The aim of these studies was to determine the metabolic adaptation to fasting and feeding during lactation. Normal lactating (L) and nonlactating (NL) women (n = 6 each) were studied using infusions of [U-13C]glucose and [2-13C]glycerol during: 1) a 24-h fast, and 2) ingestion of Sustacal (protocol 1). In addition, 8 L and 6 NL women were studied during infusion of [6,6-2H2]glucose and ingestion of a glucose meal containing [1-13C]glucose (protocol 2). Protocol 1: Glucose production rate (GPR) during fasting was 33% higher in the L women (12.5 +/- 1.0 vs. 9.4 +/- 0.5 micromol x kg(-1) x min(-1); P < 0.03). Fractional gluconeogenesis (GNG), GNG rate, glucose, lactate, beta- hydroxybutyrate, FFA, insulin, and C-peptide were similar in both groups during feeding and fasting, but glycogenolysis was 50% higher in fasting L women. Protocol 2: Although GPR was slightly increased in the L group (L, 1.8 +/- 0.2 micromol x kg(-1) x min(-1); NL, 1.2 +/- 0.2 micromol x kg(-1) x min(-1); P < 0.04), no other differences were observed in splanchnic and systemic metabolism of ingested glucose between L and NL women. Insulin concentrations were lower in L women compared with controls (L, 15 +/- 3 microU/ml; NL, 28 +/- 6 microU/ml; P = 0.05). In conclusion, the increased glucose demands of lactation are met by increased GPR as a result of increased glycogenolysis but not GNG or by increased use of FFA. During feeding, lactating women handle oral carbohydrates normally but have increased insulin sensitivity.