Background Several observational studies have been conducted to investigate the link between anaemia and adult depression but have shown inconsistent results. This systematic review and meta-analysis was performed to investigate this association.

Methods A comprehensive search of four electronic databases (PubMed, EMBASE, PsycINFO and the Cochrane Library) was conducted to identify relevant papers published up to November 2019 for the systematic review and meta-analysis. The Newcastle-Ottawa Scale was used to assess the methodological quality of selected studies. A random-effects model was used to draw metaestimates of the relationship between anaemia and adult depression.

Results A total of 9706 papers were retrieved, and 14 observational epidemiological studies (9 case–control studies and 5 prospective cohort studies) comprising 10 764 cases of depression were finally included in this meta-analysis. The mean age of the participants ranged from 38.4 to 75.0 years. A significant association was identified between low haemoglobin levels and adult depression (OR or relative risk 1.43; 95% CI 1.23 to 1.65). Subgroup analyses according to study design, mean age, diagnostic criteria of anaemia, geographical region, number of participants, methodological quality and adjustment for various confounding factors such as education, smoking, comorbid disorders, physical activity, alcohol intake and medication use showed similar results.

Conclusions The current study showed that anaemia was related to an increased risk of adult depression. One of the important limitations of our study was a moderate degree of heterogeneity due to the variety of assessment tools used to identify depression and the existence of publication bias. Another limitation of this meta-analysis was the lack of prospective cohort studies.