eggs).
Fish oil is used to attain DHA and EPA but is not high in vitamin D, unless a brand is specifically made that way. Cod liver oil is a good source, and some brands have higher levels than others.
Technically, naturally high levels of sun exposure during the summer should give us enough stores to get through the winter, with little dietary boosts. Taking supplements? There's no reason not to take some, especially in winter. There are stern discussions about the various forms but it's just that one version may absorb better than another. I don't think it's worth worrying much about. The whole thing with dosage and blood levels is a mystery to me. I try to get a couple good sun-soaks in the summer, and am active outdoors in summer, wearing sunscreen, hats and long sleeves when out for much time though. We DO make vitamin D through sunscreen, just less. I probably get 500 IU/day in my soy milk and supplements. Sometimes more. Anyway, when I had my blood levels tested they were pretty high -- 65 ng/ml (some experts are recommending 80). I just say this because I know a couple other women, with same light skin, who take 1,000/day and probably get more from other sources, and don't wear sunscreen, and have lower blood levels. Is it hormonal differences? I don't know what's going on there.
I do believe that there are more health contributions from the sun than simply vitamin D, and think that at least some portion of one's D should come from the sun.
People with black and brown skin started out much closer to the equator than many live now, the middle tones in middle reagions, and those who drifted very far north likely lost much pigmentation in order to absorb more sun. A dark-skinned person living in northern latitudes needs to make special efforts to have enough vitamin D.