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The reason why modern humans don't get enough vitamin D from the sun, is not only due to lack of exposure, but mostly due to changes in hygiene habits, over the years. When I was a kid, we didn't take a bath or shower every day - it was more like once a week, unless we got really dirty. We typically didn't wash our hands/arms/neck/face anywhere near as often as we do these days, either.
In order for the vitamin D production cycle to complete, the skin cannot be washed with soap and water, for approximately 48 hours, following exposure to the sun. A light rinse with water only, may not wash away much of the film containing the chemical reaction that is in progress, but hard wiping or scrubbing, even without using soap, can remove the chemical film, and therefore terminate the process. Soap, of course, will definitely kill the reaction. Since this a time dependent process, yes, some small benefit may accrue within a few hours after sun exposure, but to get any significant benefit, at least 10 or 12 hours should elapse, and it takes about 48 hours for a full completion of the reaction.
Virtually none of us allows enough time to pass, before we wash off the chemical film, because we don't feel "clean", with that oily film on our skin. Besides that, in today's society, one would quickly be branded as "unwashed", if we changed our habits to allow effective vitamin D transformation. Outdoor workers, such as construction workers, farmers, and ranchers, still get some benefits from the sun, because they typically don't wash much of their exposed skin until the end of the day, but since this is still a limited amount of time, it is probably the massive amount of sun exposure they receive, that allows them to derive a significant vitamin D benefit from the sun.
You are quite correct, though, that one must be cautious when using any supplements, because unlike the natural process, which is 100% safe, (as far as a risk of an overdose is concerned), most supplements carry risks. Back when vitamin D was "discovered" for the first time, a few people got carried away during the 1940s, and took megadoses which led to severe reactions. This scared the hell out of most doctors, and so they have been afraid of vitamin D, ever since. However, it was later found that those individuals were taking a daily dose on the order of something like 200,000 IU, and they did so for months.
A dose of 50,000 IU per day, for several months, can cause an overdose condition, (assuming normal absorption capability). 10,000 IU per day, however cannot cause an OD condition. Here's a good reference, for anyone concerned about vitamin D overdose risks:
http://www.vitamindcouncil.org/vitaminDToxicity.shtml
Thanks for bringing this up, because there are indeed risks involved with taking any supplement, no matter how "natural" it might be.
Tex
