Dietary alterations in protein, carbohydrates and fat increase liver protein-turnover rate and decrease overall growth rate in the rainbow trout (Oncorhynchus mykiss)

We have determined the protein-turnover rates and nucleic-acid concentratio ns in the liver of trout (Oncorhynchus mykiss) fed on two different isocalo ric diets: low-protein/high-fat and non-carbohydrate/high-fat. Compared to controls, the partial replacement of protein with fat significantly decreas ed the protein accumulation rate and protein-retention efficiency in the li ver whilst increasing the fractional protein-synthesis and protein-degradat ion rates as well as protein-synthesis efficiency. The complete replacement of carbohydrates with fat significantly lowered the protein-accumulation r ate and protein-retention efficiency, but enhanced both the protein-synthes is and protein-degradation rates as well as protein-synthesis capacity. The protein:DNA and RNA:DNA ratios decreased considerably on both diets. Total DNA decreased in fish on a low-protein/high-fat diet but did not change in those on a non-carbohydrate/high-fat diet. The absolute protein-synthesis rate registered no significant change under any of the nutritional conditio ns. Both the experimental diets did however raise the fractional protein-sy nthesis rate significantly, due to enhanced protein-synthesis efficiency wh en protein was partially replaced with fat and to enhanced protein-synthesi s capacity when carbohydrates were completely replaced with fat. Our result s show the capacity of the liver to adapt its turnover rates and conform to different nutritional conditions. They also point to the possibility of co ntrolling fish growth by dietary means.