Available data are not strong enough to show a definitive association between vitamin D levels and risk of obesity, diabetes, cancer, heart disease, or maternal-fetal health, according to a comprehensive review of available research.
"The efficacy issue [of vitamin D] remains a major question mark," said Dr. Clifford J. Rosen in an interview. He chaired the group that wrote the scientific statement published by Endocrine Society (Endocr. Rev. 2012 May 17 [doi:10.1210/er.2012-1000]).
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Do the benefits of vitamin D extend beyond bone health? Studies thus far can't say for sure.
The scientific statement takes a comprehensive look at basic and clinical evidence related to the effect of vitamin D and various organ systems.
Although some observational studies have shown that benefits of vitamin D may extend beyond bone health, the findings are inconsistent, the authors noted.
"We need large randomized controlled trials and dose-response data to test the effects of vitamin D on chronic disease outcomes including autoimmunity, obesity, diabetes, hypertension, and heart disease," said Dr. Rosen, professor of medicine at Tufts University, Boston, in a statement.
Dr. Robert H. Eckel, past president of the American Heart Association, said he agreed with the findings of the report. He said that physicians should be aware of low vitamin D levels and should correct the deficiencies, but they should not make any strong statements about vitamin D levels and risk for conditions such as heart disease or cancer, due to lack of sufficient evidence.
He added that ultimately, data from large, well-designed trials "may be informative in uncovering a relationship that’s more meaningful." Dr. Eckel, who is a professor of medicine at University of Colorado at Denver, Aurora, was not involved in the study.
Interest in vitamin D as a therapeutic option for the prevention of chronic disease has been growing in recent years, the authors noted. "In a 2-month span during the summer of 2011, there were more than 500 publications centered on vitamin D, most of which were related to its relationship to nonskeletal tissues," they wrote. But the results, they added, are confounded and difficult to interpret.
By organ or disorder, the authors came to the following conclusions:
• Skin. "There are no large-scale, randomized, placebo-controlled clinical trials demonstrating that vitamin D metabolites are superior to other types of treatment for various proliferative skin disorders or for the prevention of skin cancer."
• Obesity and diabetes mellitus. "The ever-expanding obesity epidemic has been associated with a rising prevalence of vitamin D deficiency, but a cause-and-effect relationship has not been established. ... There remains a paucity of randomized controlled trials of vitamin D for the prevention of diabetes; hence, few conclusions can be firmly established."
• Fall prevention and improvement in quality of life. Citing the public health implication of fall prevention, and the report by the Institute of Medicine, the authors wrote, "The absolute threshold level of [vitamin D] needed to prevent falls in an elderly population is not known in part because of lack of true dose-ranging studies. ... Selecting patients at risk for falls and defining the appropriate dose remains as areas in need of further research."
• Cancer. "Despite biological plausibility for a role of vitamin D in cancer prevention, most recent systematic reviews and meta-analyses, as well as a comprehensive review by the IOM Committee, have found that the evidence that vitamin D reduces cancer incidence and/or mortality is inconsistent and inconclusive as to causality."
• Cardiovascular disease. Although there is a possibility that vitamin D supplementation may lower cardiovascular risk, "additional research, particularly from randomized trials, is needed."
• The placenta and maternal-fetal health. "There is insufficient evidence to recommend a particular maternal intake of vitamin D or [serum 25-hydroxyvitamin D] blood level during pregnancy to achieve any purported nonskeletal benefit of vitamin D," the authors wrote, but they added that "the biological plausibility may be sufficient to justify clinical trials to test whether vitamin D supplementation during pregnancy will prevent type 1 diabetes in the offspring."
"We’re hopeful that some of the intervention trial [on vitamin D] will get underway, and although they’re expensive, their findings can help change practice," said Dr. Rosen.
Dr. Rosen reported no relevant conflicts of interest.