 There are many ways to arrive at a target vitamin level. We've already examined some of the problems inherent to just using deficiency disease prevention and evolutionary arguments to establish an optimal intake. If a vitamin only does one thing, then it's easy. You set the level at whatever the body needs to do that one thing best. But what if the vitamin affects dozens of different organs, like with vitamin D? Then it's more difficult. Here's a list of the target tissues affected by vitamin D in the body. In revising the recommendations, the Institute of Medicine decided to only look at one tissue, bone, which many consider to be a mistake. I did like how they went about it, though. I mean, they asked the expert, the human body. When our body senses we don't have enough active vitamin D for adequate bone health, it releases a hormone called PTH to boost our levels. And so the Institute of Medicine figured, why don't we just listen in on the body's own innate wisdom and find out which level of vitamin D it feels comfortable with for bone health? And that number is about 20 nanograms per milliliter here as 50 nanomoles per liter. Once we fall below that, our body's like, uh-oh, and starts producing this PTH to protect our bones from softening. But that's just the bone. What about the other three dozen organs affected by vitamin D?