Your body digests each macronutrient in its own way, including dietary fat. Digestion, or the breakdown of food, begins as soon as you take a bite. The chewing action that occurs inside your mouth breaks down food. As you swallow, food passes through the esophagus and into the stomach, mixing with stomach acid. Fat breakdown picks up in the small intestine, and once inside the large intestine, excess water is removed from what has become waste. The food waste is packaged and prepared for excretion. Fat serves many important functions inside the body. Your body's ability to properly break down and absorb this complex molecule is important.
Location and Enzymes
Fat digestion is complex due to the hydrophobic nature of lipids, meaning lipids repel water. This can be complex because most of the enzymes used during fat digestion are water-soluble. The stomach begins fat digestion when lingual lipase, an enzyme secreted by the salivary glands, is activated by stomach acid. Only about 10 percent of dietary fat is broken down by the stomach. The rest of fat digestion occurs in the small intestine, where the enzyme pancreatic lipase continues to break down fat molecules.
Bile
While in the small intestine, bile aids in the breakdown of fat. Bile is unique in that it is ampiphatic, which means it contains components that are hydrophobic and hydrophilic. Hydrophilic means these components are attracted to water. Having both hydrophobic and hydrophilic properties allows bile to fully engulf and break down fat molecules. This allows the bile lecithin, the hydrophobic aspect, and bile acids, the hydrophilic aspect, to completely cover and surround fat molecules to speed digestion. Otherwise, pancreatic lipase would only be working at the surface, which would make fat digestion inefficient.
Formation of Micelles
Bile and pancreatic lipase work to break down fat into a simple triglyceride. Then, lipase works even further to break down the triglyceride into two free fatty acids and a monoglyceride. After this occurs, bile steps in to form the molecules into micelles. These are small droplets made up of bile acids, free fatty acids, monoglycerides, cholesterol and fat soluble vitamins. Transforming the free fatty acids and monoglycerides into micelles makes absorption of dietary fat possible.
Absorption
Micelles pass through the brush border cells of the small intestine and transfer their lipid components to absorpative cells. The endoplasmic reticulum of these cells re-forms the lipids into triglycerides, then the golgi complex takes the triglycerides in and adds a phospholipid and protein, creating a chylomicron. The chylomicrons travel through the lymphatic system and eventually make their way into the bloodstream, where the fat is either used or stored.
References
- "Anatomy and Physiology"; Kenneth S. Saladin; 2004
- National Diabetes Information Clearinghouse: Your Digestive System and How it Works; April 2008
- Colorado State University: Secretion of Bile and the Role of Bile Acids In Digestion; R. Bowen; November 2001
- Colorado State University: Absorption of Lipids; R. Bowen; August 2007
