Verification of protein sparing by feeding carbohydrate to common carp Cyprinus carpio

A 9-week feeding trial in floating freshwater cages (1.0 mx1.0 mx2.0 m) was conducted to study the effects of different dietary levels of protein and starch on growth, body composition, and gene expression of enzymes in common carp, Cyprinus carpio (mean body weight, 36.12 +/- 1.18 g) to evaluate the protein-sparing effect of dietary carbohydrate. Four diets were formulated with corn starch as the carbohydrate source to obtain corn starch levels of 6.5%, 13%, 19.5%, or 26% and protein levels of 30.5%, 28.2%, 26.4%, and 24.2%. The results showed no differences in growth performance of fish fed the diets with different protein and corn starch levels, but body composition and glucose metabolic enzyme activity of carp were significantly affected by the different diets (P<0.05). Weight gain, specific growth rate, and the feed conversion ratio were not different in fish fed the different dietary treatments. Protein efficiency ratio increased significantly as corn starch level increased (P<0.05). Whole-body crude lipid composition increased with increasing dietary corn starch level (P<0.05). Glucokinase (GK), hexokinase, and pyruvate kinase (PK) activities increased significantly with increasing dietary corn starch level (P<0.05), whereas glucose-6-phosphate (G6Pase) activity decreased with increasing dietary corn starch level (P<0.05). GK gene expression was significantly higher in fish fed the high-corn starch diet than those fed the low-corn starch diet (P<0.05). G6pase gene expression tended to decrease with increasing starch level (P>0.05). In summary, the results indicate a protein-sparing effect by substituting carbohydrate in the diet of common carp.