Some observational studies have suggested an increased risk of breast cancer (BC) among diabetic patients. A meta-analysis was performed to assess the risk of BC in diabetic patients compared to nondiabetic patients. Studies were selected if they had a prospective design. Studies that compared BC incidence in women with diabetes to the incidence in the general population were excluded. Summary relative risks (SRR) were computed using a random-effect model. Eighteen studies were included in the meta-analysis, including 28,230,143 person-years of follow-up and 320,111 BC cases. A significantly increased risk of BC among diabetic patients compared to nondiabetic patients was found: SRR=1.13 (95% CI: 1.04, 1.24). There was a large amount of unexplained heterogeneity of results across studies (I²=95%), but no indication of publication bias. The SRR remained similar when the analysis was restricted to post-menopausal women (SRR=1.12 (95% CI: 0.99, 1.26)). Only two studies reported data in pre-menopausal women. In the 9 studies that adjusted for adiposity, both the SRR and the heterogeneity of results across studies substantially decreased (SRR=1.(95% CI: 0.97, 1.14), I²=21%). In contrast, in the 9 studies that did not adjust for adiposity, both the SRR and the heterogeneity increased (SRR=1.19 (95% CI: 1.01, 1.39), I²=98%). Only two studies included 1,723 and 14 BC cases among type 1 diabetic women. The RR for BC were 0.90 (95% CI: 0.85, 0.94) and RR=1.01 (95% CI: 0.60, 1.71), respectively. This analysis provides evidence for a moderately increased risk of BC in diabetic patients. The effect of the adjustment for BMI on the SRR and on the heterogeneity suggests that the increased BC risk appears to be linked to adiposity, and not to being diabetic. New studies should collect data allowing the assessment of Breast Cancer risk according to the duration of diabetes and treatments that favour weight loss.
M. Bota: None. P. Autier: Other Relationship; Self; Sanofi. P. Boyle: Other Relationship; Self; Sanofi.