How Normal Fats are Digested
As stated before, we will look at the digestion of triglycerides, the most common fat ingested by humans.
Mouth, Esophagus, Stomach
Digestion begins in the mouth, where food is crushed and ground into small pieces that can be easily swallowed. Fats follow the esophagus to the stomach where they are crushed more. While other macromolecules begin to be digested and broken down in the stomach, triglycerides are for the most part unchanged until they reach the small intestine. However, a small amount of short chain triglycerides from butterfat are digested in the stomach by tributyrase, a gastric lipase (Guyton 1986.) The amount of digestion in the stomach is negligible to the mass digestion in the small intestine (Guyton 1986, Jacobson 1984.)
Liver
The liver excretes bile, a substance that does not contain any digestive enzymes, but one that is nonetheless essential to fat digestion. Bile contains large quantities of bile salts, mainly ionized sodium salts that are steroid derivatives produced by the oxidation of cholesterol in the liver (Guyton 1986, McMurray 1983.) Bile salts emulsify fat molecules by surrounding fats with the sterol portion of the salt facing in toward the fat, and the carboxyl and polar portions facing outward. This in essence makes a droplet of fat that is soluble in water. Emulsification greatly reduces the interfacial tension of fat and allows that fat to be broken into small particles for easier digesting by enzymes. It also allows lipases access to the molecules; lipases are water soluble and require the presence of bile’s hydrophilic regions to get to the hydrophobic fats. With the presence of bile, 97% of ingested triglycerides are absorbed, whereas in the absence of bile, only 50-60% of fat is able to be absorbed (Guyton 1986.) Bile also plays an essential role in the transport of products of triglyceride digestion to the walls of the intestine.
Pancreas
The most important enzyme for fat digestion is pancreatic lipase. It hydrolyzes triglycerides into two fatty acid chains and a 2-monoglyceride. The hydrolysis of triglyceride into these end products allows the small intestine walls to absorb them (Guyton 1986, Tortora 1987.)
Small Intestine
Bile and pancreatic lipase is dumped into the small intestines when food is present. Bile emulsifies the fat and the lipase hydrolyzes it. The bile salts continue to emulsify the products after hydolysis for two reasons. First, the hydrolysis of triglycerides is a highly reversible chemical process. To prevent hydrolyzed fatty acids and monoglycerides from reforming triglycerides, the products must be separated from the vicinity of the other. Bile salts separate the products by emulsifying them in globules called micelles. Micelles are similar to the globules that initially surround triglycerides. The micelles consist of 20-50 bile salt molecules with the hydrophilic ends facing outward to allow the globule to dissolve in the water-based intestinal chyme (Tortora 1987.) The hydrophobic ends of the bile salts surround the fatty acids or monoglycerides on the inside of the globule (Guyton 1986.)
The second reason bile salts form micelles is to transport the triglyceride products to the intestinal walls to be absorbed into the blood stream. Read about the absorption of fatty acids and monoglycerides on the next page.
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The digestion of triglycerols. The triglycerols travel through the esophagus to the stomach, then into the small intestines where they are hydrolyzed and broken down by lipases. The products are absorbed through the intestinal walls into the lymph where they travel to the blood stream. Adapted from http://www.emc.maricopa.edu/faculty/farabee/BIOBK/digest.gif |
