- Fats and oils or collectively called lipids or tryglycerides are digested by lipases.
- Their digestion starts in the stomach and is nearly completed in the small intestine.
- Saliva contains no lipase, nor is any fat emulsifying agent present in the oral cavity.
1. Stomach:
- Stomach also lacks fat-emulsifying agent.
- However, gastric juice contains gastric lipase, which converts some fats into monolgycerides and fatty acids.
- The reaction is negligible as the enzyme is sensitive to free acid and is soon destroyed by HCl.
2. Small intestine:
- In the small intestine, food meets three secretions: bile, pancreatic juice and intestinal juice, all alkaline in nature.
a. Bile:
- It is secreted by liver and is stored and concentrated by the gall bladder.
- It contains no enzyme, and, thus has no chemical action on food.
- Its salts namely, sodium glycocholate and sodium taurocholate, reduce the surface tension of large fat droplets and break them down into small ones, the process called emulsification of fats.
- It produces a fine emulsion of fats in the aqueous intestinal contents.
- Bile salts also form a thin coating around tiny fat droplets to keep them from coalescing.
- The small droplets of fats present large surface area to lipases which increase their action upon them.
Triglycerides ———-> Emulsified triglycerides (by bile)
b. Pancreatic juice:
- It contains pancreatic lipase, which is the principal fat-digesting enzyme.
- It hydrolyzes fats in different stages.
- In the first stage, lipase separates one fatty acid molecule, changing the emulsified triglycerides into a diglyceride.
- In the second stage, the diglyceride is broken down into another fatty acid molecule and a monoglyceride.
- In the third stage, the monoglyceride is hydrolyzed to another fatty acid molecule and a glycerol molecule.
- Thus, a complete hydrolysis of fat molecule gives three fatty acid molecules and one glycerol molecule.
- However, normally the digestion of fats remains incomplete in the intestine.
- Hydrolysis of fats is a slow process, and a few hours available for the action of lipase in the intestine are not enough for the completion of fats hydrolysis.
Triglyceride ———-> Fatty acid + Diglyceride
Diglyceride ——–> Fatty acid + Monoglyceride
Monoglyceride ———-> Fatty acid + Glycerol
c. Intestinal juice:
- Intestinal glands secrete intestinal lipase.
- This enzyme occurs mainly in the intestinal epithelial cells and only a small amount is released in the intestinal juice.
- The lipase found in the intestinal juice hydrolyzes some triglycerides, diglycerides and monosaccharides to fatty acids and glycerol like the pancreatic juice.