Vitamin D for Bones and Beyond?
From Johns Hopkins Medical Letter - February 2008
Vitamin D helps the body absorb calcium, which in turn aids in the formation and maintenance of strong teeth and bones. It also helps keep muscles strong. But could this vitamin have even bigger benefits? A flood of recent research has explored the potential of vitamin D.
Vitamin D and Cancer
Two recent studies shed light on the somewhat spotty past evidence that suggests calcium and vitamin D have a role in cancer prevention. The first study, published in The American Journal of Clinical Nutrition, enrolled 1,024 healthy postmenopausal women over 55. The women were randomly assigned to receive calcium, calcium plus 1,100 IU of vitamin D, or placebo. Four years later, both supplement groups had lower rates of cancer than the placebo group, but the largest difference was between the placebo group and the group that took calcium plus vitamin D: 7% vs. 3%, respectively.
In a second study, published by the Archives of Internal Medicine, Harvard researchers revisited data collected on 31,487 women enrolled in the landmark Wonmen’s Health Study. Investigators observed diet, supplement use, and whether or not women developed breast cancer over a 10-year period. Premenopausal women who took at least 548 IU of Vitamin D a day were 35% less likely to develop breast cancer than premenopausal women who took the least amount of vitamin D — less than 162 IU. There was no difference in the development of breast cancer among postmenopausal women.
Michele Bellantoni, M.D., Associate Professor of Medicine, Medical Director, John Hopkins Bayview Care Centre, believes that if vitamin D does play a role in cancer prevention, it is a minor one, at most. She points out that the first study was very small, and so were the differences.
Although the second study found a decreased risk in premenopausal women, postmenopausal women are at greater risk for breast cancer. Moreover, explains Dr. Bellantoni, "these findings resulted from an analysis of data gathered from a study that was not designed to measure the association between vitamin D and breast cancer. Thus, it's difficult to assess how significant these new results really are."
Vitamin D and Hypertension
In 2007, the American Journal of Hypertension published findings from the Third National Health and Nutrition Examination Survey, which followed the health and dietary habits of 12,644 adults over a period of 6 years. Researchers found that blood pressure was lower in the participants with the highest blood levels of vitamin D and higher in participants who had the lowest vitamin D levels.
"The large number of study participants adds some credence to these findings," says Dr. Bellantoni. "It's not so far-fetched to think that vitamin D may play a role in blood pressure, because calcium is an important mineral for the vascular system. If you're not getting enough vitamin D, your body may not absorb calcium well, so your blood pressure might suffer a bit."
Vitamin D and Type 2 Diabetes
A 2007 study in The Journal of Clinical endocrinology and Metabolism examined a connection between vitamin D and diabetes. The evidence, collected by researchers at the Harvard School of Public Health and Tufts-New England Medical Center, suggested that taking vitamin D improved the body's response to insulin.
But, Dr. Bellantoni points out, "Vitamin D levels are not a major predictor of whether or not someone will develop diabetes. Obesity and lack of physical activity are far mor important factors. Also, this was a meta-analysis; its conclusions were drawn by grouping together data from multiple, varied studies." Meta-analyses can be helpful, especially in identifying risk factors (or risk reducers) and treatment side effects that warrant further study; however, they are less reliable than clinical trials that enroll participants to measure specific treatment effects against a controlled group.
Data Roundup
Dr. Bellantoni believes that none of these studies is definitive. But, she adds, "What this research does offer is insight into how our body functions and our metabolism of vitamins and minerals are interrelated. People should look at these studies as examples of how complex our bodies are and how multiple system overlap."
These interrelationships extend to the immune system, where vitamin D is believed to aid in the production of antimicrobial substance. This immune function may play a role in the recent finding, from European researchers writing for the Archives of Internal Medicine, that people who took vitamin D for at least 3 years had a lower risk of dying from all cause. Caveat: This study was also a meta-analysis. It’s much too soon to know if vitamin D has any impact on longevity.
The Bottom Line
Vitamin D may have broad effects because, unlike other vitamins, it is a hormone. An increasing number of researchers are trying to pinpoint the vitamin's exact benefits. "But right now, we really don't need any other reason to take vitamin D beyond its proven bone benefits," says Dr. Bellantoni. She recommends 800 IU daily for anyone over the age of 50.
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