Background: Dietary metabolizable energy (ME) and crude protein (CP) levels are important for lactation performance and metabolic responses in dairy ruminants. This study aimed to evaluate the effects of dietary ME and CP levels on lactation performance and postprandial metabolic responses in lactating dairy goats. Methods: Goats were randomly assigned to a 4 × 4 two-factor Latin square experiment consisting of four 14 d periods. The dietary treatments were high energy, high protein (HEHCP); high energy, low protein (HELCP); low energy, high protein (LEHCP); and low energy, low protein (LELCP). Serial postprandial arterial blood samples were collected at 17 daytime time points to characterize temporal changes in plasma amino acids, biochemical parameters and hormones. Results: Increasing CP supply elevated milk yield (+6%) and lactose yield (+5%) but decreased milk fat yield (−8%; p ≤ 0.04). Increasing ME supply tended to enhance milk yield and milk fat yield and increased milk lactose content only under the high CP condition (ME × CP interaction: p = 0.04), suggesting that the response to ME supply depended partly on dietary CP level. High CP increased plasma branched chain amino acid concentrations, whereas high ME reduced Leu and Val under the high CP condition. Most plasma amino acids exhibited marked postprandial dynamics, decreasing initially and then stabilizing, with CP × time interactions observed for Leu, Met, and Phe (p ≤ 0.05). High ME decreased plasma AST activity and tended to reduce urea N concentration and also reduced ALT activity and increased glucose concentration under the high CP diet. Following morning feeding, plasma urea N showed a progressive postprandial decline (p = 0.02), with glucagon decreasing and both prolactin and growth hormone increasing, despite no dietary effects on mean plasma hormone concentrations. Conclusions: Overall, dietary ME and CP levels affected lactation performance and selected plasma metabolic indicators in lactating dairy goats. Coordinated energy and protein supply should be considered when formulating diets for lactating dairy goats. Serial postprandial sampling further revealed temporal changes in plasma metabolites and hormones, providing useful information for refining precision nutrition strategies during lactation.

Background: Branched-chain amino acid (BCAA) is involved in glucose and lipid metabolism and energy homeostasis, but whether its effects on maternal metabolism and offspring growth during lactation depend on dietary protein level remains unclear. Methods: C57BL/6J lactating dams were randomized after delivery to a normal-protein control group (CN, 20% protein), low-protein group (LP, 10% protein), CN + BCAA group (CNB), or LP + BCAA group (LPB) (n = 11/group). Maternal body composition, glucose and lipid metabolism, hepatic energy status, oxidative stress indices, and pup litter weight gain were assessed. Results: The LP diet reduced maternal body weight, pup litter weight gain, and glucose responses during the intraperitoneal pyruvate tolerance test (IPPTT), and increased hepatic lipid deposition and oxidative stress. BCAA supplementation increased pup litter weight gain under normal-protein conditions, but decreased maternal food intake and offspring growth under low-protein conditions. Metabolically, BCAA supplementation reduced the homeostasis model assessment of insulin resistance (HOMA-IR) and hepatic PEPCK1 activity under normal-protein conditions, while partially restoring IPPTT glucose responses and increasing hepatic PEPCK1 activity under low-protein conditions. In addition, BCAA supplementation reduced hepatic lipid deposition and ATP content at both protein levels. Notably, under low-protein conditions, BCAA supplementation increased serum β-hydroxybutyrate (β-HB), while alleviating hepatic oxidative stress. Conclusions: BCAA supplementation exerted opposite effects on offspring growth during lactation under different maternal dietary protein levels. These effects may be related to differential changes in maternal glucose and lipid metabolism and hepatic energy status, suggesting that BCAA supplementation during lactation should be evaluated according to maternal dietary protein level.