I think that's the big one. If you've only got your hands and face exposed, I don't think I'd expect much in 2 hours. What would that be, maybe 5% of your body exposed? But I don't know. I mean, if you could synthesize 20,000 IU in an hour, 5% would still be 1,000IU so a couple of hours could net you what a lot of guys are supplementing with in this thread.
Right, I've thought about that as well. But, it seems "X" I.U.'s of vitamin D from supplements do more to increase measured D levels than "X" I.U.'s synthesized from sunlight. I have no idea what mechanism that can work through, but I don't see how 2000-5000 I.U.'s via supplements could actually be useful any other way. Yet, most of the research says 2000-5000 I.U.'s is indeed useful as far as raising measured D levels. Embedded in this observation might actually be a decent counterpoint to the observation I made about D from food above...maybe our bodies somehow "waste" a lot of the D synthesized from sunlight, but not so much from supplements/food. Pure speculation on my part.
Personally, I just think that vit D is so ridiculously cheap and so many people have been found to benefit from it that if you have to question whether you get enough sun light or not, just supplement.
Agreed.