ï»¿ Welcome to another Spotlight video, where we walk you through a Spotlight figure from your textbook. This video walks you through the Chemical Events in Digestion spotlight found in chapter 16. After watching this video, you should be able to summarize the chemical events in digestion. Have you ever wondered why eating fatty foods can trigger unpleasant reactions in some people? Here, we focus on how the three macronutrients-- carbohydrates, lipids, and proteins-- are broken down in a process called digestion. Digestive enzymes must break the chemical bonds between the nutrients' molecules before they could be absorbed into the bloodstream. Carbohydrate digestion begins in the mouth with chewing, which mechanically breaks molecular bonds and salivary amylase, an enzyme in our saliva. The breakdown of carbohydrates continues in the stomach as well as in the small intestine, where alpha-amylase is introduced by the pancreas, as polysaccharides are broken down into disaccharides and trisaccharides. Brush border enzymes in the intestinal mucosa break these down further into monosaccharides that are absorbed into the intestinal epithelium, cross the interstitial fluid, and enter into the capillaries. Lipid digestion also begins in the mouth but relies on lingual lipase released by glands on the tongue. In the small intestine, bile salts from the liver and gall bladder assist pancreatic lipase in further breaking down lipid molecules. The resulting monoglycerides and fatty acids interact with bile salts to form micelles. These lipid bile salt complexes diffuse into the intestinal epithelium, eventually forming a soluble complex called a chylomicron. Chylomicrons are secreted into the interstitial fluid via exocytosis, then into lymphatic lacteals, and finally, into the bloodstream as the thoracic duct drains into the left subclavian vein. Proteins are incredibly complex molecules to digest and require chewing, followed by exposure to low pH and pepsin in the stomach. Multiple enzymes within the small intestine, including trypsin, chymotrypsin, elastase, and carboxypeptidase break down proteins into short peptides and amino acids. Amino acids are absorbed into the intestinal epithelium and, like simple carbohydrates, are released into capillaries. In summary, the digestion of carbohydrates, lipids, and proteins is an enzymatically controlled process of breaking large molecules down into their subunits so they can be absorbed into the bloodstream. Polysaccharides and proteins are broken down into monosaccharides and amino acids, respectively, both of which pass through the intestinal epithelium into the interstitial fluid, and then into capillaries. Lipids are digested into chylomicrons that travel first into the lymphatic system and then into the bloodstream. So what? Why is it important to understand the chemical events of digestion? Well, this knowledge can help us understand the common problems of gallstones, which afflict up to 15% of Americans. Sometimes digestive fluids harden into deposits called gallstones that can clog the gallbladder and biliary ducts. When the gallbladder contracts to release bile to digest lipids, a person with gallstones can experience intense pain in the upper-right abdomen. Some gallstone attacks can be prevented by eating less fat. However, many patients require surgical removal of the gallbladder, called a cholecystectomy. We can live without a gallbladder because bile is produced in the liver and only stored in the gallbladder. Dietary adaptations may be necessary, though, since bile can no longer be stored. This can lead to gastrointestinal discomfort as undigested lipids progress through the intestines.