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 from 2011, I noted that the relationship between vitamin D levels and mortality appeared to be a U-shaped curve, meaning low levels of vitamin D were associated with increased mortality, but so were levels that were too high, with the apparent sweet spot around 75 or 80 nmol/L based on individual studies. (See Vitamin D and Mortality May Be a U-Shaped Curve for more on this.)
Why might higher vitamin D levels be associated with higher risk? Well, the study I profile in my video How Much Vitamin D Should You Take? was a population study, so we can’t be sure which came first. Maybe the higher vitamin D higher risk, or perhaps higher risk led to higher vitamin D levels, meaning maybe those who weren’t doing as well were prescribed vitamin D. Maybe it’s because it was a Scandinavian study, where individuals tend to take a lot of cod liver oil as a vitamin D supplement, one spoonful of which could exceed the tolerable upper daily limit of intake for vitamin A, which could have negative consequences.
Anyway, 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 what’s the point? As well, 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 laboratories around the world? You’d perhaps expect a little variation, but results from the same sample ranged anywhere from less than 20 to over 100 nmol/L. Depending on what laboratory your doctor sent your blood sample to, the results could vary dramatically, so one could argue the test isn’t 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 nmol/L (which is 30 ng/mL), but by most people, researchers mean 50 percent of people. To get around 85 percent of the U.S. population up to 75 nmol/L would require 2,000 IU a day. Two thousand IU a day would shift the curve so that the average person would fall into the desired range without fear of toxicity. We can take too much vitamin D, however, but problems don’t tend to be seen until blood levels get up around 250 nmol/L, which would take consistent daily doses in excess of 10,000 IU.
Note that if you’re overweight, you may want to take 3,000 IU and even more than that if you’re obese. If you’re over age 70 and not getting enough sun, it may take 3,500 IU to get that same 85 percent chance of bumping up your levels above the target. Again, there’s no need for the average person to test and retest, since a few thousand IU per day should bring up almost everyone without risking toxicity.
Given this, why then did the Institute of Medicine set the Recommended Daily Allowance at 600 to 800 IU? In fact, official recommendations are all over the map, ranging from just 200 IU a day all the way up to 10,000 IU a day. I’ll try to cut through the confusion in my next post.
After all that work plowing through the new science, the same 2,000 IU per day recommendation I made in 2011 remains (for those not getting enough sun): http://nutritionfacts.org/2011/09/12/dr-gregers-2011-optimum-nutrition-recommendations/.
The other videos in this series include:
I also explore Vitamin D as it relates to specific diseases:
Michael Greger, M.D.
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