Maternal Rumen-Protected Leucine Supplementation Enhances Placental Nutrient Transport Capacity and Increases Birth Weight in Hu Sheep

Leucine, an essential branched-chain amino acid, serves not only as a substrate for protein synthesis but also as a key regulator of placental function and fetal development. This study investigated the effects of dietary supplementation with RP-Leu during late gestation on placental development and offspring performance in Hu sheep. Sixty twin-pregnant ewes at day 80 of pregnancy were randomly assigned to either a control group (fed a basal diet) or an RP-Leu group (fed a basal diet supplemented with 19 g/day RP-Leu). The feeding trial lasted for 60 d. The ewes were slaughtered at day 140 of gestation. Maternal slaughter traits and fetal organ weights were recorded. Blood and milk samples were collected for milk composition analysis and targeted metabolomic profiling. Leucine supplementation significantly increased the percentage of milk fat content, total solid content, and the birth weight of lambs (p < 0.05). Improvements in placental morphology and antioxidant capacity were observed, including a significant increase in cotyledon density and a significant enhancement of catalase (CAT) activity (p < 0.05). Gene expression analysis indicated that the NOS3, SLC38A1 and FABP4 genes in the placental cotyledons (p < 0.05), and the VEGFA, NOS3, SLC27A1 and FABP4 genes were significantly upregulated in the maternal caruncles (p < 0.05). Plasma metabolomic profiling revealed increased L-glutamic acid levels and alterations in several amino acids, with pathway enrichment indicating involvement in amino acid metabolism and membrane transport processes. Transcriptomic analysis identified 739 differentially expressed genes, which were mainly enriched in the PI3K/Akt signaling pathway, ECM–receptor interaction pathway, and cytokine–cytokine receptor interaction pathway. Collectively, these findings suggest that RP-Leu supplementation during late gestation may enhance offspring growth by modulating amino acid metabolism, promoting placental development, and improving placental nutrient transport capacity, thereby supporting fetal growth and development.