If everyone took 2,000 units of vitamin D a day, it could shift the curve from average blood levels in the mid-50s to about a 110 nmol/L, which some estimate could add years to our life expectancy. Data derived from randomized clinical trials have convinced some influential experts, such as Harvard’s Chair of Nutrition, that we should shoot for this kind of range, levels that about nine out of ten people fail to reach because it may necessitate taking 1,800 to 4,000 units of vitamin D a day.

The Institute of Medicine (IOM), however, considered blood levels of 50 nmol/L to be sufficient and therefore recommended only 600 to 800 units a day for those with little or no sun exposure. Why so low? Because the IOM was only considering bone health. Even if we cared just about our bones and not our lifespan, we’d still probably want to shoot for a 75 nmol/L threshold, because there’s evidence from hundreds of autopsies of people who died in car accidents, for instance, showing osteomalacia, or softening of the bones, in 18 to 39 percent of people who reach the IOM target of 50 nmol/L, but failed to make it to 75 nmol/L.

There has even been a charge that the Institute of Medicine simply made a mistake in its calculations, and, based on its own criteria, should be recommending thousands of units a day, as well. However, the mere absence of soft bones “can hardly be considered an adequate definition either of health or of vitamin D sufficiency.” It’s like saying you only need 10 mg of vitamin C to avoid scurvy—yes, but we need way more than that for optimal health.

The Institute of Medicine took the position that the burden of proof “fell on anyone who claimed benefits for intakes higher than the panel’s [minimal] recommendations,” which is a good approach for drugs. For unnatural substances, less is more until proven otherwise. For nutrients, however, shouldn’t the starting point at least be the natural levels to which our bodies have become finely tuned for millions of years? I explore this question in my video The Optimal Dose of Vitamin D Based on Natural Levels.

The target level of 75 nmol/L only sounds high compared to average levels today, but in modern times, we often practice unnatural activities like working at a desk job, or even wearing clothes! We evolved running around naked in equatorial Africa getting sun all day long. If we measure vitamin D levels in those living traditional lives in the cradle of humanity, a normal vitamin D level would be over 100 nmol/L. So, maybe that should be the starting point until proven otherwise—a concept, regrettably, many guideline committees seem to have ignored.

The natural level, however, isn’t necessarily the optimal level. Maybe the body would have thrived with less, so we still have to look at what levels correspond to the lowest disease rates. And, when we do, the higher levels do indeed seem to correlate with less disease.

When I was doing pediatrics, it always struck me that breastfed babies required vitamin D drops. Shouldn’t human breast milk be a perfect food? Of course, for the medical profession, the solution is simple: Provide the baby supplements, the vitamin D drops. But it seems like we shouldn’t have to. If we measure human breast milk these days, however, it has virtually no vitamin D and would cause rickets unless the mom has vitamin D levels up around the level natural for our species, which of course makes total sense. The way we live in our modern world is like an environmental mismatch. It helps to think of vitamin D as what it truly is: a hormone, not a vitamin. If you think of it as a hormone, then it would be reasonable to have normal levels. We physicians try to maintain blood pressure and all sorts of parameters within normal limits, “but why so little attention is paid to the status of the hormone ‘vitamin D’?”


If one is going to make an evolutionary argument for what a “natural” vitamin D level may be, how about getting vitamin D in the way nature intended—that is, from the sun instead of supplements? That’s the subject of my video The Best Way to Get Vitamin D: Sun, Supplements, or Salons?.

For the other videos in this series, check out:

I also explore vitamin D as it relates to specific diseases:

In health,
Michael Greger, M.D.

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