BROWN (US) — Postmenopausal women receive no additional mortality benefit from vitamin D after controlling for health risk factors such as abdominal obesity, a new study finds.
The research, consistent with advice issued last fall by the Institute of Medicine, found no evidence linking vitamin D to the prevention of cancer, cardiovascular disease, or other causes of death after controlling for health factors like abdominal obesity.
“What we have is clinical trial evidence that for the most part vitamin D doesn’t seem to be helpful for conditions where people thought it might,” says Charles Eaton, professor of family medicine and of epidemiology at Brown University.
“The best we can tell is that there isn’t an association. Once we took into account these other factors, high levels didn’t provide a benefit and low levels didn’t put you at risk.”
In the study, published online Oct. 26 in the American Journal of Clinical Nutrition, Eaton led an analysis of data from 2,429 postmenopausal women aged 50 to 79 who participated in the broad-based Women’s Health Initiative study, that tracked blood levels of vitamin D in the women and their mortality over a 10-year period. They not only looked at death from all causes but also focused on cancer and cardiovascular disease.
In all, 225 of the women died, including 79 from cardiovascular disease and 62 from cancer.
The researchers expected to find some protective effect against such mortality from vitamin D, and at first glance—controlling only for age, ethnicity, and whether women took part in a calcium and vitamin D supplement trial—that’s what the data showed.
But what was apparent in the data was that the women with the lowest levels of vitamin D also had a lot of other negative health indicators.
The team therefore controlled for several more key health factors, such as smoking, history of cardiovascular disease, history of cancer, alcohol consumption, and waist circumference. The additional controls, especially waist circumference, which is a measure of abdominal obesity, eroded the statistical significance of vitamin D’s seemingly protective effects down to nothing.
The one exception was that women with thinner waistlines (less than 35 inches) and with the lowest vitamin D levels seemed to have a greater risk of “all-cause” mortality within the 10-year analysis period. That result, however, was right on the borderline of statistical significance.
“If you are thin, this data suggest that maybe low vitamin D levels are potentially harmful and you should talk to your doctor about what to do about them,” Eaton says.
The researchers say they can only speculate about why abdominal obesity was an especially important and powerful factor to control for in their analysis. The study shows that abdominal obesity is associated with several negative health indicators that may overwhelm any modest benefit vitamin D might have. Also, fat tissue can store vitamin D, possibly meaning that women with larger waistlines are storing more of the vitamin than their blood serum levels alone would reveal.
More research into the connections between abdominal fat and the health effects of vitamin D could help resolve the question, Eaton says. He also says that a major new trial of vitamin D supplements and health called “VITAL” is getting under way and will likely inform the broader controversy about the benefits of vitamin D.
But for now, Eaton says, “there’s not enough evidence to do anything about our vitamin D levels if it’s not in regard to bone health.”
Researchers from Northwestern University, University of Pittsburgh, University of California–San Diego, University of California–Los Angeles, University of Iowa, George Washington University, University of Tennessee, University of Hawaii, and Harvard University contributed to the study.
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