Fat digestion is more than just enzymatic breakdown of fatty foods in out diet unlike carbohydrate and protein digestion which mainly involve the activity of enzymes in splitting them into their constituent molecules. Fats are insoluble in the watery environment of the intestinal fluids and require processing before the digestive enzymes can act on them. Learn what these additional steps in fat digestion are and what digestive enzymes participate in lipid digestion.
Fats in our diet are largely represented by triglycerides and to a smaller extent by phospholipids and cholesterol. Triglycerides are molecules made of three fatty acids linked together by one molecule of glycerol. Digestion of fats includes breakdown of triglycerides to release fatty acids that can be absorbed by the intestinal cells.
Fat Emulsification – First Step in the Digestion of Fats
The problem with fat digestion is that the digestive enzymes are water-soluble and so cannot dissolve into globules of fat to act on the triglycerides they contain. They can only act on the surfaces of the fat globules in the food. This means that fat droplets have to first be broken down into much smaller sizes so that these enzymes have plenty of surface area to work on. This process is called emulsification of fat.
Fat globules are somewhat fragmented when they get churned with other food in the stomach. But for the most part, emulsification of fat is achieved by the phospholipid lecithin and bile salts, both secreted by the liver in bile. These molecules actually act in a way very similar to how detergents work to remove oil and grease stains.
Bile salts and lecithin have both water soluble and fat soluble portions. The fat soluble portions dissolve in the fat globules while their water soluble portions protrude from the surface of the globules, soluble in the watery fluids in the lumen of the small intestines. What this does is to make the fat globule very unstable and prone to being broken into smaller fragments when agitated. This process results in the production of particles less than 1 micrometer which are ready to be acted upon by the lipase enzymes.
Lipase Enzymes and Lipid Digestion
Lipases are the enzymes required in the breakdown of lipids and are secreted by various parts of the digestive system:
- Lingual lipase – secreted in the mouth by the lingual glands
- Pancreatic lipase – secreted in pancreatic juice
- Enteric lipase – present in small quantities in enterocytes or intestinal cells
Of these, pancreatic lipase is the most important and responsible for most of the fat digestion that occurs in the intestines as it is secreted in large quantities. Pancreatic lipase acts on emulsified fat splitting the triglycerides into free fatty acids and 2-monoglycerides by a process called hydrolysis.
Bile Salts and ‘Micelle’ Formation in Fat Digestion
After the enzymatic action and hydrolysis of the triglycerides, bile salts have yet another role to play in the fat digestion process. Triglyceride hydrolysis being a highly reversible reaction, the products of this reaction have to be quickly removed from the surrounding area. To add to the problem, the fatty acids and monoglycerides produced are mostly insoluble in water.
Bile salts help by forming what are called ‘micelles’. Micelles are tiny globules, each 3 to 6 nanometers in diameter, made up of about 20 to 40 molecules of bile salts. As already mentioned, bile salts have both fat and water soluble parts – a fat-soluble sterol nucleus and a water-soluble polar group. So a number of these molecules collect around the products of fat digestion and form micelles, the sterol nuclei in the centre and the polar groups on the outside.
Since the surface of these micelles is entirely made of water-soluble polar groups, they dissolve easily in the digestive fluids in the intestines. So not only do these micelles help in quickly removing products of fat digestion from the area of their production, they also help transport them to the intestinal cells which absorb them, an otherwise difficult task as they are water insoluble.
To summarise fat digestion, fatty foods in our diet first undergo fat emulsification where they are broken down into small droplets. These are acted upon by the enzyme lipase and free fatty acids released. Bile salts then form micelles around these fatty acids and assist in their absorption by intestinal cells.
Murray R.K., Granner D.K., et al, “Harper’s Illustrated Biochemistry” (McGraw Hill; 26th edition)
Guyton and Hall, “Textbook of Medical Physiology” (Saunders; 10th edition)