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David Katz, M.D.

David Katz, M.D.

Posted: December 2, 2010 08:10 AM

Vitamin D and Calcium: Is the IOM Right to Recommend We Get Less?


You are likely aware that a committee of the Institute of Medicine has just issued recommendations for calcium and vitamin D intake. The big news is that the committee is recommending not as much more of both nutrients as enthusiasts might have hoped, and sounds a precautionary note about excess dosing.

Are the supplement enthusiasts right, and IOM wrong -- or vice versa? Is the IOM report a reliable basis for your own decisions?

Let's start with the strong points of both the IOM in general, and this particular report. The Dietary Reference Intakes -- of which the new report is a small part, and home to the RDAs -- are evidence-based. As a scientist and physician, I consider that a good thing, but it comes with caveats nonetheless.

An evidence review is only as good as the available evidence. While the IOM committee report on calcium and vitamin D refers to "1,000 papers reviewed," it says nothing (at least not before accessing the fine print) about the quality of those papers. But since I know this literature fairly well, I can tell you: not great.

We simply do not have large scale, long-term intervention trials with all the bells and whistles -- randomization, double-blinding, placebo-control -- to tell us what dose of calcium or vitamin D is truly optimal for health. The science we do have, no matter how many papers are cited, has major gaps in it, which must be filled with judgment.

The judgment of the IOM panel is sound, as are their cautious conclusions which, fundamentally, suggest that we stick close to the calcium intake previously recommended about a decade ago and not go higher, and roughly double our intake of vitamin D daily (to between 400 and 600 IU), but not more.

These cautious conclusions are based on studies that fail to show clear benefits of higher doses, and studies that suggest (but do not prove) the possibility of harm. They are also based on the prime directive of biomedicine -- "first do no harm," and its cousin, the precautionary principle. The precautionary principle basically says to take the path of least risk when in doubt, and that is what the IOM committee appears, quite reasonably, to have done.

But of course, being cautious does not reliably mean being right. While there is some potential evidence for absence of benefit from calcium and vitamin D supplements, there is to a much greater degree absence of evidence. Again, the definitive trials simply haven't been conducted, mainly due to cost and other difficulties.

When evidence is in shorter supply than one might like, science routinely turns to models and theories to guide the judgment required to plug the gaps. Two such models are handy: transcultural comparisons and paleoanthropology.

Transcultural comparisons allow us to see variations in human health associated with variations in exposures to nutrients, among other things. Such observational assessments cannot prove cause and effect, but they are useful for general guidance.

Transcultural comparisons fully back up the IOM's conclusion about calcium. Most populations around the world actually consume less than we do in the U.S., yet have fewer cases of osteoporosis. This may be due to more weight-bearing exercise elsewhere, less protein and acid in the diet, and more sun exposure -- and thus higher levels of vitamin D. We don't really know, but we do know it is possible to have healthy bones without increasing calcium intake above the RDA in the former and current IOM reports -- and indeed, to get there with less.

But vitamin D is another story. Paleoanthropology and transcultural comparisons both suggest that humans with more sun exposure nearer the equator live with higher vitamin D levels than their house-bound, temperate climate counter-parts. We find ourselves relying on dietary vitamin D to compensate to a marked reduction in levels 'normally' produced by the work of sunlight on our skin.

The back story here is fascinating. All humans were originally dark-skinned, or black if you will. A genetic mutation resulted in pale (white) skin, and that spread in populations away from the equator because it conferred a survival advantage (the reason mutations spread). The particular advantage was more efficient production of vitamin D in limited light by paler skin.

So I think there is still a theoretical basis for more vitamin D than clinical trials permit us to recommend with confidence. The IOM may, in other words, have been a bit too cautious in this case.

Here's where all of this leaves us: Haphazard fortification of the food supply with the darling nutrients du jour is a bad idea, and always was. When this is done, there is no predicting what dose or unbalanced combination of nutrients you may consume over the course of a day. Some judicious fortification makes sense, but when every processed food contains calcium, or vitamin D, you are indeed at risk of inappropriate doses. The IOM report rightly sounds an alarm about these prevalent and misleading practices.

Calcium supplementation by adolescent girls and adult women may make sense, although calcium from foods, including low and non fat dairy, is likely preferable. There are other therapeutic roles for calcium as well, such as treating PMS. It would be very appropriate for individualized decision making, ideally based on a discussion between each woman and her gynecologist or primary care physician.

I am less convinced by the IOM's cautious interpretation of the vitamin D literature, however. I find that many of my patients, when tested, do indeed have very low blood levels. Sun exposure is limited in much of the U.S. during much of the year. And while definitive evidence to support high dose vitamin D supplementation is lacking, there are hints of benefits in many studies with dosing above the IOM recommendation of 400 IU daily.

My advice about vitamin D, therefore, remains much as it was: Get outdoor activity whenever possible, and let sunlight work its magic. If you can't get a good 20 minutes a day of sun exposure, dietary vitamin D is essential. It can come from fortified food, but a supplement is a very reasonable insurance policy. A supplement of 400 IU daily ensures you will get the recommended dose, at least. Higher doses may be warranted, but should be discussed with your physician. You are unlikely to suffer any harm from doses up to 2,000 IU per day, but I hasten to add that we don't have long term intervention trials to prove harmlessness any more than we do to prove benefit.

Calcium and vitamin D are important nutrients. As with all nutrients, enough is good -- too little or too much is bad. The IOM invokes the precautionary principle to offer recommendations that are reasonable, and willfully conservative. But a relative absence of evidence means that guidance is as much about judgment as science.

My judgment, and familiarity with the literature, leaves me quite comfortable with the IOM conclusion about calcium, but with a bit more doubt regarding their conclusions for vitamin D.

In the absence of decisive evidence, your own decisions must also depend in the end on the evidence presented to you, and your own good judgment. Apply at will.


Dr. David L. Katz; www.davidkatzmd.com
www.turnthetidefoundation.org

 

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