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.

Anthropometric measurements Data was recorded using customized case report forms. Obstetric data, such as glycemic tests, gestational age, and mode of delivery, were retrieved from the medical records. Pre-pregnancy weight was self-reported. Mothers' height was measured twice to the nearest 0.1 cm with a wall-mounted stadiometer. Mothers' weight was measured twice to the nearest 0.1 kg with a medical balance scale before delivery, on postpartum days 42 and 90. The weight, length, and head circumference of the infants were measured at birth, on days 42 and 90. The infants were weighed twice in nude using a precision scale (Seca, CA, USA). Body length and head circumference were measured twice to the nearest 0.1 cm with a length board and non-stretchable measuring tapes (Seca, CA, USA). The mean values of the two readings were used for data analysis.

Milk sample collection, processing, and laboratory tests Colostrum samples were collected between 8 a.m. and 9 a.m. before infant feeding on the third day after delivery. Mature milk, including both foremilk and hindmilk, was delivered and collected from one breast before infant feeding using an electric pump (Medela, Baar, Switzerland) between 2 p.m. and 4 p.m. on days 42 and 90 in the clinics. The milk samples were frozen immediately in sterilized plastic tubes at -80°C. Before quantifying the hormones in BM, the samples were thawed at 4°C, sonicated, and centrifuged. The samples were sonicated using a sonicator (Braun-sonic sonicator, B. Braun, Melsungen, Germany) at 50 watts for 3 bursts with 10-second intervals, and centrifuged at 100,000g for 1 hour at 4°C. The supernatant fat was discarded and the skim milk was used for quantifying adiponectin, leptin, insulin, and ghrelin by ELISA at the Key Laboratory of Endocrinology in the Peking Union Medical College Hospital. The assay had an intra- and inter-assay CV of \<5.4 and \<8.5% for adiponectin, and \<7.4% and \<9.3% for leptin, respectively. The insulin assay had no cross-reactivity to proinsulin (\<0.05%), and had sensitivity of 0.5 mU/L and an inter-assay CV of \<9.0%. Total ghrelin was tested using the total human ghrelin ELISA kit (Millipore, USA). The intra- and inter-assay CVs for the ghrelin assay were \<1.9% and \<7.7%.