Dietary Guanidinoacetic Acid Improves Meat Tenderness and Antioxidant Capacity in Rabbits via Modulating Muscle Fiber Characteristics and Fat Metabolism

Guanidinoacetic acid (GAA) is a direct precursor of creatine and plays a key role in en-ergy and protein metabolism. Rabbit meat is increasingly recognized as a healthy food source due to its high protein and low fat content, and improving its quality is of growing interest to consumers and producers alike. This study investigated the effects of dietary GAA supplementation on meat quality, antioxidant capacity, muscle fiber characteristics and fatty acid metabolism in rabbits. A total of 960 weaned male rabbits were assigned to two age groups (40 ± 2 days, 1.19 ± 0.09 kg; 60 ± 2 days, 1.82 ± 0.15 kg). Within each age group, rabbits were randomly allocated to a control diet or a diet supplemented with 100 mg/kg GAA (CON-40, GAA-40, CON-60, GAA-60). After a 45-day feeding period, GAA supplementation significantly improved meat tenderness, as evidenced by reduced shear force in 60-day-old rabbits (p < 0.01), and decreased muscle fiber area and density in 40-day-old rabbits (p < 0.05). Moreover, GAA enhanced systemic antioxidant capacity, increasing serum superoxide dismutase (SOD) activity and total antioxidant capacity (T-AOC) (p < 0.05), while reducing malondialdehyde (MDA) levels in 60-day-old rabbits (p < 0.05). GAA also modulated the expression of genes involved in lipid metabolism (FAS, HSL, ACC) in intramuscular and perirenal fat, suggesting a regulatory role in fatty acid metabolism. In conclusion, dietary GAA supplementation improves meat tenderness and antioxidant capacity in rabbits without compromising growth performance. These findings support the potential of GAA as a nutritional strategy to enhance the quality of rabbit meat as a functional food for human consumption.