TROUT CAN COPE ON SOME CARBS

Abstract

Trout have voracious appetites, which the aquaculture industry satisfies with fishmeal and oil from marine fisheries. However, Biju Kamalam from the French National Institute for Agricultural Research explains that the fisheries that supply aquaculture are under increasing pressure, forcing aquaculturists to consider alternative protein and energy sources. ‘The alternative is to replace fish meal with plant ingredients’, says Kamalam, although plant ingredients are rich in carbohydrates that trout find difficult to use for fuel. However, while breeding two new trout lines with altered muscle fat content (one with a high muscle lipid content while the other had a low muscle lipid content), Françoise Medale and her colleagues found that the lines appeared to metabolise fuel differently from each other. The fish with high levels of muscle fat seemed to express the genes that code for proteins that metabolise carbohydrates at high levels. Could the high muscle fat trout handle a plant-based, carbohydrate-rich diet better than fish that are currently being farmed? Kamalam, Medale and Kamalam's thesis advisor, Stephane Panserat, decided to investigate the fish's metabolism (p. 2567).Kamalam fed both the high muscle fat fish and the lean muscle fish either a fish meal supplemented with gelatinised starch diet or an unsupplimented diet for 10 weeks and then collected the livers, blood, muscles and adipose tissue from all of the fish for analysis. However, when the team compared the growth of the two fish lines they were disappointed to see that the lean fish grew better than the fat fish on both diets. ‘We had thought that if the fat muscle fish was able to use the carbohydrate as an efficient energy substrate, the protein in the diet would be used for muscle growth and improve the growth of the fish’, says Kamalam. However, both fish were capable of metabolising carbohydrate.Next, the team compared the fish's blood glucose levels, suspecting that the fat fish would regulate their glucose levels better than the lean fish, only to find that the two lines of fish managed their glucose levels equally well. The fat fish also expressed more of the genes involved in fat production, suggesting that they converted excess glucose from their diet into fat in the liver before transporting it to other tissues. And when they analysed the gene expression pattern in the liver, they realised that the fat fish expressed more of the genes involved in producing the healthy omega-3 fatty acids, which are an essential component of the human diet. However, when they analysed the gene expression patterns in the fat fish's muscle and fat, neither tissue overexpressed genes involved in glucose metabolism, suggesting that they are not adapted to metabolise carbohydrate from their diet.Finally, Kamalam analysed the activation by phosphorylation of two other proteins – one involved in nutrient sensing (mTOR) and the other involved in energy sensing (AMP kinase) – in the fish lines and found that the livers of the fat and lean fish are both able to use carbohydrate as an energy source. ‘The preferred substrate of carnivorous trout will still be protein and lipids, but when we need to substitute the protein and lipid content of the diet with carbohydrates, they can adapt’, says Kamalam. ‘We think trout can't use much carbohydrate, but we provide evidence that they can use optimal levels of carbohydrate and use it for an energy substrate’, he adds.