Iron intake, serum iron indices and risk of colorectal adenomas: a meta-analysis of observational studies

To provide a quantitative assessment of the association between iron intake, serum iron indices and the risk of colorectal adenoma (CRA), we summarised the evidence from observational studies. Relevant studies were identified in MEDLINE and EMBASE until 31 March 2015. Summary relative risks (SRRs) with 95% confidence intervals (CIs) were pooled with a random-effects model. A total of 10 articles, involving 3318 subjects with CRA, were used in this meta-analysis. The SRR of CRA was 0.83 (95% CI: 0.71-0.98; P_{heterogeneity}  = 0.694, I²  = 0) for the highest versus the lowest level of dietary iron intake. The SRR was 0.93 (95% CI: 0.62-1.42) for total (dietary and supplemental) iron intake, 1.23 (95% CI: 1.03-1.48) for haem iron intake and 0.73 (95% CI: 0.54-0.97) for supplemental iron intake. Serum iron indices were not associated with CRA risk (serum ferritin: SRR = 1.16, 95% CI: 0.81-1.66; serum iron: SRR = 1.02, 95% CI: 0.75-1.38; serum transferrin saturation: SRR = 1.01; 95% CI: 0.82-1.50). Increased intake of haem iron is associated with significantly increased risk of CRA, whereas intake of non-haem or supplemental iron is inversely associated with risk of CRA. Limited data indicate null associations between serum iron indices and CRA risk.