DIGESTION AND ABSORPTION OF FATS Detergent Action as Well as Enzymatic Action Is Necessary for the Digestion and Absorption of Lipids
Lipids, or fats, present a special digestive problem to the animal because they do not dissolve in water, the major medium in which most body processes» including digestion, occur.
Detergent action is necessary to emulsify or dissolve lipids so that they may be subjected to the actions of water-soluble hydrolytic enzymes in the gut. The problem of solubility makes the mechanics of digestion and absorption of lipids somewhat different from that of proteins and carbohydrates. For that reason, lipid assimilation is discussed here in a separate section.Lipids make up a large portion of the diets of carnivores and omnivores, whereas they usually form a minor portion of the natural diets of adult herbivores. Nonetheless, it appears that herbivorous species have the capacity to digest and absorb lipids in quantities considerably higher than found in their natural diets, and frequently, supplemental lipids are added to the diets of performance horses and high-producing dairy cows. The neonates of all mammalian species have a high capacity for lipid digestion and absorption because milk has a high fat content.
The primary dietary lipid is triglyceride, which may originate from either plant or animal sources. Other important dietary lipids are cholesterol and cholesteryl ester from animal sources, waxes from plant sources, and phospholipids from both plant and animal sources. Figure 30-24 illustrates the structures of these dietary lipids. In addition, the lipid-soluble vitamins A, D, E, and K are absorbed along with the other dietary lipids.
Lipid assimilation can be divided into four phases: (1) emulsification, (2) hydrolysis, (3) micelle formation, and (4) absorption. Enndsification is the process of reducing lipid droplets to a size that forms stable suspensions in water or water-based solutions. In the gut the emulsification phase begins in the stomach as the lipids are warmed to body temperature and subjected to the intense mixing, agitating, and sieving actions of the distal stomach.
This distal-stomach activity tends to break lipid globules up into droplets that pass into the small intestine. In the small intestine, emulsification is completed by the detergent action of bile acids and phospholipids. (See Chapter 29 for a discussion of bile formation and secretion.) These bile products reduce the surface tension of the lipids and allow the droplets to become even further divided and reduced in size (Figure 30-25).While in the bile-coated, or emulsified-droplet, stage, the lipids are subject to the actions of hydrolytic enzymes. Hydrolysis of triglyceride, the major dietary lipid component, occurs because of the combined action of the pancreatic enzymes lipase and co-liρase. Lipase is an enzyme secreted, in its active form, from the pancreas. However, lipase cannot directly attack the emulsified lipid droplets in the gut because it cannot penetrate the coat of bile products surrounding the droplets. The function of co-lipase, a relatively short peptide, is to “clear a path" through the bile products, giving lipase access to the underlying triglycerides. Lipase cleaves the fatty acids off each end of the triglyceride molecule but does not attack the central fatty acid, resulting in the formation of two free, or Iionesterified, fatty acids and a nιonoglyceride from each molecule of triglyceride hydrolyzed (Figure 30-26).
Other lipid-digesting pancreatic enzymes are cholesterol esterase and phospholipase. The products of these enzymes are nonesterified fatty acids, cholesterol, and Iysophospholipids.
The products of hydrolytic lipid digestion (fatty acids, monoglycerides, etc.) combine with bile acids and phospholipids to form micelles, small water-soluble aggregations of bile acids and lipids. Micelles are considerably smaller than the emulsified fat droplets from which they are derived (see Figure 30-25). The soluble micelles allow the lipids to diffuse through the gut lumen into the unstirred water layer and into close contact with the absorptive surface of the apical membrane (see Figures 30-26 and 30-27).