Steatorrhea and Lipid Digestion Part 2
August 21st 2008 09:09
LIPID DIGESTION
The bulk of dietary lipid is triglyceride, composed of a glycerol backbone with each carbon linked to a fatty acid . Foodstuffs typically also contain phospholipids, cholesterol, and others (Figures 1, 2, and 3). Once we have taken in food through our mouth, digestion already takes place. There are two types of digestion: the mechanical and the chemical digestion. And the fats from the food we eat are mechanically digested and broken down into smaller pieces by chewing and by the agitations produced in the walls of the gut. Nevertheless such process at times, is just not enough in order for us to optimally absorb the nutrients. And so chemical digestion has to take place.
Figure 1. General Structure of Triglyceride
Figure 2. General Structure of Cholesterol
Figure 3. Phospholipid
The mouth and the stomach may play roles in lipid breakdown. However in the said locations, minimal digestion takes place. The bulk of lipid digestion occurs along the small intestine. Nevertheless in the mouth, an enzyme called the lingual lipase acts on triglycerides with short to medium fatty acid chains. Linguinal lypase is secreted by the Ebner’s glands located at the root of the tongue and parotid glands. It hydrolyzes triglycerides into a diacylglyceride and a fatty acid. Such action of the lingual lipase continues as the food enters the stomach despite of the acidic environment (since its optimal pH is 4). In the stomach another enzyme exerts its effect on the lipids or triglycerides containing small to medium fatty acid chains by the same hydrolyzing effect. This time the enzyme is called gastric lipase (range of optiml pH is 3 to 6), produced by the gland cells of the fundic stomach.
The bigger amount of fats is digested along the small intestine. And so chances are the fats being mobilized from stomach to small intestine remain to be large aggregates of triglycerides, cholesterol ester, and phospholipids, and would need further breakdown. Such role is being performed by the enzymes secreted by the pancreas into the small intestine namely the pancreatic lipase, the phospholipase, and the cholesterol esterase.
We all know that the pancreatic enzymes, most especially the lipases are water-soluble enzymes, and therefore are short-lived. And so after meal, these lipases have to be released immediately into the small intestine in huge amount to ensure faster action. However if the fat aggregates are too large for the enzymes, then it may take long before these enzymes finish their jobs. By that time, most of them would have lost their potency. Therefore, the lipid aggregates have to be torn down into smaller bits or droplets. This would increase lipase-lipid interaction thus, increasing efficiency and speed of digestion. This process is being done by the bile/ bile acids from the liver and gall bladder.
Bile acids play their first critical role in lipid assimilation by promoting emulsification. As derivatives of cholesterol, bile acids have both hydrophilic and hydrophobic domains (i.e. they are amphipathic- Figure 4). On exposure to a large aggregate of triglyceride, the hydrophobic portions of bile acids intercalate into the lipid, with the hydrophilic domains remaining at the surface. Such coating with bile acids aids in breakdown of large aggregates or droplets into smaller and smaller droplets (Figure 5). The "detergent" function of bile salts is essential to fat digestion, for the lipase can "attack" the fat globules only on their surfaces. The smaller the fat particles, the better digestion.
Figure 4. Bile Acids
Figure 5. Emulsification
After lipid emulsification, the pancreatic enzymes finally would have optimal effect on digestion. The pancreatic lipase breaks triglyceride down to one 2-monoglyceride and two fatty acid chains (Figure 6). In order for the lipase to carry out such function, it should not be inhibited by bile acid action. And so upon emulsification of a lipid particle, bile acids have to be displaced away from the emulsified lipids. Displacement of such is done by the colipase, which is also a protein found in pancreatic secretions.
Figure 6
Phospholipids are in turn broken down into components by the phospholipase. Phospholipids are broken down into glycerol, fatty acids, phosphoric acid and others (e.g. choline).
The dietary cholesterol, which occurs frequently as cholesterol ester, has do be broken down into its fatty acid and free cholesterol components. This has to be done in order for the cholesterol to be effectively absorbed later on. And this is the job of the cholesterol esterase.
The whole process of digestion in the small intestine is triggered by the release of cholecystokinin hormones, thus stimulating the pancreas and liver to release their respective secretions.
