Since much of the human body is water, foods must be digested in an aqueous solution. This presents a problem for fats that are not water soluble. To aid fat digestion, the body breaks down fat by first using emulsifiers that help the fat to suspend in a liquid and then using enzymes that break it down. Once it is broken down, fat can be absorbed and distributed to the various parts of the body.
Normally, digestion starts in the mouth as enzymes in saliva start to break food down. This process is not as effective for fat digestion. Instead, fat is sent down to the stomach largely unchanged. In the stomach, food usually mixes with gastric juices and is transformed into chyme. Again, the watery solution of the stomach is not conducive to breaking down fat, and so true digestion of fat doesn't take place until it reaches the small intestine.
There are two organs in the body that secrete substances essential to fat digestion: the liver and the pancreas. The pancreas secretes lipase, a substance that breaks down food including fat, and the liver secretes bile, a substance that emulsifies fat. Without bile, the lipase would not be able to digest fat efficiently. When fat, or other food, enters the small intestine lipase is secreted into the area via the main pancreatic duct through the hepatopancreatic ampulla. Lipase then breaks down food into simpler particles.
The small intestine, however, is also an aqueous area. The lipase is only able to attack and break down the outer layer of fatty particles. This is where bile comes into the fat digestion equation. Though bile is secreted by the liver, it is stored in the gallbladder. When fat hits the small intestine, bile is released through the bile duct and joins lipase in the hepatopancreatic ampulla. It is this substance that allows the fat to be broken down into smaller particles so the lipase can attack it more thoroughly.
Bile is an emulsifier. This means that it can take fatty substances and suspend them in a watery solution, resulting in what is called an emulsification. A common household emulsification is mayonnaise. Basic mayonnaise is made up of oil, egg yolks, and an acid, like vinegar. By whipping the egg yolks — an emulsifier — and the oil together and then adding vinegar slowly, the oil and water, in this case vinegar, are held together. The same process occurs in fat digestion when bile is introduced — the fatty substances become suspended in the watery environment.
Once the fat is broken down and suspended, the lipase can get to work breaking it down. After the lipase does its work, the fatty substances can then pass through the lymphatics. Then, these substances can be sent into the blood stream where they are delivered to other cells or sent to the liver for further processing.
