In this article we will discuss about the Digestion and Absorption of Fats from Gastrointestinal Tract.
Digestion of Fats:
Lipase present in the stomach is unable to hydrolyze fats owing to the high acidity of the gastric contents. Therefore, the major part of the ingested fat is digested in the small intestine.
The ingested fat reaching the duodenum is mixed with the bile and pancreatic juice which contains lipase. The bile salts emulsify the fat before the action of lipase. The emulsification is also brought about by monoglycerides, phospholipid and lysolecithin.
The secreted inactive pancreatic lipase is activated by bile and Ca. The surface area of the emulsified fat becomes increased for which the rate of reaction of lipase is increased. Pancreatic lipase hydrolyzes 1- and 3-positions of the triglycerides leaving a mixture of 2- monoglycerides, 1, 2- and 2, 3-diglycerides as well as the soaps of the free fatty acids.
The pancreatic juice also contains phospholipase and cholesterol-esterase which hydrolyze phospholipid and esterified cholesterol. Intestinal juice also contains a lipase whose action is not of much importance as most of the fat is hydrolyzed by the pancreatic lipase.
Absorption of Fats:
Several theories have been proposed for the mechanism of absorption of fats after digestion.
The important theories are:
A. Lipolytic hypothesis.
B. Partition theory.
C. More recent theory.
A. Lipolytic Hypothesis:
1. According to this theory, fat is completely hydrolyzed to fatty acids and glycerol which are absorbed.
2. The fatty acids combining with bile salts form a miscible complex which is absorbed into the intestinal mucosa.
3. The fatty acids are then separated from bile acids and converted into triglycerides by combining with glycerol.
4. The triglycerides are passed to the lacteals. They then enter the lymphatic’s and reach the systemic circulation via thoracic duct.
B. Partition Theory:
1. According to this theory, 30 per cent of the triglycerides are hydrolyzed to fatty acids and glycerol while 70 per cent remain un-hydrolyzed.
2. The un-hydrolyzed triglycerides are emulsified by monoglycerides and diglycerides in combination with bile salts to form minute particles known as “micelles” of size about 0.1 to 0.5µ.
3. The resulting mixture is absorbed into the intestines, passed on to the lacteals and then to the lymphatic’s. The mixture then reaches the systemic circulation via thoracic duct.
4. The free fatty acids are absorbed as bile salt-fatty acid complex into the intestinal mucosa. The fatty acids are absorbed into the portal blood to reach the liver.
C. Recent Theory:
1. The removal of the ester group of 2- mono-acylglycerol requires isomerization to a primary ester linkage. This is a slow process. As a result, monoacylglycerols are the major end products of fat digestion and less than one-fourth of the ingested fat is completely broken down to glycerol and fatty acids.
2. Within the intestinal wall, 2-monoacylglycerols are converted to triacylglycerol’s and l-monoacylglycerols are further hydrolyzed to form free glycerol and fatty acids.
3. The fatty acids are then activated by thiokinase in presence of ATP and coenzyme A for the resynthesize of triacylglycerol’s.
4. The free glycerol in the intestinal lumen is about 22 per cent of total amount of triacylglycerol originally present. This passes directly to the portal vein.
5. The glycerol within the intestinal wall is activated by glycerokinase in presence of ATP to form glycerol-3-phosphate for the synthesis of triacylglycerol followed by the combination with acyl-CoA present in the intestinal wall.
6. All long chain fatty acids present in the intestinal wall are reincorporated into triacylglycerol’s which are transported to the lymphatic vessels of the abdominal region (the so-called lacteals) for distribution to the rest of the body.
7. The great majority of absorbed fat appears in the form of chylomicrons which appear first at the lymphatic vessels of the abdominal region and later in the systemic blood. The chylomicrons contain triacylglycerol, free and esterified cholesterol, phospholipid and 0.5 per cent protein.
All of the factors relating to digestion and absorption of fat are mentioned in Fig. 16.8.
Absorption of Phospholipids:
Phospholipids are split by phospholipases and their acyl chains are incorporated into chylomicrons, choline, the hydrophilic component, may be transported directly to the liver via the hepatic portal vein.
Absorption of Cholesterol:
It is absorbed into the lymphatic’s and recovered mainly as cholesteryl esters.
In this abnormality, the patient excretes milky urine because of the presence of an abnormal connection between the urinary tract and the lymphatic drainage system of the intestine, a so-called “chylous fistula”.