Dietary methionine and choline mediate mobilisation of plasma lipids to improve feed efficiency for the diets of giant grouper (Epinephelus lanceolatus)

Giant grouper (Epinephelus lanceolatus) has become an increasingly popular candidate for aquaculture due to its high growth rates, disease resistance, and high production value. Despite these positive attributes its potential as an aquaculture species is limited by extensive knowledge gaps in its specific nutrient requirements needed for appropriate feed formulation. Methionine, an essential nutrient in feed ingredients, plays an important role in fish nutrition. Its complex metabolism suggests that its requirement may be influenced by other nutrients, such as the vitamin-like compound choline. This study aimed to investigate the interaction between dietary methionine and choline in regulating plasma lipid mobilisation, and how this affects feed efficiency in juvenile giant grouper. Two series of diets were tested: one with a constant methionine and varying choline levels, and another with a fixed choline (2.71 mg/g) and increasing levels of methionine. Both methionine and choline affected FCR, which appears to have been mediated by the utilisation of energy substrates, with shifts in circulating low-density lipoproteins (LDL), and triglycerides (TAG). These results appear to be related to energy derived from improved mobilisation of lipids to growing tissue, resulting in improved feed efficiency with increasing dietary methionine and choline. From this investigation, it is recommended that juvenile E. lanceolatus diets are supplemented with both dietary methionine and choline no less than 12.7 mg/g and 4.3 mg/g, respectively. Given that these results demonstrate that choline and methionine levels alter lipid mobilisation, they raise the potential to capture the growth and efficiency effects of higher lipid levels than previously found optimal for this species.