 In 1822, a Polish physician was the first to publish that sunlight could cure the vitamin D deficiency disease, rickets. His work was ignored by mainstream medicine for a century, not coming into widespread use until the 20th century, when wire cages were fixed to tenement buildings so babies could benefit from the sun. Are we in a similar situation now where the medical profession is just not caught up with the science? Researchers have documented correlations between all sorts of good things to higher vitamin D levels, even to the point of seeing whether vitamin D supplementation might reduce the adverse effects of earthquakes. Seems to help with everything else, so why not? It's actually not as silly as it sounds. Traumatic events like natural disasters can have a significant psychological impact, which may be affected by vitamin D status. When researchers put supplements to the test, the purported links often didn't pan out. This lack of effect may exist in part because low vitamin D levels may just be a marker for things like aging, obesity, smoking, and inactivity. Or maybe low vitamin D didn't lead to disease, but maybe disease led to low vitamin D. Inflammation can drop D levels within the body. So just because low D levels and disease seem to be correlated doesn't mean that vitamin D deficiency is the cause. While the majority of observational studies may show a link where you just measure vitamin D levels and disease rates, in only a handful of conditions have interventional studies proven vitamin D to be effective, where you give people D supplements or placebo and see what happens. But one of those conditions for which vitamin D supplements appear to genuinely work is helping to prevent mortality. 56 randomized clinical trials involving nearly 100,000 people between the ages of 18 and 107, mostly women, randomized to four years of vitamin D supplements or sugar pills, put all the studies together, and those given vitamin D supplements lived longer. Also specifically lowering the risk of dying from cancer. Note this effect appeared limited to vitamin D3, though, the type derived from plants and animals, not D2, the type derived from yeast and mushrooms. How large an effect was it? It would take 150 people taking vitamin D supplements for five years to save one life. And so if we were talking about a drug, you'd have to weigh that against the cost and side effects of dosing so many people. But when we're talking about something as safe and cheap as vitamin D supplements, it seems like a bargain to me. Similar analysis pegged the benefit at 11% in terms of reduction of total mortality, which is pretty substantial, potentially offering a life extension benefit on par with exercize. Though no, it does not seem to reduce the adverse effects of earthquakes. The only concern that was raised is that it may give people a license to order an extra donut or something. We still have to eat healthy. Any longevity benefit from vitamin D would just be a small adjunct to a healthy lifestyle. But for those of us who want all the help they can get, the question then becomes, OK, how much should we take? 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 may be expect a little variation, but not this. Results from the same sample range to 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.