Abstract
Glutamine (Gln) supplementation during the weaning period can alleviate stress in piglets. However, free Gln has poor stability and low absorption in the small intestine. Glycyl-glutamine (Gly-Gln), a stable dipeptide form of Gln, has been evaluated as a potential alternative in pig nutrition. This study investigated the effects of Gly-Gln at 0, 0.125%, 0.25%, 0.375%, and 0.50%, as well as a Gly + Gln positive control, on growth performance, intestinal morphology, immunity, antioxidant status, and nutrient apparent digestibility in weaned piglets. The results showed that dietary supplementation with 0.25%, 0.375%, or 0.50% Gly-Gln significantly increased average daily gain, average daily feed intake, and final weight (p < 0.05). Linear and quadratic effects (p < 0.05) were observed for growth performance indicators, suggesting that moderate supplementation levels yielded optimal benefits. Dietary Gly-Gln supplementation with 0.25%, 0.375%, or 0.50% Gly-Gln significantly increased serum immunoglobulin (IgG, IgA, and IgM), insulin, insulin growth factor 1, growth hormone, and T4 and T3 contents, and decreased IFN-γ and IL-1β contents (p < 0.05). Diets supplemented with 0.25, 0.375, or 0.50% Gly-Gln increased total antioxidant capacity and superoxide dismutase content in serum and liver, and decreased MDA content (p < 0.05). Compared with the negative control group, dietary supplementation of 0.25%, 0.375%, or 0.50% Gly-Gln significantly increased the mRNA expression of ZO-1, Occludin, and Claudin-1 in the jejunum (p < 0.05). Furthermore, crude protein digestibility was significantly improved in piglets receiving 0.375% and 0.5% Gly-Gln (p < 0.05), with a significant linear relationship between Gly-Gln level and digestibility. In conclusion, 0.25% is the minimum effective dose of Gly-Gln for improving weaning outcomes. Gly-Gln is more effective than equivalent doses of free glycine and glutamine in enhancing growth performance, gut barrier integrity, and nutrient utilization in weaned piglets.