 Is there a way we can ask the body how much vitamin D it wants overall? Scientists came up with two ways. First, let's say you give a whopping dose, and I mean whopping 100,000 units, something that could be toxic, if done on a daily basis. The question is, what's our body's saturation point? Of this massive dose, how much does our body actually use and how much does it sock away in storage for use later on down the road? Here's the graph. 30 people followed for four months after the mega-dose. Here's the flood of D coming into their system. But the solid circles represent the pool of vitamin D our body is keeping in our bloodstream for activation, and the rest is likely stored away to be used on an as-needed, ongoing basis. Note that in this setting of abundance, the body is keeping our levels right around that sweet spot dip found in the U-shaped mortality curve. You can do the same thing at the other end of the spectrum, too. Instead of a mega-dose, you can start by giving really tiny doses and gradually kind of working your way up. When you do that, you get a graph like this, showing a so-called biphasic pattern, really steep at first, but then leveling out. When you take in just a little bit, your body zips it into circulation, you know, desperately kneading it. But then as you increase the dose at a certain point, you kind of, you know, turn the corner when the crisis is averted, and your body seems happy enough with your levels that, you know, as you take more in, your levels still rise, but it's not such an emergency. Now, if this plateau was flat, completely horizontal, then there'd be no risk of toxicity. But because your body can't help but let some in, your levels continue to rise with increasing intake, and you can run into vitamin D toxicity problems if we take too much. But what's this level here, kind of right at the corner where your body takes a big sigh of relief that, you know, you're doing pretty good on vitamin D? Well, you know, kind of working in from both ends, the level at which our body appears satisfied translates to about 2,000 international units a day, which should get us right into that new-shaped longevity sweet spot, whereas the Institute of Medicine recommendation appears too low, and the 10,000 recommendation put forth by others appears too high.