Lipid digestion in animals
Lipids make up a large portion of diet in carnivores, where as they form only a minor portion of diets in herbivores.
Primary dietary lipid is triglyceride; other lipids include cholesterol and cholesterol esters from animal sources, waxes from plant sources and phospholipids from both plant and animal sources.
Lipid digestion occurs in four phases; emulsification, hydrolysis, micelle formation and absorption.
Emulsification is a process of reducing lipid droplets to a smaller size for their suspension in water.
In the gut, lipid globules are broken down to droplets by the mixing and agitating actions of distal stomach.
Emulsification is completed in the small intestine by the detergent action of bile acids and phospholipids.
Bile salts reduce the surface tension of the lipid droplets and further, reduce in size of the fat droplets.
The bile coated or emulsified droplets are subjected to hydrolytic enzyme action.
Triglyceridesare the major dietary lipid, undergo hydrolysis by the action of gastric, pancreatic lipase and co-lipase, which are secreted as active form.
The co-lipase “make a pathway” through the bile product coating the emulsified lipid droplet, giving access to the lipase to reach the underlying triglyceride.
Lipase cleaves the fatty acids from the end of triglyceride molecule resulting in the formation of two free nonesterified fatty acids and a monoglyceride.
Cholesterol esterase and phospholipase are the other lipid digesting enzymes of pancreas.
The products of these enzymes are nonesterified fatty acids, cholesterol and lysophopholipid. The fatty acids, monoglycerides etc., combine with bile acids and phospholipids to form very small lipid droplets, micelles . The micelles are water soluble allow the lipids to diffuse through glycocalyx and into close contact with absorptive surface of the enterocytes.