Guanidinoacetic acid in laying hen diets with varying dietary energy: Productivity, antioxidant status, yolk fatty acid profile, hepatic lipid metabolism, and gut health.

This study aimed to investigate the effects of GAA supplementation in diets differing in ME levels on productive performance, egg quality, blood parameters, yolk fatty acid profiles, hepatic expression of genes related to lipid metabolism, gut morphology, and nutrient digestibility in laying hens during their post-peak production phase. Over a 12-week period (52-64 weeks of age), 288 laying hens were randomly assigned to 6 treatments. Each treatment consisted of 8 replicates, with 6 hens per replicate. The experimental treatments were assigned in a 2 × 3 factorial arrangement, comprising 2 levels of dietary ME (a recommended level and a low level, the latter characterized by a 100 kcal/kg reduction in ME) and 3 levels of GAA supplementation (0, 0.6, and 1.2 g/kg). The results showed significant interaction effects (P < 0.05) between GAA supplementation and dietary ME levels on laying rate, egg mass, feed conversion ratio, crude protein digestibility, and AMEn. In hens fed the low-ME diet, GAA supplementation, particularly at 1.2 g/kg, significantly improved laying performance. Moreover, at both 0.6 and 1.2 g/kg under low-ME conditions, GAA significantly enhanced crude protein digestibility and AMEn. The low-ME diet was associated with decreased expression of key lipogenic genes, including sterol regulatory element-binding transcription factor 1 (SREBF1), acetyl-coenzyme A carboxylase (ACC), and fatty acid synthase (FAS), alongside increased expression of genes involved in fatty acid oxidation, such as peroxisome proliferator-activated receptor alpha (PPARα) and carnitine palmitoyltransferase 1 (CPT1). Regardless of ME content, GAA supplementation linearly improved eggshell strength, enhanced the polyunsaturated-to-saturated fatty acid ratio in the yolk, elevated serum levels of creatine and total antioxidant capacity, improved intestinal morphology, and increased radical scavenging activity in the yolk (P < 0.05). Furthermore, GAA supplementation linearly increased the relative mRNA expression of several metabolic genes, including SREBF1, ACC, PPARα, and ApoB (P < 0.05). In conclusion, GAA supplementation enhanced productive performance in low-ME diets and exerted positive effects on egg characteristics and lipid metabolism, regardless of dietary ME content.