Skip to content 
This article will focus on the lipid metabolism of fish and the demand of fish for fatty acids. It will analyze the specific needs of fish for cholesterol, phospholipids and essential fatty acids.
Fish lipid metabolism
Lipids are essential nutrients for fish. The basic pathways of lipid metabolism in fish mainly include lipid digestion, absorption, transport, synthesis and decomposition.
Lipid digestion and absorption
The main sites of lipid absorption in fish are the proximal small intestine and pyloric caeca. The hepatopancreas of fish is the main organ that produces digestive enzymes, triglycerides and phospholipases. The basic physical process of lipid digestion and absorption includes two steps: bile acid emulsification and hydrolysis product transport.
Lipid transport
Lipoproteins are the main transport form of lipids in fish blood. Female fish produce vitellogenin, which transports fat to oocytes as lipoproteins during yolk formation. The intracellular transport of fatty acids depends on fatty acid binding proteins in the cytoplasm, which bind long-chain fatty acids and other hydrophobic ligands.
Lipid synthesis
Lipid synthesis refers to the biosynthesis reaction of endogenous fats. Fish do not have the ability to synthesize long-chain polyunsaturated fatty acids de novo in the body, so they must obtain them from food. The synthesis process of saturated fatty acids, monounsaturated fatty acids and polyunsaturated fatty acids in fish is similar to that of mammals. Fatty acids can be esterified into phospholipids and triglycerides, etc.
Decomposition of lipids
Fish energy mainly comes from the decomposition of fatty acids, which occurs in organelles such as mitochondria and peroxisomes. In vivo experiments on the determination of fatty acid deposition in fish have shown that the higher the concentration of a certain fatty acid in the feed, the lower its relative deposition in the body, indicating that an increase in the concentration of fatty acids can lead to an increase in their oxidation degree.
Fish fat requirements
Protein source is the most expensive raw material in commercial feed formula. Adding an appropriate amount of fat to the feed can “save” protein. Commercial feed formulas usually use triglyceride-rich lipids as feed fat sources to ensure that the fish’s needs for polyunsaturated fatty acids and long-chain polyunsaturated fatty acids are met.
For fish, a certain range of feed fat requirements can be obtained through scientific experiments. The fat requirement of fish varies depending on the type of fish, the growth stage of the fish, the living environment, the source of feed lipids, and other nutrient levels.
Essential fatty acid requirements of fish
Additional ω-3 and ω-6 PUFAs need to be added to fish feed to meet the fish’s need for essential fatty acids. Long-chain PUFAs such as ARA, EPA, and DHA play an important role in the growth of fish.
The amount of essential fatty acids required by fish varies with species and the fatty acid composition of feed. At present, research on the essential fatty acid requirements of fish focuses on determining the appropriate levels and proportions of ARA, EPA, and DHA in marine larvae, juveniles, and young and sub-adult fish.
In addition, the need for essential fatty acids also varies with developmental and physiological stages, which increases the difficulty of determining the absolute amount of essential fatty acids required.
Freshwater and migratory fish
Adding about 1% C18 PUFA to the feed of juvenile and sub-adult freshwater and migratory fish can meet their needs for essential fatty acids (see the table in the book animal lipid science for details).
According to the difference in the need for essential fatty acids, freshwater and migratory fish can be divided into three categories: cold-water fish with a higher demand for C18:3 ω-3, warm-water fish with a higher demand for C18:2 ω-6, and species with a high demand for both of the above (see the table in the book animal lipid science for details).
Marine fish
C18 PUFA in feed cannot meet the needs of marine fish juveniles and sub-adults for essential fatty acids. Therefore, ω-3 PUFA (EPA and DHA) are considered to be essential fatty acids for marine fish. The ratio between different types of essential fatty acids is very important for marine larvae and juveniles. The need for ω-3 PUFA in fish decreases with the increase of the DHA/EPA ratio.
Biochemical and molecular basis of fish essential fatty acid requirements
According to different feeding habits, fish can be divided into herbivorous fish, omnivorous fish, carnivorous fish and piscivorous fish. Among them, herbivorous fish can synthesize LCPUFA by themselves, while other species do not have this ability. Therefore, trophic level and (or) diet are related to the ability of fish to synthesize LCPUFA, and may even be the determining factor of the ability of fish to synthesize LCPUFA.
Fish’s need for phospholipids
Fish demand for dietary phospholipids
Adding complete phospholipids to feed can promote the growth of fry and early juveniles, improve the survival rate of fry, reduce the deformity rate, and improve the stress resistance of various fish. Because the phospholipids used in the experiment are not uniform in production process, there are differences in phospholipid content and various phospholipid components (see the table in the book for details).
The requirement of phospholipids for fry is generally as high as 8%~12%, while the requirement of juveniles is much lower (2%~4%), and adult fish do not need to add phospholipids to feed.
Biochemical basis of fish phospholipid demand
Phospholipids promote lipid emulsification, absorption, transport and intestinal digestive enzyme activity. In the early development stage of fish, complete phospholipids must be added to the feed so that fatty acids and lipids can be effectively transported from the intestine to other tissues.
Cholesterol requirements of fish
As the amount of vegetable protein and vegetable oil in the feed increases, the cholesterol content in the feed decreases, while the plant sterol content increases. Studies have shown that the addition of cholesterol to feed has no significant effect on the specific growth rate, mortality, apparent digestibility and total lipid content of Atlantic salmon, but the cholesterol content and liver-to-body ratio in the liver increase.
