Effects of different dietary metabolizable energy levels on growth performance, liver metabolism, and gut microbiota in growing broilers

This study investigated how graded dietary metabolizable energy (ME) levels affect growth performance, and metabolic regulation in broilers, with particular focus on oxidative status, hepatic metabolism, and gut microbiota. A total of 480 one-day-old male AA broilers were reared under standard conditions until 21 days of age, after which they were assigned to four dietary treatments containing 3050, 3150, 3250, or 3350 kcal/kg. Growth performance was not significantly affected by dietary ME, as body weight, average daily gain, average daily feed intake, and feed conversion ratio were similar among treatments ( P ANOVA > 0.05). However, dietary ME significantly influenced relative gizzard weight and immune organ indices, including the thymus, spleen, and bursa of Fabricius ( P ANOVA < 0.05). Higher dietary ME enhanced antioxidant capacity, reflected by increased serum SOD and GSH-Px activities, while TNF-α decreased with increasing ME ( P ANOVA < 0.05). Liver expression of genes involved in glucose metabolism, lipid metabolism, and insulin signaling was modulated by dietary ME. In the ileum, FABP6 expression increased with increasing dietary ME ( P ANOVA < 0.05), whereas FBP1 and INSR expression decreased ( P ANOVA < 0.05). Collectively, these findings suggest that moderate dietary ME levels may support metabolic stability and antioxidant capacity without affecting growth performance.