Fish species can be divided into the categories of lean fish, medium-fat fish and fatty fish, based on how much fat is stored in the muscle tissue (fillet). Lean fish, such as cod, do not store fat in the muscle tissue, but in the liver. Fatty fish, such as salmon and trout, store fat in the muscle tissue and around the inner organs, but not in the liver. However, the composition of nutrients in the fish feed can also affect how fat is stored in fish. When fatty fish store lipids in the liver, it is one of the first signs of malnutrition and/or an imbalance between the nutrients in the feed. We do not yet know how much lipids fatty fish can store in their liver before they develop liver damage and inflammation. In humans and rodents, more than 5% lipids in the liver is deemed to constitute liver damage (NAFLD: non-alcoholic fatty liver disease) that can lead to health challenges such as diabetes.
Increased intestinal lipid storage in rodents and humans can result in chronic low-degree inflammation that has obvious negative health effects. Fatty fish species also store surplus energy as fat around their inner organs, and a lack of some nutrients leads to more fat being stored around the inner organs. However, we know too little about how increased intestinal fat storage (adiposity) affects health and disease resistance in fish.
Which nutrients are important?
Too little or too much of several nutrients can result in fish changing their lipid storage pattern. In particular, too little of the amino acid methionine, marine omega-3 fatty acids and phospholipids in the fish feed increases the storage of lipids in the liver of Atlantic salmon. Too low dietary levels of of the amino acid methionine also increases fat storage around the inner organs in salmon. Recent research also indicates that high levels of plant sterols, low levels of saturated fat and cholesterol can increase lipid storage in the liver and lipid levels in blood in salmon. It is also possible to increase salmon’s ability to utilise fatty acids for energy by for example by adding more of the essential amino acid arginine in the feed or giving them extra marine omega-3.
NIFES’s research endeavours to increase our knowledge about the amount of various minerals, vitamins, fatty acids, amino acids and other fish feed components farmed fish need to minimise adiposity to ensure production of robust fish.