13C values of glycolytic amino acids as indicators of carbohydrate utilization in carnivorous fish

Abstract

BackgroundStable isotope analysis of single amino acids (AA) is usually applied in food web studies for tracing biosynthetic origins of AA carbon backbones and establishing trophic positions of consumers, but the method is also showing promise for characterizing quantity and quality of dietary lipids and carbohydrates.MethodsTo investigate whether changes in high- and low-digestible carbohydrates affectδ13C values of glycolytic AA, i.e., AA carbon backbones sourced from the glycolytic pathway, we compared Atlantic salmon (Salmo salar) from a feeding experiment with and without dietary inclusion of the red macroalgaPalmaria palmata. The Control and experimental diets had similar relative proportions of macronutrients, but their ingredients differed; in the experimental treatment, 15%Palmariainclusion substituted proteins from fishmeal and carbohydrates from corn starch.ResultsWe found that13C values of the glycolytic AA were highly sensitive to substitution of corn starch withPalmaria. Theδ13C offsets of glycolytic AA between salmon and their diets were significantly greater in thePalmariainclusion than Control treatment. This greater offset can be attributed to the different utilization of high- vs. low-digestible carbohydrate sources, i.e., corn starch vs.Palmaria, in the two treatments, and metabolic routing of dietary lipids. In addition, similarδ13C values of essential AA between treatments indicate similar nutrient assimilation efficiency for all terrestrial (pea protein concentrate and wheat gluten meal) and marine (fishmeal and red alga) derived protein sources. These results show thatδ13CAAanalysis is a promising tool for improving our understanding of how carnivorous fish utilize macronutrient and route metabolic intermediates to tissue.