Preventive Medicine Column
Dr. David L. Katz
A committee of the Institute of Medicine has revised recommended intake levels for calcium and vitamin D. The RDA range for calcium is fairly similar to before- around 1000mg per day. The RDA for vitamin D has been revised upward, to between 400 and 600IU per day, but less than vitamin D enthusiasts would like.
The Dietary Reference Intakes- home to the RDAs- are evidence-based. As a scientist and physician, I consider that a good thing. But an evidence review is only as good as the available evidence. While the widely circulated summary of the IOM committee report on calcium and vitamin D refers to “1000 papers reviewed,” it does not address the quality or rigor of that research. But since I know this literature fairly well, I can tell you there are significant limitations.
We simply do not have definitive large-scale, long-term intervention trials to tell us what dose of calcium or vitamin D is truly optimal for health in diverse populations. The science we do have has gaps in it- which must be filled with judgment.
The judgment of the IOM panel is fundamentally sound, as are their cautious conclusions- which suggest that we stick close to the calcium intake previously recommended about a decade ago, and roughly double our intake of vitamin D daily (to between 400 and 600 IU)- but not more.
These cautious conclusions are based on the weight of evidence that fails to show reliable benefits of higher doses, and studies that suggest the possibility of harm. They are also based on the precautionary principle which 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 being cautious does not 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- and due to cost and other difficulties, may not be.
When evidence is in shorter supply than one might like, models and theories can help guide the judgment required to plug the gaps. Trans-cultural 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.
Trans-cultural comparisons back up the IOM’s conclusion about calcium. Most populations around the world actually consume less calcium than we do in the U.S., yet have less 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 know it is possible to have healthy bones without increasing calcium intake above the RDA – and indeed, to get there with less.
But vitamin D is another story. Paleo-anthropology and trans-cultural 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.
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 combinations of nutrients you may consume over the course of a day.
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. There are hints of potential benefit in many studies with dosing above the IOM recommendation.
My advice about vitamin D, therefore, remains much as it was: get outdoor activity whenever possible, and let sunlight work its magic. Dietary vitamin D can come from fortified food, but a supplement is a very reasonable insurance policy. A supplement of 400IU 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 2000IU 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.
The IOM committee is right to remind us of the precautionary principle, and to caution us against our tendency to fall in love with a silver bullet nutrient du jour. Vitamin D is not a panacea.
But the case for vitamin D predicated on a combination of epidemiology, intervention research, on-going studies, and compelling theory remains. Maybe the honeymoon is over, but it’s certainly not time for divorce.
Dr. David L. Katz; www.davidkatzmd.com