Effects of variable protein and lipid proportion in gilthead sea bream (Sparus aurata) diets on fillet structure and quality

Juvenile gilthead sea bream (Sparus aurata) were fed with seven experimental diets containing different proportions of protein and lipid (P/L): 38/29, 41/26, 44/25, 47/23, 50/22, 54/21 and 57/20 per cent of dry matter, respectively. After 12 weeks, fish fed on diet 38/29 (P/L) showed the lowest food efficiency and final weight. A correlation between the hepatosomatic index and fat content in the diets was observed, together with a decrease in muscle glycogen in animals fed with a 38/29 (P/L) diet. Whilst no statistical differences were found in flesh texture, a clear relationship was observed between elasticity and fibre density. Fillet Chroma was altered in the animals fed with the 38/29 (P/L) diet. The same group showed a decrease in hepatic growth factor receptor (c‐met) and proliferating cell nuclear antigen (PCNA) expression in white muscle. Protein content in the diets correlated with mammalian target of rapamycin (mTOR) phosphorylation in muscle. In conclusion, we have shown that the substitution of protein by lipid in sea bream diets can be performed up to a certain percentage without affecting fish growth, and we have demonstrated that several molecules such as MyoD, mTOR, PCNA and c‐met are affected by the dietary ratio of protein to lipid and could be potential flesh quality markers.