LIPID DIGESTION AND ABSORPTION

Large oil droplets form in water. Most lipids are not soluble in water and tend to clump together.

EMULSIFICATION a process that allows lipids—fats—to mix with water

To carry out their functions in the body, most dietary lipids must first be digested. A small amount of fat digestion takes place in the mouth and stomach (via enzymes called lipases), but most occurs in the small intestine. However, a problem must be overcome for digestion to proceed. Fats cannot mix with water because lipids are not water soluble; the fat tends to clump together in the intestine’s watery environment. The body’s solution to that problem is emulsification.

LIPASE enzyme that removes fatty acids from the glycerol backbone of triglycerides

Emulsification aids digestion by breaking up large fat globules into smaller droplets so that fat-digesting enzymes can operate efficiently. Bile acids produced in the liver make emulsification possible. Bile acids have a water-soluble and fat-soluble “face.” These two-faced molecules attach to lipids, so that the lipids remain suspended in water instead of clumping together. Lipases produced in the pancreas can then digest triglycerides into monoglycerides and free fatty acids. These lipids, along with bile acids, form structures called micelles that deliver dietary lipids to the mucosal cells of the small intestine so that they can be absorbed.

Once inside mucosal cells, fatty acids and monoglycerides (which, as their name implies, contain only one fatty acid chain) are reassembled into triglycerides. (INFOGRAPHIC 6.5)

INFOGRAPHIC 6.5 Lipid Digestion and Absorption The majority of the lipids in our diet are in the form of triglycerides, which must be digested before they can be absorbed. Most triglyceride digestion occurs in the small intestine by pancreatic lipase, with the assistance of bile acids produced by the liver.

Question 6.5

Why might someone who has had their gallbladder surgically removed need to be careful to avoid consuming excess fat?

The gallbladder aids in the digestion of fat by storing bile.