Vitamin D and Calcium Supplements: What Do the Data Show? Who Should Be Treated?
Vitamin D and calcium supplementation may lower fracture risk in some individuals, but data regarding its effects on cancer are inconclusive.
Vitamin D is essential for human health. Deficiencies are associated with osteomalacia in adults and rickets in children and have been postulated to contribute to various cancers. The three major sources of vitamin D are diet; ultraviolet B radiation, primarily from the sun; and pharmacologic supplements. The extent to which vitamin D supplements, with or without calcium, can prevent various cancers and reduce fracture incidence in high-risk individuals has been controversial. The authors of this meta-analysis reviewed randomized, controlled trials and observational studies reporting incidence of or death from cancer and fracture outcomes.
Results of three randomized clinical trials of the effect of vitamin D supplementation showed no discernable improvement in outcomes of breast, prostate, or all cancers compared with placebo, although such effects could not be definitively excluded. In randomized, controlled trials of combined vitamin D and calcium supplementation, results were conflicting. An analysis of the association between cancer and vitamin D status in 28 observational trials showed an increased risk for total cancer mortality among men with higher baseline vitamin D concentrations, but not among women. When different cancer types were examined individually, most studies found that higher vitamin D levels were associated with lower risk for colorectal cancer. No relationships between vitamin D and breast or prostate cancer were identified.
Sixteen randomized clinical trials evaluated vitamin D effects on fractures. Compared with placebo, vitamin D supplementation alone did not diminish fracture risk, but combined vitamin D calcium supplementation was associated with a decrease in fractures. Subgroup analysis found the greatest reduction among institutionalized individuals and less improvement among postmenopausal women and community-dwelling elders. No reduction in fracture risk occurred among community-dwelling women who had already had a fracture.
Comment: These results largely confirm conclusions of the Institute of Medicine that vitamin D and calcium supplementation may lower fracture risk in some individuals, but data regarding its effects on cancer are inconclusive. This does not mean such effects do not exist, merely that studies so far have been unable to identify a consistent relationship between vitamin D supplementation and cancer. Unresolved issues in measuring vitamin D status, setting optimal vitamin D blood concentrations, defining patient populations, and identifying vitamin D doses complicate matters. It should be noted that vitamin D supplementation is not without adverse effects. The Women’s Health Initiative, which enrolled 161,808 generally healthy postmenopausal women, demonstrated an association of vitamin D supplementation (with or without calcium) with increases in kidney and urinary tract stones. Until more-conclusive information is available on this complex topic, vitamin D and calcium supplementation should be reserved for individuals, such as those with high risk for colon cancer and institutionalized elders, in whom a benefit is clear cut.
Craig A. Elmets, MD
Published inJournal Watch Dermatology January 27, 2012