How Are Lipids Digested in the Watery Digestive Tract

Digestion is a watery affair and has been loosely compared to whitewater rafting. In addition to the water-based fluids we drink, liters of water-based fluid enter the digestive tract daily as part of saliva and other digestive juices. Dissolved in those fluids that our body provides are digestive enzymes. This means that our digestive enzymes are water soluble, while their task is to interact with and break down water-insoluble lipids for absorption. This presents an interesting yet readily solved problem.

When lipids are present in the small intestine the natural course would be for these substances to clump together. This is analogous to oil clumping together in the kitchen sink when we wash dishes, or to the separation of oil from the watery portion of traditional salad dressings. If lipids remain clumped together in the small intestine, surely the efficient digestion of these substances would be hindered? To solve this potential problem, bile is delivered to the small intestine and serves as an emulsifier or detergent during lipid digestion. Here, components of bile coat smaller droplets of lipid, rendering them water soluble, as depicted in Figure 5.5. Bile activity keeps larger lipid droplets from reforming. So instead of having a few very large droplets of lipids, the result is many tiny droplets. When lipids are present as tiny droplets, digestive enzymes have no problem attacking them and efficiently doing their job.

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