A new study of a quarter million Copenhagen residents found that those with Vitamin D blood levels of 40-70 nmol/L [16-28 ng/ml] had the lowest death rate. People with lower and higher amounts had higher death rates, in other words. The death rate versus blood level function has a reverse-J shape, i.e., too little is worse than too much. About 1% of the sample had levels above 140 nmol/L [56 ng/ml], for practical purposes a “high” level.
Because Vitamin D3 seems to have a big time-of-day-dependent effect on sleep (Vitamin D in the morning improves sleep, Vitamin D in the evening makes sleep worse) it is plausible that people with high Vitamin D levels were more likely to take it in the evening than those with moderate levels and this is why they had higher mortality. Likewise, it is plausible that those with moderate levels were more likely to take Vitamin D in the morning than those with low levels and better sleep explains the lower mortality. Although epidemiologists adjust for smoking in studies like this, they don’t yet adjust for sleep quality. It is also plausible that people who were more sick took more Vitamin D — hoping it would improve their health.
I think I have a better way to decide how much Vitamin D3 to take: choose the minimum amount that produces the best sleep. Sleep is so strongly connected with health that I wouldn’t want to choose worse sleep over better sleep simply because of epidemiology. At the same time that I greatly improved my sleep, I stopped getting easy-to-notice colds. Apparently my immune system was doing a better job of fighting them off.
There is evidence that Vitamin D improves immune function independently of its effect on sleep. A 2009 survey found that “those with less than 10 nanograms of vitamin D per milliliter [25 nmol/L] of blood, considered low, were nearly 40 percent more likely to have had a respiratory infection [over what period of time?] than those with vitamin D levels of 30 ng/ml [75 nmol/L] or higher.”
So those three studies (epidemiology, lab, epidemiology) taken together make a good case that my Vitamin D levels should be at least 25 ng/ml. I will have my Vitamin D level measured soon and it will be interesting to see how much an approach based purely on self-measurement (find the minimum amount of Vitamin D that optimizes sleep) agrees with this.
Thanks to Chase Saunders.
More. In an earlier version of this post I confused ng/ml with nmol/L.