Effects of l-carnitine supplementation in diet on growth, lipid metabolism, and carnitine biosynthesis in juvenile giant grouper (Epinephelus lanceolatus) fed high plant protein diets

The present study investigated the effects of supplementing a high plant-protein diet with carnitine on growth performance, body composition, physiological parameters, and lipid metabolism in juvenile giant grouper (Epinephelus lanceolatus). l-carnitine was added to a basal diet with soy protein concentrate (SPC), which replaced 50 % of the fish meal protein, at analyzed carnitine levels of 0.07, 0.18, 0.83, 1.58, and 2.13 g/kg. An all fish meal diet (FM) was used as a reference. Lysine and methionine levels in SPC diets were adjusted to be comparable to those in the reference diet. Triplicate groups of juvenile grouper (initial weight: 19.50 ± 0.03 g) were fed one of six experimental diets and reared in a recirculating aquaculture system for 8 weeks. Weight gain and fish in-fish out ratio were higher (p < 0.05) in fish fed the reference diet than those in fish fed diets containing SPC. Carnitine supplementation in the diet had no significant impact on growth performance, body composition, hepatic oxidative status, or blood biochemical indices. Increasing dietary carnitine levels were associated with a decreasing trend in hepatic gamma-butyrobetaine hydroxylase 1 (BBOX1) gene expression, and an increasing trend in hepatic and muscle carnitine concentrations. This study confirms that giant grouper can endogenously synthesize carnitine via lysine metabolism and BBOX1 activity to meet physiological requirements. Even when fed diets with SPC and deficient in carnitine, juvenile giant grouper do not require additional dietary carnitine supplementation when adequate amounts of methionine and lysine are supplemented.