Dietary iron intake and the risk of type 2 diabetes: a systematic review and dose–response meta-analysis of prospective cohort studies

PurposeWe aimed to assess the long-term association of total, heme, non-heme, and supplemental iron intake and risk of type 2 diabetes (T2D).MethodsPubMed, Scopus, and Web of Science were searched to October 2021. Two researchers extracted data in duplicate and rated the certainty in the estimates using the GRADE approach. Random-effects models were applied to estimate the relative risks (RRs) and 95% CIs. Dose–response associations were modeled by a one-stage weighted mixed-effects meta-analysis.ResultsEleven prospective cohort studies 323,788 participants and 28,837 incident cases of T2D were included. High versus low category meta-analysis indicated that higher heme iron intake was associated with a 20% higher risk of T2D (95% CI 1.07, 1.35; I2 = 77%, n = 11; GRADE = moderate). Dose–response analysis indicated a positive monotonic association, wherein each 1 mg/day increment in heme iron intake was related to a 16% higher risk (95% CI 1.03, 1.30). No significant relationship was detected between dietary intakes of total, non-heme, and supplemental iron and risk of T2D (GRADE = very low).ConclusionsIn summary, higher heme iron intake was associated with a higher risk of T2D. Our results are in line with existing evidence indicating that adopting a Western-style dietary pattern, rich in dietary sources of heme iron, was associated with a higher risk of T2D.Registry and registry numberThe protocol of this systematic review was registered at PROSPERO (registration number: CRD42021226835).