Fats and oils constitute the main energy source of animals and possess the highest caloric value of all nutrients, with almost 3 times higher apparent metabolizable energy. Hence, fats are widely added to animal diets to meet energy requirements. However, their digestion, and absorption, are poorly developed in young animals and ultimately provide lower available energy to fish. Hence the addition of an emulsifier in feed could enhance lipid digestibility…
By G. Sanjay Gopal, Mir Ishfaq Nazir, C. Sathishkumar, T. Bhuvaneshwaran
Tamil Nadu Dr. J. Jayalalithaa Fisheries University
Development of the cost-effective nutritionally balanced diets for fish is the major factor that affects intensive aquaculture due to its influence on growth, health, and production costs. Nutrients and energy are needed in the diet for proper growth and maintenance. Fish are known to utilize protein preferentially over lipids or carbohydrates as an energy source protein utilization can be improved by increasing the dietary energy level. Since protein sources are among the most expensive component of feed ingredients, it is economically desirable to minimize the protein level in the diet. Excess protein can be replaced by an energy-rich compound such as lipid. Lipid is an important nutrient for fish which provides a source of energy, essential fatty acid incorporation of well-balanced levels of lipid in fish diets can maximize protein sparing which can be used for reducing the feed cost and minimizing nitrogenous waste output from fish farm effluents into the aquatic environment this article describes the lipid utilization and the additives used for the improvisation of lipid.
IMPORTANCE OF LIPIDS
Lipids supply energy, act as a structural component of cell membranes, and reduce the protein content in the diets of aquatic animals. Lipids have been reported to play a significant role in fish’s optimal growth and 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.
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 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:
– 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 mixed micelles, these micelles break up when contacting with intestinal villi, LCFA is absorbed into intestine epithelia, while bile acids return to the liver via enterohepatic circulation.
Role of phospholipids on lipid utilization
Phospholipids are a kind of surface-active agent or surfactant, and the main industrial use of lecithin is as a lipid emulsifier, particularly in processed foods. Fish diets, of course, do not require the presence of phospholipids as emulsifiers, but their presence in the formulation may improve lipid emulsification and aid digestion in the intestine of the fish after consumption. Improved lipid digestibility has been reported in salmon fed diets containing soybean lecithin has been attributed to emulsification properties of the phospholipids, and other studies have shown that dietary phospholipids has increased the digestibility in juvenile fish (Craig and Gatlin, 1997). In a study conducted by (Koven et al., 1993) in gilthead sea bream larvae, a lecithin–supplemented diet substantially increased the uptake of labelled lipids from the diet via improved emulsification.
There is a general beneficial effect of PL addition to micro diets on the growth and survival of several fish species such as ayu, carp. PC (phosphatidyl choline) are more effective in promoting growth in larval Japanese flounder (Kanazawa 1993a) and carp whereas PI (Phosphotidylinositol) seems to be more effective than PC in enhancing the survival. The addition of soybean lecithin to diets enhances ingestion rates in prawn and gilthead seabream. Total lipid digestibility is increased by the addition of soybean PC to diets for carp.
There is considerable evidence that dietary phospholipids can improve feed intake and act as an emulsifier in the gut (Koven et al., 1993), reduce lipid peroxidation, it stimulates the lipoprotein synthesis in gut enterocytes.
Cholic acid is a naturally occurring, primary bile acid that represents a major component of the total bile acid pool in humans. Cholic acid is synthesized from cholesterol in the liver and it is conjugated to either with glycine (glycocholic acid) or taurine (taurocholic acid) before the secretion in the bile.
Choline is considered as a lipotropic factor preventing excessive lipid accumulation and development of fatty livers. Several studies reported that fish fed the diets containing supplemental choline had significantly lowered the total lipid in liver compared with the fish fed diets without supplemental choline, and total lipid in muscle significantly increased by supplemental choline. Furthermore, studies have investigated the effects of dietary choline level on lipid deposition in fish, the underlying molecular mechanisms remained unknown (Li et al., 2014). Reduced hepatic lipid contents in fish fed increasing dietary choline levels have been reported in a study conducted by (Griffin et al., 1994).
Fats and oils constitute the main energy source of animals and possess the highest caloric value of all nutrients, with almost 3 times higher apparent metabolizable energy. Hence, fats are widely added to animal diets to meet energy requirements. However, their digestion, and absorption, are poorly developed in young animals and ultimately provide lower available energy to fish. Hence the addition of an emulsifier in feed could enhance lipid digestibility and thereby protein sparing effect can reduce the cost of costly protein in the feedstuff so further investigation of emulsifiers as feed additives is required.
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