There's a reason you can't gain weight from rubbing oil into your skin. The body must absorb dietary fat exclusively through the digestive tract. It does this by first breaking down the fats into smaller components and then absorbing those building blocks using sophisticated cellular machinery.
The journey of a fat begins in the mouth, moves into the stomach and then empties into the small intestine where it is ultimately absorbed. The inner linings of these digestive organs secrete enzymes, which are proteins that perform biochemical functions such as breaking apart molecules into smaller pieces. Once the fat enters the stomach, enzymes in the digestive tract called lipases begin their hard work.
In her book "Biochemical and Physiological Aspects of Human Nutrition," Martha Stipanuk explains that a fat, otherwise known as a lipid, is made up of two distinct chemical components. The first is a fatty acid, which is a long chain of carbons and hydrogens that vary in length from four carbons to more than 20 carbons. A lipid is composed of three of these fatty acid chains, each bound to a single molecule called glycerol.
According to the book "Human Physiology: An Integrated Approach," about 10 percent of fat digestion takes place in the stomach using gastric lipase. Gastric lipase removes two of the three fatty acids from the glycerol by breaking the bond between them, leaving two free fatty acids and a monoglyceride. Because fat does not dissolve in water, it forms droplets in the otherwise water-based slurry of digested food. Once it is in the small intestine, a substance called bile is secreted from the gall bladder to help break apart those droplets into even smaller ones. This process is known as emulsification, which can be replicated in everyday life by shaking up a bottle of Italian salad dressing.
Digestion in the Small Intestine
An enzyme in the small intestine called pancreatic lipase breaks down the remainder of the fat. The pancreatic lipase is assisted by a protein called colipase, which helps the enzyme access the fat inside the droplets. Once again, the lipase breaks the lipid into two fatty acids and a monoglyceride. Throughout this process, the fatty acids, monoglycerides, bile, and other fats form new droplets called micelles. This enables them to move into intestinal cells, where they are prepared for absorption.
Absorption into Bloodstream
The free fatty acids and the monoglycerides diffuse into the intestinal cells at the border between the intestine and the bloodstream. Once inside the intestinal cell, the fatty acids and monoglycerides are combined back into lipids again. The lipids are then enclosed in packages known as chylomicrons. Chylomicrons do not pass directly into the blood. They must first travel through another bodily fluid called lymph, which empties into the blood. Once in the blood, the chylomicrons are broken apart, and the fats are ready to be used for nourishing the body.