The plasma level of retinol, vitamins A, C and α-tocopherol could reduce breast cancer risk? A meta-analysis and meta-regression

Plasma antioxidants are supposed to be directly related to breast cancer risk. However, the results remain inconsistent. Herein, we carried this meta-analysis to comprehensively summarize the associations between plasma retinol, vitamins A, C and alpha-tocopherol and breast cancer risk. We searched PubMed, Embase and the Cochrane Databases (through September 24, 2014) and the reference lists of the retrieved articles in English with sufficient information to estimate relative risk or odds ratio and the 95 % confidence intervals (CIs), or with mean serum/plasma level of vitamins and SD/SEM/p value in breast cancer and controls. Two reviewers independently extracted data using a standardized form, with any discrepancy adjudicated by the third reviewer. Forty studies entered this meta-analysis. For the pooled OR, no significant association between plasma retinol and breast cancer was observed (p = 0.13). Significant association was observed between plasma alpha-tocopherol and breast cancer (pooled OR 0.42, 95 % CI 0.25, 0.72, p = 0.00) in the subgroup with the median lowest level of 5.74-9.16 mu mol/L. For the weighted mean difference (WMD), the plasma alpha-tocopherol and vitamin C level between breast cancer and controls were significantly different [WMD = -0.93 mu mol/L (95 % CI -1.26, -0.61, p = 0.00) and -2.51 mu mol/L (95 % CI -4.00, -1.02, p = 0.00), respectively]. No significant association between plasma retinol and vitamin A and breast cancer was observed. Severe alpha-tocopherol deficiency could increase breast cancer risk. The association between plasma vitamin C and breast cancer was only significant in case-control studies. There was no significant association between other vitamins and breast cancer risk.