Sunday, May 3, 2009

The Women's Health Initiative Confirms That Vitamin A Intakes Are Only Associated With Osteoporosis At Low Vitamin D Intakes

by Chris Masterjohn

The Winter 2005/Spring 2006 issue of Wise Traditions carried my article on vitamin A and osteoporosis, in which I argued that vitamin A only contributes to osteoporosis when vitamin D intakes are very low. I suggested that vitamin A intakes would not be correlated with osteoporosis risk in people consuming adequate vitamin D. This prediction has now been confirmed by a study of subjects participating in the Women's Health Initiative.

This is the third prospective study evaluating the association between vitamin A intakes and the risk of osteoporosis. Previously, the Nurses' Health Study found that high intakes of vitamin A were associated with an increased risk of osteoporosis. The Iowa Women's Health Study, by contrast, found that use of vitamin A supplements was associated with a 17 percent increase in risk, but found no association with total vitamin A intake or the amount of vitamin A in the supplements. Until now, no study had separated the subjects into high and low vitamin D intakes and looked for associations with vitamin A in the separate groups.

In the Women's Health Initiative, a modest ten percent increase in the risk of fracture was seen in women consuming more than 2,000 IU of preformed vitamin A (retinol) per day, but the risk did not continue increasing in retinol intakes higher than this. When the investigators made statistical adjustments for intakes of vitamin D and calcium, the association disappeared.

When the investigators divided the subjects into those consuming less than 440 IU of vitamin D per day and those consuming more than this amount, vitamin A intake was only associated with an increased risk of fracture in women with low intakes of vitamin D. Among women consuming less than 440 IU of vitamin D per day, the twenty percent with the highest intake of retinol, who averaged more than 8,000 IU per day, were 15 percent more likely to suffer a fracture over the course of the study than women with lower retinol intakes. Among women consuming more than 440 IU of vitamin D per day, however, the twenty percent with the highest intake of retinol were five percent less likely to suffer a fracture, although the difference was not statistically significant.

As most readers of this blog probably know, an intake of 440 IU of vitamin D is pitifully low. Ideal intakes are probably 2,000-4,000 IU in the absence of sunlight. Blood levels of 25(OH)D are a much better indicator of vitamin D status than dietary intake, since they account for both diet and sun exposure. Adequate blood levels appear to be 30-35 ng/mL. The ideal epidemiological study would separate women into those with 25(OH)D levels below this amount and those above this amount. It would be even better to add a third category of those with blood levels below 12 ng/mL, which indicates a more severe level of deficiency. We may find, perhaps, that high retinol intakes actually protect against osteoporosis when blood levels of vitamin D are ideal.

Read more about the author, Chris Masterjohn, PhD, here.