 The Institute of Medicine's target vitamin D level corresponds with what one finds out in the general population. Normal people getting about an hour or a sun a day have about 20 to 30 nanograms per milliliter. This is in nanomoles per liter, which corresponds to about 27 nanograms per milliliter. Whereas lifeguards who spend more like 8 hours in the sun every day have abnormally high levels, like over 60. Others, however, interpret this data differently, suggesting that the vitamin D levels in the lifeguards are the ones that are normal, and the quote-unquote normals are actually vitamin D deficient. We did, after all, live as naked creatures in the East African tropics for about a million years before we began using animal skins as capes to cover our shoulders. But tailored clothing, something like we know it today, was not devised until about 40,000 years ago when needles first appear in the archaeological record. The invention of tailored clothing may have been an important factor enabling the first modern human beings to settle permanently in Europe with its cold winters about 30,000 years ago. In Africa, there was plenty of sunshine, plenty of vitamin D, not so in Europe where there were long winters and people covered in clothing. This must have been when our species first began to evolve a lighter skin as an adaptation to the shortage of sunshine and vitamin D. And it wasn't until we started living in the sunless alleys of smog-ridden cities did rickets rear its ugly head and we had to start fortifying our food supply with D. So instead of a blood level of 20, maybe we should shoot for what? You know, farmers in Puerto Rico, or lifeguards from Israel and St. Louis, right? Just because those levels might really be normal for our species, though, doesn't necessarily mean that they're the best. There's a reason people tan. That's our body producing more melanin to protect itself. There's a reason we, as a species, evolved with a built-in SPF-15 in our beautiful black African skin. So, well, maybe normal now is too low. Maybe normal then was too high.