Simple SummaryIn recent years, affected by the international trade situation, China’s corn import volume and import price have risen sharply, which has increased the production cost of poultry farming. Poultry nutritionists have been encouraged to use different grains instead of corn. However, it is unclear whether feeding broiler chickens with different starch sources affects the utilization of digestible lysine (dLys) in their body due to differences in digestion rate, thereby affecting broiler breast muscle protein and liver lipid metabolism. Therefore, this study aims at the above-mentioned problems. Here, we found that the waxy corn starch diet resulted in significantly higher expression levels of fat-synthesis-related genes than lipolysis-related genes, leading to abdominal fat deposition in broilers. Increasing the level of dLys in the diet increased the protein content in muscle by promoting protein synthesis and inhibiting protein degradation and also promoted the expression of lipolysis-related genes, thereby degrading the generation of abdominal fat in broilers. In conclusion, our findings signify that increasing the dLys level to 1.32% when using the waxy corn starch diet could improve carcass traits.AbstractThis study investigated the effects of digestible lysine (dLys) in different dietary starch sources on liver lipid metabolism and breast muscle protein metabolism in broiler chickens. The experimental design was a 3 × 3 two-factor completely randomized design. A total of 702 one-day-old maleArbor Acres Plusbroilers were randomly divided into nine treatments of six replicate cages with thirteen birds each. The treatments consisted of three different starch sources (corn, cassava and waxy corn) with three different dLys levels (1.08%, 1.20% and 1.32%). The trial lasted from 1 to 21 days. Carcass traits, serum metabolites, breast muscle protein and liver lipid metabolism were evaluated. A significant interaction effect (p< 0.05) for dietary starch sources and dLys levels was noted in the percentage of abdominal fat and gene expression related to breast muscle protein metabolism throughout the experimental period. The waxy corn starch diet and a 1.08% dLys level in the diet increased both the percentage of abdominal fat (p< 0.01) and blood total cholesterol (p< 0.05) in the broilers. The waxy corn starch diet significantly upregulated the mRNA expressions ofEif4E,AMPK,FABP1,ACCandCPT1(p< 0.05). The 1.32% dLys level significantly upregulated the mRNA expressions ofmTOR,S6K1,Eif4E,AMPKandPPARα(p< 0.05) and significantly downregulated the mRNA expressions ofMuRFandAtrogin-1(p< 0.05). In summary, the waxy corn starch diet resulted in significantly higher expression levels of fat-synthesis-related genes than lipolysis-related genes, leading to abdominal fat deposition in broilers. Increasing the level of dLys in the diet increased the protein content in muscle by promoting protein synthesis and inhibiting protein degradation and also promoted the expression of lipolysis-related genes, thereby degrading the generation of abdominal fat in broilers. Our findings signify that increasing the dLys level to 1.32% when using the waxy corn starch diet could improve carcass traits.