Subjects Nulliparous women with GDM and healthy women, who intended to exclusively breastfeed their singletons, were recruited consecutively from the obstetric wards at the Peking Union Medical College Hospital and the Beijing Obstetrics and Gynecology Hospital during the 37th gestational week. The exclusion criteria were: pre-pregnancy diabetes, fetal anomaly, gestational hypertension, preeclampsia, fetal growth restriction, ruptured membranes, postpartum glucose abnormalities (see below), and introduction of formula feeding during the follow-ups. Women with plasma glucose>7.8mmol/L in the 1-hour 50g glucose load test (GLT) during the 24th-28th gestational weeks underwent a 3-hour 100g diagnostic oral glucose tolerance test (OGTT) following a 12-hour overnight fast. GDM was diagnosed if two or more plasma glucose reads in the OGTT equaled or exceeded the threshold according to Carpenter and Coustan. All subjects diagnosed with GDM initially received dietary therapy to attain glycemic targets: 3.3-5.6 mmol/L at fasting, 3.3-5.8 mmol/L pre-prandially, 4.4-6.7 mmol/L 2-hours post-prandially, and 4.4-6.7 mmol/L at night. The participants were followed by dietitians to ensure euglycemia, appropriate weight gain, and adequate nutritional intake. The participants who did not attain glycemic targets in 2 weeks after starting the dietary therapy were given insulin via injection. As macronutrients in BM were mainly determined by the maternal glucose metabolic status, the investigators excluded women with postpartum glucose abnormalities, i.e., impaired glucose tolerance (IGT) with the 2-hour plasma glucose between 7.8 and 11.0 mmol/l and type 2 diabetes with the 2-hour plasma glucose ≥11.1 mmol/l in the 75g OGTT on postpartum day 42.