 Randomized controlled clinical trials have found that vitamin D supplements extend one's lifespan. What is the optimal dose? What blood levels associated with living longest? In my nine-part video series on vitamin D back in 2011, I noted that the relationship between vitamin D levels and mortality appeared to be a U-shaped curve, meaning low vitamin D levels were associated with increased mortality, but so were levels that were too high with the apparent sweet spot around 75 or 80 nanomoles per liter, based on individual studies like this one. Why might higher D levels be associated with higher risk? Well, this was a population study, so you can't be sure which came first. Maybe the vitamin D led to higher risk, or maybe higher risk led to the vitamin D, meaning maybe those who weren't doing as well were prescribed vitamin D. Maybe it's because these were Scandinavian studies where they tend to take a lot of cod liver oil as the vitamin D supplement, one spoonful of which could exceed the tolerable upper daily limit of intake for vitamin A, which could have negative consequences even if you don't inject it into your penis. I was surprised to see cod liver oil listed among the long list of things men have tried to inject into themselves, because they felt they were coming up short, though may have ended up even shorter after all the reconstructive surgery. Anyways, the U-shaped curve is old data. An updated meta-analysis has shown that as population vitamin D levels go up, mortality appears to go down and stay down, which is good, because then we don't have to test to see if we're hitting just the right level. Routine testing of vitamin D levels is not recommended. Why? Well, it costs money, and in most people, levels come right up to where you want them with sufficient sun or supplementation, so they figure what's the point. But also, because the test is not very good, results can be all over the place. What happens when you send a single sample to a thousand different labs around the world? You'd maybe expect a little variation, but not this. Results from the same sample ranged anywhere from less than 20 to over 100. So depending on what lab your doctor sent your blood sample to, the results could have placed you here or here. So not necessarily very helpful. So what's a safe dose that will likely get us to the purported optimal level? A thousand units a day should get most people up to the target 75 nanomoles per liter, which is 30 nanograms per milliliter. But by most people, they mean 50%. To get around 85% of the population up to 75 would crowd 2,000 a day. 2,000 international units a day would shift the curve from here to here. That way, we can take the average person into the desired range without fear of toxicity. You can take too much vitamin D, but you don't tend to see problems until blood levels get up around 250, which would take consistent daily doses in excess of 10,000. Note that if you're overweight, you may want to take 3,000, or if obese, even more than that. If you're over age 70 and not getting enough sun, it may take 3,500 units to get that same 85% chance of bumping your levels past the target. Again, no need for the average person to test and retest since a few thousand a day should bring almost everyone up without risking toxicity. Okay, but then why did the Institute of Medicine set the recommended daily allowance at 6 to 800 units? In fact, official recommendations are all over the map, ranging from just 200 a day all the way up to 10,000 a day. I'll try to cut through the confusion next.