SPECIAL STORY FEED & ADDITIVE MAGAZINE March 2023 59 health. Among several lipids, phospholipids (PLs) are important components for maintaining the structure and function of cellular membranes, emulsifying the lipids in the gut, and improving intestinal absorption of long-chain fatty acids. PLs are a source of fatty acids for the synthesis of eicosanoids, a wide range of bioactive compounds with multiple functions. PROTEIN SPARING EFFECT The imperative need to reduce feed costs in aquaculture has been reported by (Jauncey, 1998) the protein component of the feed is responsible for the high cost (Shiau and Lin, 1993) and especially fishmeal is a very costly ingredient (NRC, 1993). The ability or capacity to utilize carbohydrates and lipids for energy varies between species the inclusion of non-protein energy sources in a diet formulation allows the formulator to reduce the protein content of the diet this capacity is called the protein-sparing effect. THE PROBLEM IN HIGH FAT DIET An increase in dietary lipid content in fish feed enhances feed efficiency and growth performance in fish. Thus, fat-rich diets have been extensively used in the fish farming system. However, the excess dietary lipid often leads to unwanted fat deposition in the liver (Lu et al. 2013), resulting in a high mortality rate, poor growth performances, and immune suppression of the fish (Bolla et al., 2011; Lu et al., 2014a). In order to prevent excessive lipid deposition, various ways have been researched by the fish nutritionist. Some additives have been studied extensively and successfully used in controlling excess fat accumulation in the liver, and they have been shown to regulate the abnormal expression of key genes involved in lipid metabolism. EMULSIFIER An emulsifier is a molecule with water-soluble (hydrophilic) and lipid-soluble (lipophilic) parts that can be adsorbed at the oil–water interface. The combination of the two parts in one molecule gives the emulsifiers a unique ability to dissolve equally well in both lipid and water and can aid in mixing the two fractions. Currently, there are different kinds of emulsifiers, such as cholic acid, lecithin, lysolecithin, bile acids, and Tween- 80, that are available and can be used to improve lipid digestibility. These emulsifiers have been approved as feed additives that can improve the growth performances and feed utilization of several fish species, such as rainbow trout, sturgeon, amberjack, brown trout, turbot, and golden mahseer. EMULSIFIERS THAT IMPROVE LIPID UTILIZATION IN FISH Bile acid Bile is a collection of amphipathic molecules that are synthesized in the liver from cholesterol and stored in the gallbladder, the most common bile acid and alcohol being cholic acid and cyprinol, respectively. Bile contains lesser amounts of fats (0.4–0.5%), including fatty acids, cholesterol and phospholipids. The characteristic yellowish-to-green appearance of bile is due to the presence of bilirubin (yellow) and biliverdin (green) (Hofmann et al. 2010). Role of bile as a feed additive The two primary ways bile salts improve lipid digestion is (i) via their emulsifying properties that form micelles, which thus provide a higher surface area for lipase to digest lipids by catalyzing the hydrolysis of ester bonds (Wang & Hartsock 1993) and (ii) being essential to activate the bile salt-activated lipase. Function: Promote the Emulsification, digestion, and absorption of fat Fat digestion is a complicated process and occurs in 3 main steps: 1. Emulsification 2. Digestion 3. Absorption - Firstly, during emulsification, fat is emulsified into little droplets by bile acids with both hydrophilic and hydrophobic groups, which help to enlarge the contact surface between fat and lipase. - Secondly, bile acids have the function to activate lipase, which contributes to hydrolyzing fat droplets into fatty acids, glycerin, and monoglycerides. - Thirdly, SCFA can be absorbed directly in the intestine, while LCFA bind with bile acids to form
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