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Objective: Gestational diabetes (GDM) is one of the most common complications during pregnancy, up to 12% or more of all pregnant women. It is well established evidence that women affected by GDM during pregnancy are high risk of the development of diabetes after delivery. Several studies showed that breastfeeding has a protective effect against the development of diabetes in general population and in women with GDM during pregnancy. However, underlying mechanism regarding the protective effect has not been clearly understood. Therefore, in this study, we investigated the effect of breastfeeding on insulin resistance during the first year postpartum in Japanese women with current gestational diabetes. Methods: In this retrospective study, we included women who were diagnosed as having GDM after 24 weeks' gestation during pregnancy and underwent postpartum 75g oral glucose tolerance test (OGTT) during the first year postpartum at a single tertiary perinatal care center in Japan. We used IADPSG criteria and WHO criteria to make a diagnosis of GDM during pregnancy and abnormal glucose tolerance (AGT) after delivery, respectively. We defined AGT as impaired glucose intolerance and diabetes by WHO criteria. Regarding breastfeeding conditions, we defined highintensity breastfeeding (HIB) as the condition in which infants were fed by breastfeeding alone or 80% or more of the volume, and other statuses, including partial and non-breastfeeding, as non-HIB. We investigated the effect of HIB on the prevalence of postpartum abnormal glucose tolerance and the postpartum homeostasis model of assessment of insulin resistance (HOMA-IR), after controlling for confounders, including prepregnancy obesity and weight changes during pregnancy and postpartum. Results: We included 88 women affected by GDM during pregnancy, and among them, 46 (52%) women had AGT during the postpartum period. Among the all participants, 70 (79.5%) women fed their infants by HIB at least 6 weeks or more during postpartum period (HIB group), and the other 18 (20.5%) women were in the non-HIB group. HIB women were significantly less likely to have AGT than non-HIB women (46% vs. 78%, p<0.02). Women in the HIB group were also associated with a lower HOMA-IR at one year postpartum than those in the non-HIB group (1.41±1.02 vs. 2.28±1.05, p=0.035). Regarding the duration of breastfeeding, we found that at least six months of HIB had a significant effect of both reducing the prevalence of the AGT and improving HOMA-IR during one year postpartum, in comparison with non-HIB. Conclusions: In Japanese women with GDM, HIB at least 6 months or more had a protective effect against the development of AGT during the first year postpartum through improving insulin resistance, independent of obesity and postpartum weight change. [ABSTRACT FROM AUTHOR]