The only thing I can assure, is that sun exposure is not the clue, because I live in Spain, in an Island and we have too much sun. This island is Ibiza, and if you have heard of it, its famous for sun and beaches. And sun is not the clue.
Yes, I agree, sun exposure is not the answer, for the following reasons:
- At most latitudes in the northern hemisphere (North America & Europe) during winter, your skin cannot produce ANY vitamin D, regardless if the sun is shinning or no. The angle of the sun is too steep and UVB rays that are responsible for producing vitamin D are blocked by the thickness of the atmosphere. So even if you live in a sunny climate your skin cannot produce Vitamin D during winter so pretty much everyone's vitamin D levels reduce during winter unless you are vacationing to the middle latitudes or southern hemisphere during our winter months.
- Most people these days have been instructed to lather on sun block to protect your skin from premature aging and skin cancer, so even if you do sunbath at the beach during summer, if you are applying tons of sun block you are again blocking the beneficial effects of UV (vitamin D production) on the skin. I am not saying one shouldn't apply sun block at all, but for occasional sun exposure outside of peak UV hours, it is probably best to not apply sun block to actually get some UV exposure (of course this depends on one's skin type and sensitivity)
Also, even assuming lots of sun exposure in summer without sunblock, I think it is likely some portion of the population may have an impaired ability to produce Vitamin D in the skin, hence the acne, since diet alone cannot supply enough of it for optimal health. (This is my own conjecture here, not fact)
Anyhow, yes I have heard of Ibiza, I believe it is well known for its party scene. You are lucky to live in such a beautiful place. If you do end up trying Vitamin D, please come back and let us know if it works for you.