A new study by Groot et al., fed rainbow trout either an amino acid (AA) balanced diet or one imbalanced through insufficient lysine and methionine.
As expected, the imbalanced diet cut growth, lowered protein retention, and pushed more energy into fat storage. But one number stayed remarkably constant: the efficiency with which trout converted digestible energy into growth (kgDE, ~79%) was identical across both diets. Whether protein ended up built into muscle or diverted to fat deposition and ATP production, the fish extracted energy from it with equal efficiency.
This lines up with Bar et al.’s (2007) dynamic model of nutrient pathways in fish, which places amino acids at the centre of the tricarboxylic acid (TCA) cycle. In fish, amino acids are the preferred catabolic fuel: when they’re available, they’re routed straight into the TCA cycle for ATP production. Fatty acids and carbohydrates only get pulled into play when amino acids aren’t available for catabolism.
A related study, Chowdhury et al. (2014), measured the bioavailability of arginine from Indian mustard protein concentrate, Indian mustard meal and soy protein concentrate against crystalline L-arginine in rainbow trout, using a slope-ratio assay.
Raising dietary arginine increased both protein deposition (PD) and lipid deposition (LD) linearly across every source (R² = 0.86 for both) — but LD consistently outpaced PD at every arginine level. In the highest-arginine Indian mustard concentrate diet, for example, fish deposited 20.8 g protein but 21.6 g lipid per fish.
Lead author Kabir Chowdhury points to a likely contributor: the diets used NaH2PO4 rather than MCP or DCP as the phosphorus source, raising dietary sodium contents. Because several amino acids, arginine included, cross the intestinal brush border via Na+-coupled transporters into the hepatic portal vein, the extra sodium may have boosted amino acid absorption beyond what protein synthesis could use, pushing the surplus toward catabolism and fat deposition — consistent with the higher intraperitoneal fat observed in the fish fed the high-arginine diets.
Takeaway for feed formulation: getting the amino acid profile right, on both a dietary and a digestible basis, still matters hugely for growth and body composition, even though it doesn’t move the needle on digestible-energy utilisation efficiency itself.
Please DM if you need the other two articles.
Sources:
Groot, Lyons & Schrama, Aquaculture Reports 50 (2026) 103786.
Bar et al., Can. J. Fish. Aquat. Sci. 64 (2007) 1669–1682.
Chowdhury et al., Aquaculture Research (2014) 1–12, doi:10.1111/are.12365.
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