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Dietary lipid Metabolism
Introduction

Dietary lipid digestion in chemical terms does not occur in the mouth or stomach. Chewing, of course, is very important, as larger pieces of fat as well as other nutrients are broken down into smaller pieces. As food reaches the stomach, its muscular grinding action helps to further prepare the food for delivery to its main destination for digestion, the small intestine. In the the first portion of the small intestine, the duododenum, emulsification of fats occurs. By the time dietary fats reach the duodenum, they exist in smaller droplets. Since enzymes cannot penetrate the lipid droplet, they must hydrolyze fats from the surface of the droplet. This would take quite some time, if allowed to proceed unaided. Emulsification speedens this process. Emulsification is the process by which individual fats are made available to be hydrolyzed by enzymes from the surface of the lipid droplet. Emulsification is possible by the detergent action of bile salts. Bile salts are producted by the liver and stored in the gallbladder. When a meal with fat is consumed, the gallbladder is stimulated to release bile into the duodenum. Bile mixes readily with fats, increasing the surface area and thereby helping facilitate enzyme action on them. The enzymes are produced in the pancreas. These enzymes are also released into the duodenum, and together, help to break apart lipid droplets. This enzyme action and the peristaltic action of the muscular duododenum, help in releasing individual fats, so that they can be transported out of the intestinal lumen and into the bloodstream.

Bile salts are derivatives of cholesterol. This is accomplished by adding A molecule of glycine or taurine to cholesterol. The result is a molecule that can act as an emulsifying agent not only with fats, but also with water or aqueous solutions in the intestine. This dual function molecule is useful because as the fatty tissue (eg., of a meal) is broken down into smaller particles, the emulsifying bile salt prevents the fats from coalescing (gathering together).

As the lipids are emulsified, they can easily be degraded by enzymes. These enzymes are produced and stored in the pancreas. When the pancreas is stimulated, it secretes the needed enzymes into the lumen of the intestine. The secretion of enzymes from the pancreas is in turn stimulated by a hormone called cholecystokinin (also called pancreozymin). Cholecystokinin is produced by intestinal cells when they in turn are stimulated from fats and proteins entering the duodenum (1st part of small intestine) and jejunum (2nd part). The cholecystokinin released from the intestinal cells, not only stimulates the pancreas to secrete digestive enzymes, but also stimulates the gallbladder to secrete bile. As mentioned earlier, the bile emulsifies lipids while the pancreatic enzymes digest the lipids. Basically, the enzymes and bile salts work together as a team, one emulsifying the lipids, while the other (enzyme) digests. Cholecystokinin also slows emptying of stomach contents into the intestines. This insures more complete digestion, rather than moving an entire meal into the small intesting at once.

The same intestinal cells producing cholecystokinin also produce a second hormone called secretin. Secretin is produced and secreted in response to a decrease in pH (acidity) from chyme (food that has just entered the intestine). Secretin stimulates the pancreas to release an aqueous bicarbonate solution, which helps neutralize the acidic contents of the intestine. This corrects the acidity, so that the enzymes responsible for digesting lipids can work correctly. Keep in mind that enzymes are sensitive to temperature, acidity, and other factors.

Let's take a look at the figure below. This is an example of a common lipid, called a fat.



A "free" fatty acid. A free fatty acid is a solitary molecule which remains (by itself) after emulsification and digestion in the intestinal lumen. Fats need to be free or solitary in order to pass through the intestinal wall and into circulation for use by the body.

Nutritional fats are all quite similar. They have a carbon chain of varying length and a head, as ilustrated above. Since the illustrated fat above is solitary, or "free", it is called a free fatty acid. The name free fatty acid is simply descriptive of this molecule. "Free", because it is solitary (on its own), "fatty", because it is a fat, and "acid" because this word describes some of its chemical attributes. It would be nice if all the fats that we obtained in our diet were free fatty acids. If this were the case, our intestines would be able to simply absorb them immediately, to be used for metabolism in the body. However, fats that we obtain in the diet are not usually in this form.

Triacyglycerol


When we consume food, the way fats are presented can vary. Keep in mind that fat structure is similar, as we discussed above. The difference in fat presentation is how they are stored or arranged in a food source. For example, if we consume animal flesh, some of the tissue will no doubt be fat. Even in the leanest of meats, we cannot escape eating fat. This is because the membranes of the muscle cells are composed of fat. There is also stores of excess fat which the animal uses to store energy. Much of this is what we see in the skin of the animal. And as you may note, this fat layer in the skin is usually thicker in the winter. We refer to this tissue as adipose tissue. The free fatty acids that we need are not usually free in the food source we consume, but rather, are bonded to other molecules. It is up to our digestive enzymes to separate these fats from other molecules, so that free fatty acids are available to be absorbed for use.

Fats can be bonded in several ways. Depending on the fat and the type of molecule that the fatty acid is bonded to, dictates the name, and hence, class of the resulting molecule. One type of molecule we consume and digest to release fats is called triglyceride, or triacylglycerol. Don't let these terms scare you, they are simply descriptive. In the word triglyceride, "tri"=three and "glyceride" is another word for glycerol. Glycerol is simply a molecule that can hold three free fatty acids (see illustration below).


A triacylglycerol molecule. Three fatty acids bonded to a glycerol "backbone". A glycerol molecule can hold up to 3 fatty acids. As we can see, a triglyceride is a large molecule, and cannot pass through the gut without first being disassembled in the doudenum and jejunum with the assistance of pancreatic enzymes and bile. Once these fats are removed from the glycerol, they are then free to pass through the intestines as free fatty acids. To disassemble a triacylglycerol in the intestine, the pancrease liberates an enzyme called pancreatic lipase. Pancreatic lipase removes the fatty acids at positions 1 and 3 (see illustration below).




The enzyme pancreatic lipase acts on positions 1 and 3 of the triacyglycerol, releasing the bound fatty acids. Note that H2O (water) is required for this reaction to occur.


Once the reaction with pancreatic lipase and water occur on positions 1 and 3 of the triacylglycerol, the result is 2 free fatty acids and a glycerol backbone with 1 remaining bonded fatty acid. This molecule of glycerol and one fatty acid is now called 2-monoacylglycerol (2-monoglyceride, old name). Like the other terms we've reviewed, 2-monoacylglycerol is a descriptive term as well. The number "2" indicates the position of the remaining fatty acid on the glycerol, "mono" denotes a "single ("mono"=1) fatty acid, and "glycerol" indicates the backbone in which the fatty acid is attached. The "acyl" is a chemical description of a chemical group in the molecule (see illustration below). Another enzyme called colipase is liberated by the pancreas during this process. Colipase functions in stabilising the pancreatic lipase in the area of the aqueous-lipid interface.



2-monoacylglycerol. As we can see, 2-monoglycerol is composed of a single remaining fatty acid bonded to the number 2 position of the glycerol backbone. The fatty acids at positions 1 and 3 were already removed by pancreatic lipase. The remaining fatty acid can now be removed by another pancreatic enzyme called acylglycerol lipase.


There are other enzymes which help to digest the various types of lipids as well. The remaining 2-monoacylglycerol is acted upon by another pancreatic enzyme called acylgycerol lipase. This enzyme cleaves the remaining fatty acid from the second position of the glycerol backbone. The result is a single free fatty acid, and a "free" glycerol backbone. The free fatty acids are now able to pass through the intestinal wall into circulation for use.


Dietary lipid metabolism of Phospholipids

Dietary lipid metabolism of Glycolipids

Cholesterol and steroids

Classes of lipids



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