Randomized, controlled clinical trials have found that vitamin D supplements extend one’s lifespan. What is the optimal dose? What blood level is 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 [nmol/L], 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 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 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? 1,000 units a day should get most people up to the target 75 nmol/L (which is 30 ng/mL).
But by most people, they mean 50%. To get around 85% of the population up to 75 would require 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 600 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.
Michael Greger, M.D.
Michael Greger, M.D., is a physician, New York Times bestselling author, and internationally recognized professional speaker on a number of important public health issues. Dr. Greger has lectured at the Conference on World Affairs, the National Institutes of Health, and the International Bird Flu Summit, testified before Congress, appeared on The Dr. Oz Show and The Colbert Report, and was invited as an expert witness in defense of Oprah Winfrey at the infamous “meat defamation” trial. Currently Dr. Greger proudly serves as the Director of Public Health and Animal Agriculture at the Humane Society of the United