Does vitamin D supplementation prevent cancer?
The available evidence on vitamin D and cancer occurrence is intriguing but inconclusive. Many observational studies as well as randomised trials suggest that high vitamin D levels in the blood are related to reduced cancer occurrence. However, results of randomised trials testing the effect of vitamin D supplementation for cancer prevention are contradictory.
The aim of this systematic review was to analyse the benefits and harms of the different forms of vitamin D especially on cancer occurrence. A total of 18 trials provided data for this review; 50,623 participants were randomly assigned to either vitamin D or placebo or no treatment. All trials were conducted in high-income countries.
The age range of the participants was 47 to 97 years and on average 81% were women. The majority of the included participants did not have vitamin D deficiency. Vitamin D administration lasted on average six years and most trial investigators used vitamin D₃(cholecalciferol). We did not find firm evidence that vitamin D supplementation decreases or increases cancer occurrence in predominantly elderly community-dwelling women. We observed decreases in all-cause mortality and cancer-related mortality among the vitamin D/D₃ treated participants in comparison with the participants in the control groups. However, using trial sequential analysis, a statistical approach to reconfirm or question these findings, we conclude that these results could be due to random errors (play of chance). We also found evidence that combined vitamin D₃ and calcium supplements increased renal stone occurrence, but it remains unclear from the included trials whether vitamin D₃, calcium, or both were responsible for this effect. Moreover, these results could also be due to random errors (play of chance).
Quality of the evidence
A large number of the study participants left the trials before completion, and this raises concerns regarding the validity of the results. Most of the trials were judged not to be well and fairly conducted so that the results were likely to be biased (that is, possibly an overestimation of benefits and underestimation of harms).
Currentness of evidence
This evidence is up to date as of February 2014.