Fats of the Diet. By far the most abundant fats of the diet are the neutral fats, also known as triglycerides, each molecule of which is composed of a glycerol nucleus and three fatty acid side chains, as shown in Figure 1. Neutral fat is a major constituent in food of animal origin but much less so in food of plant origin.
Fig1. Hydrolysis of neutral fat catalyzed by lipase.
Small quantities of phospholipids, cholesterol, and cholesterol esters are also present in the usual diet. The phospholipids and cholesterol esters contain fatty acid and therefore can be considered fats. Cholesterol is a sterol compound that contains no fatty acid, but it does exhibit some of the physical and chemical characteristics of fats; in addition, it is derived from fats and is metabolized similarly to fats. Therefore, cholesterol is considered, from a dietary point of view, to be a fat.
Digestion of Fats Occurs Mainly in the Small Intestine. A small amount of triglycerides is digested in the stomach by lingual lipase secreted by lingual glands in the mouth and swallowed with the saliva. This amount of digestion is less than 10 percent and is generally unimportant. Instead, essentially all fat digestion occurs in the small intestine as follows.
The First Step in Fat Digestion Is Emulsification by Bile Acids and Lecithin. The first step in fat digestion is to physically break the fat globules into small sizes so that the water-soluble digestive enzymes can act on the globule surfaces. This process is called emulsification of the fat, and it begins by agitation in the stomach to mix the fat with the products of stomach digestion.
Most of the emulsification then occurs in the duodenum under the influence of bile, the secretion from the liver that does not contain any digestive enzymes. How ever, bile does contain a large quantity of bile salts, as well as the phospholipid lecithin. Both of these substances, but especially the lecithin, are extremely important for emulsification of the fat. The polar parts (i.e., the points where ionization occurs in water) of the bile salts and lecithin molecules are highly soluble in water, whereas most of the remaining portions of their molecules are highly soluble in fat. Therefore, the fat-soluble portions of these liver secretions dissolve in the surface layer of the fat globules, with the polar portions projecting. The polar projections, in turn, are soluble in the surrounding watery fluids, which greatly decreases the interfacial tension of the fat and makes it soluble as well.
When the interfacial tension of a globule of non miscible fluid is low, this non-miscible fluid, upon agitation, can be broken up into many tiny particles far more easily than it can when the interfacial tension is great. Consequently, a major function of the bile salts and lecithin (especially the lecithin) in the bile is to make the fat globules readily fragmentable by agitation with the water in the small bowel. This action is the same as that of many detergents that are widely used in household cleaners for removing grease.
Each time the diameters of the fat globules are significantly decreased as a result of agitation in the small intestine, the total surface area of the fat increases manyfold. Because the average diameter of the fat particles in the intestine after emulsification has occurred is less than 1 micrometer, this represents an increase of as much as 1000-fold in total surface areas of the fats caused by the emulsification process.
The lipase enzymes are water-soluble compounds and can attack the fat globules only on their surfaces. Consequently, this detergent function of bile salts and lecithin is very important for digestion of fats.
Triglycerides Are Digested by Pancreatic Lipase. By far the most important enzyme for digestion of the tri glycerides is pancreatic lipase, present in enormous quantities in pancreatic juice, enough to digest within 1 minute all triglycerides that it can reach. The enterocytes of the small intestine contain additional lipase, known as enteric lipase, but it is usually not needed.
End Products of Fat Digestion Are Free Fatty Acids. Most of the triglycerides of the diet are split by pancreatic lipase into free fatty acids and 2-monoglycerides, as shown in Figure 2.
Fig2. Digestion of fats.
Bile Salts Form Micelles That Accelerate Fat Digestion. The hydrolysis of triglycerides is a highly reversible process; therefore, accumulation of monoglycerides and free fatty acids in the vicinity of digesting fats quickly blocks further digestion. However, the bile salts play the additional important role of removing the monoglycerides and free fatty acids from the vicinity of the digesting fat globules almost as rapidly as these end products of digestion are formed. This process occurs in the following way.
When bile salts are of a high enough concentration in water, they have the propensity to form micelles, which are small spherical, cylindrical globules 3 to 6 nanometers in diameter composed of 20 to 40 molecules of bile salt. These micelles develop because each bile salt molecule is composed of a sterol nucleus that is highly fat-soluble and a polar group that is highly water-soluble. The sterol nucleus encompasses the fat digestate, forming a small fat globule in the middle of a resulting micelle, with polar groups of bile salts projecting outward to cover the surface of the micelle. Because these polar groups are negatively charged, they allow the entire micelle globule to dissolve in the water of the digestive fluids and to remain in stable solution until the fat is absorbed into the blood.
The bile salt micelles also act as a transport medium to carry the monoglycerides and free fatty acids, both of which would otherwise be relatively insoluble, to the brush borders of the intestinal epithelial cells. There the monoglycerides and free fatty acids are absorbed into the blood, as discussed later, but the bile salts are released back into the chyme to be used again and again for this “ferrying” process.
Digestion of Cholesterol Esters and Phospholipids. Most cholesterol in the diet is in the form of cholesterol esters, which are combinations of free cholesterol and one molecule of fatty acid. Phospholipids also contain fatty acid within their molecules. Both the cholesterol esters and the phospholipids are hydrolyzed by two other lipases in the pancreatic secretion that free the fatty acids—the enzyme cholesterol ester hydrolase to hydrolyze the cholesterol ester, and phospholipase A2 to hydrolyze the phospholipid.
The bile salt micelles play the same role in “ferrying” free cholesterol and phospholipid molecule digestates that they play in ferrying monoglycerides and free fatty acids. Indeed, essentially no cholesterol is absorbed without this function of the micelles.
