EMULSIFIERS IN FISH NUTRITION AND THEIR IMPLICATIONS FOR PLANT-BASED DIETS

Supplies of fish oil are limited and continued growth in aquaculture production directs that substitutes must be found that do not compromise fish health and product quality. As a result, several potential alternative oil sources (plant, animal, algal, etc.) are being evaluated and used in the aquafeed industry. However, their digestion, absorption, and assimilation are typically lower than fish oil and ultimately provide lower available energy to fish. An alternative approach, the addition of emulsifier in feed could enhance lipid digestibility. Emulsifiers have two polar (hydrophilic) and nonpolar (lipophilic) heads helping water and fat molecules to bind, leading to solubilization of fatty acids and enhanced oil contact with digestive lipase enzyme. Emulsifiers can be divided into natural and synthetic groups. Bile salts are one of the common natural emulsifiers, which helps for the digestion and absorption of dietary lipids and fat-soluble vitamins from the intestine of fish. Bile salts are synthesized by hepatocytes of the liver through cholesterol conversion, stored in the gallbladder, and secreted to the intestine. These salts emulsify dietary fat, leading to a larger pancreatic lipase activity. Fat transportation is another role of emulsifiers, which transport fat from intestinal epithelium to hemolymph and reduce the accumulation of lipid in the intestine. After digestion, the intestinal bile acids are absorbed along with the nutrients and returned to the liver via the portal blood. Dietary plant ingredients can affect this circulation. An increased plant ingredient, such as soybean, reduces gallbladder somatic indices in different species (Figure 1) and also changes bile salt composition and reduces blood cholesterol levels (Table 1), which will have negative impacts on fat digestion and absorption.

Dietary supplementation of emulsifiers, e.g. synthetic (Twine 80) and/or natural (bovine salt or lecithin) can alleviate the negative impact of plant-based diets on the reduction of emulsification capacity of the bile acid pool. This supplementation also prevents the occurrence of histological abnormalities in the distal intestine and liver which may be a proper alternative to reduce soybean meal-induced enteritis in fish. Evidence suggests that fish require emulsifiers when feeding low-digestible oil and/or high-plant protein diets. Supplementation of emulsifier at the level of 1.5-2% (natural) or 2-4% (synthetic) is suitable for carnivorous fish to compensate for low fat digestibility caused by plant-based diets.