Effects of dietary protein sources on growth performance and feed intake regulation of grass carp (Ctenopharyngodon idellus)

This experiment was conducted to study the effects of different protein sources on growth performance and feed intake regulation of pre-adult grass carp (initial body weight: 153.4 ± 0.3 g), a herbivorous species. Two isonitrogenous, isoenergetic with essential amino acids and available phosphorus balanced diets based on a fish meal (FM) or plant protein blend (PPB) were designed. All fish were fed with FM diet for 4 w to acclimatize to the system and the feeding rhythm. The plasma, hypothalamus, intestinal bulb, and mid-intestine samples were collected at postprandial 3 h (P3h) and 24 h (P24h) after feeding for 7 w. The results showed that final body weight, weight gain rate and specific growth rate of fish in PPB group were significantly higher than those in FM group, and the feed conversion rate was significantly decreased. The dramatic changes in weekly feed intake for PPB group were observed, with significantly lower at 1–3 w, then kept flat at 4 w, and finally higher than that in FM group at 5–7 w. The mRNA levels of feeding regulatory genes in the periphery and central nervous system (CNS) showed that the feeding enhancing genes, ghrelin and AgRP were upregulated, combined with feeding-suppressing genes, like leptin, POMC, PYY were down-regulated in PPB group. Similar trends were validated by metabolic levels, with the increased plasma ghrelin and NPY and the decreased leptin contents in PPB group at P24h. It can be concluded that the differential regulations of appetite factors in periphery and CNS were involved in the reduced appetite of the FM group and actively foraging status of PPB group. Further analyses on taste receptors in the mid-intestine suggested that upregulation of sweet receptors T1R2a was the main reason for the selective preference for the PPB diet after 7 w feeding. Taken together, the feeding habits shift can provide insights into grass carp evolution and vegetarian adaptation.