Potential protective effect of lactation against incidence of type 2 diabetes mellitus in women with previous gestational diabetes mellitus: A systematic review and meta‐analysis

Lactation may protect women with previous gestational diabetes mellitus (GDM) from developing type 2 diabetes mellitus, but the results of existing studies are inconsistent, ranging from null to beneficial. We aimed to conduct a systematic review to gather available evidence. Databases MEDLINE, CINAHL, PubMed, and EMBASE were searched on December 15, 2015, without restriction of language or publication year. A manual search was also conducted. We included observational studies (cross‐sectional, case‐control, and cohort study) with information on lactation and type 2 diabetes mellitus incidence among women with previous GDM. We excluded case studies without control data. Data synthesis was conducted by random‐effect meta‐analysis. Fourteen reports of 9 studies were included. Overall risk of bias using RoBANS ranged from low to unclear. Longer lactation for more than 4 to 12 weeks postpartum had risk reduction of type 2 diabetes mellitus compared with shorter lactation (OR 0.77, 95% CI 0.01‐55.86; OR 0.56, 95% CI 0.35‐0.89; OR 0.22, 95% CI 0.13‐0.36; type 2 diabetes mellitus evaluation time < 2 y, 2‐5 y, and >5 y, respectively). Exclusive lactation for more than 6 to 9 weeks postpartum also had lower risk of type 2 diabetes mellitus compared with exclusive formula (OR 0.42, 95% CI 0.22‐0.81). The findings support the evidence that longer and exclusive lactation may be beneficial for type 2 diabetes mellitus prevention in women with previous GDM. However, the evidence relies only on observational studies. Therefore, further studies are required to address the true causal effect.