The bulk of dietary lipid is triglyceride, composed of a glycerol backbone with each carbon linked to a fatty acid . Foodstuffs typically also contain phospholipids, cholesterol, and others (Figures 1, 2, and 3). Once we have taken in food through our mouth, digestion already takes place. There are two types of digestion: the mechanical and the chemical digestion. And the fats from the food we eat are mechanically digested and broken down into smaller pieces by chewing and by the agitations produced in the walls of the gut. Nevertheless such process at times, is just not enough in order for us to optimally absorb the nutrients. And so chemical digestion has to take place.
Figure 1. General Structure of Triglyceride
Figure 2. General Structure of Cholesterol
Figure 3. Phospholipid
The mouth and the stomach may play roles in lipid breakdown. However in the said locations, minimal digestion takes place. The bulk of lipid digestion occurs along the small intestine. Nevertheless in the mouth, an enzyme called the lingual lipase acts on triglycerides with short to medium fatty acid chains. Linguinal lypase is secreted by the Ebner’s glands located at the root of the tongue and parotid glands. It hydrolyzes triglycerides into a diacylglyceride and a fatty acid. Such action of the lingual lipase continues as the food enters the stomach despite of the acidic environment (since its optimal pH is 4). In the stomach another enzyme exerts its effect on the lipids or triglycerides containing small to medium fatty acid chains by the same hydrolyzing effect. This time the enzyme is called gastric lipase (range of optiml pH is 3 to 6), produced by the gland cells of the fundic stomach.
The bigger amount of fats is digested along the small intestine. And so chances are the fats being mobilized from stomach to small intestine remain to be large aggregates of triglycerides, cholesterol ester, and phospholipids, and would need further breakdown. Such role is being performed by the enzymes secreted by the pancreas into the small intestine namely the pancreatic lipase, the phospholipase, and the cholesterol esterase.
We all know that the pancreatic enzymes, most especially the lipases are water-soluble enzymes, and therefore are short-lived. And so after meal, these lipases have to be released immediately into the small intestine in huge amount to ensure faster action. However if the fat aggregates are too large for the enzymes, then it may take long before these enzymes finish their jobs. By that time, most of them would have lost their potency. Therefore, the lipid aggregates have to be torn down into smaller bits or droplets. This would increase lipase-lipid interaction thus, increasing efficiency and speed of digestion. This process is being done by the bile/ bile acids from the liver and gall bladder.
Bile acids play their first critical role in lipid assimilation by promoting emulsification. As derivatives of cholesterol, bile acids have both hydrophilic and hydrophobic domains (i.e. they are amphipathic- Figure 4). On exposure to a large aggregate of triglyceride, the hydrophobic portions of bile acids intercalate into the lipid, with the hydrophilic domains remaining at the surface. Such coating with bile acids aids in breakdown of large aggregates or droplets into smaller and smaller droplets (Figure 5). The "detergent" function of bile salts is essential to fat digestion, for the lipase can "attack" the fat globules only on their surfaces. The smaller the fat particles, the better digestion.
Figure 4. Bile Acids
Figure 5. Emulsification
After lipid emulsification, the pancreatic enzymes finally would have optimal effect on digestion. The pancreatic lipase breaks triglyceride down to one 2-monoglyceride and two fatty acid chains (Figure 6). In order for the lipase to carry out such function, it should not be inhibited by bile acid action. And so upon emulsification of a lipid particle, bile acids have to be displaced away from the emulsified lipids. Displacement of such is done by the colipase, which is also a protein found in pancreatic secretions.
Figure 6
Phospholipids are in turn broken down into components by the phospholipase. Phospholipids are broken down into glycerol, fatty acids, phosphoric acid and others (e.g. choline).
The dietary cholesterol, which occurs frequently as cholesterol ester, has do be broken down into its fatty acid and free cholesterol components. This has to be done in order for the cholesterol to be effectively absorbed later on. And this is the job of the cholesterol esterase.
The whole process of digestion in the small intestine is triggered by the release of cholecystokinin hormones, thus stimulating the pancreas and liver to release their respective secretions.
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