The last time, we talked about the functions of fats in the body, and why it is necessary that dietary fat be included in our meals. Today we’ll be looking at how dietary fats enter the body; in other words, how dietary fat is digested.
Although some initial fat digestion is done in the stomach, most digestion and virtually all nutrient absorption of fats takes place in the small intestines. Both animal fats and vegetable fats appear to be equally well-digested and equally well-absorbed. The digestive actions are facilitated by an enzyme secreted by the pancreas, and by an important substance that comes from the liver.
We are most familiar with the pancreas as the organ that secretes insulin, but the pancreas (a long, narrow gland that lies below and behind the stomach) also produces several enzymes for the digestion of proteins, carbohydrates, and fats. The pancreatic enzyme, called pancreatic lipase or steapsin, is the major fat-digesting substance. It functions to split the dietary fat back into its component parts, glycerol and fatty acids. The digestive substance from the liver is called bile, and it works hand-in-hand with the lipase.
Since fats and oils cannot be mixed with water or water soluble substances, they must be altered before they can be digested. The alteration is done in a process known as emulsification. (Recall that we first talked about this process in a previous article when we talked about how soap makes it possible to wash grease from dishes.) In the process of emulsification, each droplet of fat is surrounded with a droplet of the emulsifying substance, which holds the fat in solution long enough for it to be used. The primary emulsification substance in our bodies is the bile produced by our livers.
Bile is a greenish or yellowish liquid that serves two major purposes in our bodies, as it is basically composed of two kinds of compounds. These compounds are called bile pigments and bile salts.
Bile pigments are a waste product that comes from the normal destruction of red blood cells, as the older cells die and are replaced by new ones. The bile pigments give color to the bile, and they give normal coloration to the feces. The pigments enter the small intestines, and are then passed out of the body with bowel movements. If the bile pigments build up in the liver because they are being produced too fast (such as when there is a disease that is causing red blood cells to be destroyed in an abnormal way), or because the bile can’t be excreted due to a blockage (such as gallstones), many body tissues, including the skin and the whites of the eyes, turn yellow. This condition is called jaundice.
The other important factor in bile is the bile salts. They are the chemicals that act in fat metabolism to emulsify dietary fats. The emulsified fat droplets present a lot of surface area for the action of the enzymes, which are water soluble. As a result, fat and fat-soluble vitamins are able to be absorbed through the walls of the small intestines. After the bile salts do their work, the liver reabsorbs them from the blood so that they can be used again.
The liver is the largest organ in the body and is located to the right of the upper part of the stomach. Among its many functions is the production of bile. The formation of bile is continuous, as much as 1000 ml per day, but since it is not needed unless there is undigested fat present in the upper small intestines, the bile is stored in the gallbladder between digestive periods.
Bile leaves the liver and flows down a tube that enters the small intestines. But on the way, most of it is diverted into a side channel called the cystic duct. This duct leads to the gallbladder, a small sack located under the liver. The gallbladder has only one function: it collects and concentrates the bile. And since it has no other function, we can be entirely certain that fat belongs in the human diet. In other words, if fat was not a natural part of our diet, human beings would not have a gallbladder.
During the period of time that the bile is being stored in the gallbladder, water from the bile is absorbed into the blood, thereby making the bile more concentrated and improving its action.
In a person who has had his/her gallbladder removed, bile from the liver runs directly into the small intestines as it is produced. It may or may not encounter any fats that need digestion when it gets there, but even if there is fat present, the bile will be less than optimal in its function since it will not be of the concentrated strength gained by bile that has been stored in the gall bladder for a period of time. And if fats can’t be acted upon by sufficient bile, much of them will proceed through the intestinal tract undigested.
Now, this may seem like a good idea at first blush, since most undigesteddietary fat will not provide dietary calories, but if fat is not digested, much of the fat-soluble vitamins A, D, E, and K, much of the calcium, and much of the essential fatty acids will be lost. People who have had their gallbladders removed should strongly consider taking a product containing bile salts as a food supplement with each meal.
Once the dietary fats have been digested and absorbed through the walls of the small intestines, some of the fatty acids reform with the glycerol that was separated from them. These simplest-form fats are now referred to as triglycerides.
As we have already seen, since both fats and fatty acids are insoluble in water, special conditions must exist to transport the fat in the blood. A large proportion is transported as triglycerides, some binds with protein to make lipoproteins, and some binds with cholesterol.
Some fats are stored in the liver for a time, where they are used to form more complex lipid structures, such as cholesterol and other steroids, or lecithin and other phospholipids.
And just in case the whole process is starting to seem understandable, here’s a little complication to keep you on your toes: unlike carbohydrates and protein, which must be fully digested before they can be absorbed, not all fats needs to be fully digested before they can be absorbed.
Some dietary fats enter the lymphatic system, which is a sort-of secondary blood system, without being digested. We will talk more about the lymphatic system as we move along.
Next time, we’ll continue talking about fats, and we’ll look at ketones and the controversy surrounding them.