Fat digestion is somewhat complicated for humans and animals both, but the process can generally be reduced to three steps. First the fats are emulsified, which means that they are suspended in a liquid, typically the acids of the small intestine. From here they are broken down with a series of enzymes and proteins, and finally they are absorbed and distributed. A number of internal organs, particularly the liver, gallbladder, and pancreas, play essential roles in this process. People who have problems digesting fat, whether chronically or just temporarily, often have a number issues, most of which are characterized by abdominal pain and trouble passing “normal” or predominantly solid stools.
Much of the human body is made of water, which means that food usually has to be digested in an aqueous solution. This can present something of a problem for fats that aren’t water-soluble, though, and many aren’t. Digestion starts for many types of food right in the mouth, where the saliva starts processing food and preparing it for the stomach. The stomach’s gastric juices then turn many foods into something called chyme, which is basically a semi-digested solid with more readily available nutrients.
This process doesn’t usually work for fats, though. These tend to be unaffected by the enzymes in saliva which means they reach the stomach largely unchanged, and the stomach acids aren’t always strong enough to alter their basic composition. As a result, they don’t usually begin digesting at all until they hit the small intestine.
There are two organs in the body that secrete substances essential to the digestion of fats: the liver and the pancreas. The pancreas secretes lipase, a substance that breaks down nearly all foods, including fat, and the liver secretes bile, a substance that emulsifies fat in particular. When food enters the small intestine, lipase is secreted into the area via the main pancreatic duct through the hepatopancreatic ampulla. It then breaks down food into simpler particles.
The small intestine, however, is also an aqueous area, and the lipase is only able to attack and break down the outer layer of most fatty particles. This is where bile comes into the 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.
This process is generally referred to as emulsification. On a basic level, all that this means is that fatty substances have been suspended in a watery solution in order to end up with a relatively smooth or seamless liquid. Many people find the concept easier to understand by thinking about common household emulsifications like mayonnaise. 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 or, in this case, vinegar, are held together in a mostly permanent way. The same process occurs in fat digestion when bile is introduced. The fatty substances become suspended in the watery environment and stay that way for the rest of their time in the body.
Once the fat is suspended like this, the lipase can get to work breaking it down. Lipase is a digestive enzyme that works in many of the same ways as enzymes elsewhere along the digestive tract, particularly in the saliva and the stomach. Fat that has been emulsified is usually a lot easier for lipase to break down and in most cases the decomposition is complete, meaning that all parts of the fat molecules are exposed and deconstructed into particles that can be easily absorbed by the bloodstream.
Absorption and Distribution
Fatty acids, cholesterol, and other products of fat digestion are typically absorbed into the bloodstream through the walls of the small intestine. This organ determines how much of the substances to distribute to the body and how much to send out as waste. A lot of this depends on a person’s overall health and general level of activity, since the body will make adjustments when it comes to how much fat it needs to perform certain tasks. In general, though, about half of the cholesterol that enters the small intestine never makes it into the bloodstream, and the ratio is only slightly higher for most fatty acids.
Once fat hits the bloodstream, it can go almost anywhere in the body, and has a profound impact on blood sugar as a whole. People usually feel full based on the chemistry of their blood sugar rather than the actual contents of their stomach or how much they’ve eaten by volume. In most cases the quality and type of food are more important to satiety than the actual amount consumed.
Common Digestive Problems
A number of things can go wrong in the fat digestive process, though problems are more likely when the system is overloaded with fats all at once, or when something is wrong with either the bile or lipase production centers. People with chronic problems with their livers or gallbladders often have to be really careful to control the fats they eat, since fat digestion tends to be slower and more cumbersome in these cases. Undigested or improperly digested fats typically lead to abdominal cramping and loose, watery stools.