Potential Benefits of Glycine, Proline and Hydroxyproline on Growth and Flesh Quality of Mirror Carp (Cyprinus carpio var. specularis)

Collagen content is a key determinant of flesh quality and directly influences consumer acceptance. This study investigated the effects of the main collagen amino acids, glycine, proline and hydroxyproline, on the growth performance and flesh quality of mirror carp. A total of 240 fish (initial weight: 174.4 g) were randomly assigned to a control and three treatment groups, each receiving 5 g of glycine, proline, or hydroxyproline per kilogram of feed for eight weeks. Measured parameters included growth performance, serum hormones, flesh quality traits, free amino acid profiles, and proteomics. Glycine and hydroxyproline increased serum GH and IGF-1 levels (p < 0.001), while somatostatin was differentially regulated across treatments. Hydroxyproline notably improved flesh quality by reducing drip loss and increasing muscle moisture and protein content (p = 0.022; p = 0.040; p = 0.026). Collagen levels in muscle and skin were elevated in all treatment groups (p < 0.05). Free amino acid analysis revealed elevated levels of leucine and taurine in the glycine group, increased taurine in the proline group, and elevated methionine and taurine in the hydroxyproline group (p < 0.05). Proteomic analysis identified differentially expressed proteins enriched in pathways including oxidative phosphorylation, glutathione metabolism, and valine, leucine and isoleucine degradation. These findings suggest that hydroxyproline plays a regulatory role in hormone secretion and muscle quality enhancement, offering new insights for optimizing aquafeed formulations.