Folate intake and the risk of prostate cancer: a systematic review and meta-analysis

There is conflicting evidence regarding the role of folate on the risk of developing prostate cancer. We performed a systematic review and quantitative meta-analysis of folate blood levels and folate intake, and the risk of prostate cancer. Four electronic databases (Medline, PubMed, Embase and Current Contents Connect) were searched to 11 October 2013, with no language restrictions for observational studies that measured folate intake or blood levels and the risk of prostate cancer. Pooled odds ratios (ORs) and 95% confidence intervals (95% Cls) were calculated using a random effects model. The dietary folate meta-analysis comprising 11 studies with 15 336 cases and a total sample size of 146 782 found no statistically significant association with prostate cancer, with an OR of 0.97 (95% Cl 0.89-1.06). The total folate meta-analysis comprising of 5 studies with 7114 cases and a total sample size of 93 781 also found no statistically significant association with prostate cancer, with an OR of 0.99 (95% Cl 0.82-1.19). The blood folate meta-analysis comprising of seven studies with 6122 cases and a total sample size of 10 232 found an increased risk of prostate cancer with high blood folate levels, with an OR of 1.43 (95% Cl 1.06-1.93). There was significant heterogeneity (l(2)= 79.5%, P<0.01). Removal of an outlier study removed the heterogeneity (l(2)= 0.0%, P=0.54) and the association remained significant with an OR of 1.14 (95% Cl 1.02-1.28). Dietary and total folate intake do not appear to be significantly associated with the risk of prostate cancer. High blood folate levels are associated with an increased risk of prostate cancer. These conclusions are limited by the predominance of included studies originating from developed countries with mostly Caucasian populations. Further research in populations with a high prevalence of non-Caucasian backgrounds is needed.