BACKGROUND: Studies suggest that vitamin D supplementation may reduce cancer and fracture risks.
PURPOSE: To examine the benefits and harms of vitamin D with or without calcium supplementation on clinical outcomes of cancer and fractures in adults.
DATA SOURCES: English-language studies identified from MEDLINE and the Cochrane Central Register of Controlled Trials through July 2011.
STUDY SELECTION: Randomized, controlled trials (RCTs), prospective cohort studies, and nested case-control studies reporting incidence of or death from cancer and fracture outcomes.
DATA EXTRACTION: Multiple reviewers extracted details about participant characteristics, including baseline vitamin D status and use of supplements; details of statistical analyses, including adjustments for confounding; and methodological quality. Differences were resolved by consensus.
DATA SYNTHESIS: 19 RCTs (3 for cancer and 16 for fracture outcomes) and 28 observational studies (for cancer outcomes) were analyzed. Limited data from RCTs suggested that high-dose (1000 IU/d) vitamin D supplementation can reduce the risk for total cancer, and data from observational studies suggested that higher blood 25-hydroxyvitamin D (25-[OH]D) concentrations might be associated with increased risk for cancer. Mixed-effects dose-response meta-analyses showed that each 10-nmol/L increase in blood 25-(OH)D concentration was associated with a 6% (95% CI, 3% to 9%) reduced risk for colorectal cancer but no statistically significant dose-response relationships for prostate and breast cancer. Random-effects model meta-analysis showed that combined vitamin D and calcium supplementation reduced fracture risk (pooled relative risk, 0.88 [CI, 0.78 to 0.99]) in older adults, but the effects differed according to study setting: institution (relative risk, 0.71 [CI, 0.57 to 0.89]) versus community-dwelling (relative risk, 0.89 [CI, 0.76 to 1.04]). One RCT showed adverse outcomes associated with supplementation, including increased risk for renal and urinary tract stones.
LIMITATIONS: Most trial participants were older (aged=65 years) postmenopausal women. Observational studies were heterogeneous and were limited by potential confounders.
CONCLUSION: Combined vitamin D and calcium supplementation can reduce fracture risk, but the effects may be smaller among community-dwelling older adults than among institutionalized elderly persons. Appropriate dose and dosing regimens, however, require further study. Evidence is not sufficiently robust to draw conclusions regarding the benefits or harms of vitamin D supplementation for the prevention of cancer.
PRIMARY FUNDING SOURCE: Agency for Healthcare Research and Quality.
This meta-analysis matches what many of us suspected, namely that vitamin D is of some help with bone health, and perhaps more with patients who are less physically active, but not by a large margin.
Useful summary of what we know and why we don't know about vitamin D supplementation.
Vitamin D has been a hot topic for a while now and this article brings to light the data specifically regarding how it (plus calcium) affects fracture risk and cancer cases. The fact that many studies, including the few RCTs found, were of fair to poor quality is notable. Heterogeneity was high, perhaps because the key questions were somewhat unfocused. For example, the conclusion that no clear evidence exists for the effects on cancer cases is true, but there were significant differences between the types of cancer investigated (breast, prostate, CRC). There was very little evidence regarding potential harm. All in all, I think it's very helpful to know what evidence does and doesn't exist for therapies that many patients consider to be highly effective and clearly helpful for them, even if it is heterogeneous, observational, and lacking data regarding harm.
The study is interesting and important because it raises questions that unfortunately are not answered, yet. Further studies are needed.