Fat has a bad reputation, but it is an important part of your diet. It provides more than twice as much energy per gram as carbohydrates or protein. Fat helps your body absorb certain vitamins, and certain fats help you maintain a healthy heart. A complex process allows your body to use fat from food, and it hinges on an operation that forces oil to mix with water. The changes occur physically and chemically, and inhibiting any part of the process can have noticeable effects.
Ingestion of Fat
Chewing is the first step in the digestion of any food, breaking down the physical form of the food to provide a larger surface area for digestive enzymes to work on. The amylase in your saliva begins digesting carbohydrates in your mouth, but fat is immune to amylase, so it passes through your esophagus into your stomach unchanged. The involuntary movements of your stomach mix the swallowed food with gastric juices that contain chemicals to help break the food down into smaller molecules. Most fats are immune to this as well -- only a few short-chain triglycerides found in butter succumb before the resulting mash is fed into the small intestine.
Emulsification
Your liver sends bile to your small intestine, and the bile mixes with fat droplets to begin the emulsification process, by which fat mixes with water. Fat and water are generally unmixable, but the bile acids attach to the surface of the fat droplets in such a way that the large fat droplets are broken down into successively smaller droplets until the fat can mix freely with water in the intestine. Without bile, your body would absorb 47 percent less fat because the droplets wouldn't become small enough to allow the next step of digestion to take place.
Hydrolysis
Next, your pancreas releases lipase, which attaches to the surface of the fat droplets. This is why the action of bile is so important -- by breaking down the large fat droplets, bile creates more surface area for the lipase to attach to, allowing it to work more efficiently. The lipase breaks fat droplets apart on a molecular level, into fatty acids and monoglycerides. The molecules are now small enough to pass through the walls of the small intestine and into the bloodstream. Diet pills called "fat blockers" work by disabling lipase, so much of the fat remains unbroken and therefore unable to be absorbed. This results in reduced calorie intake, because fat is much richer in calories than other nutrients.
Absorption
At this point, fat molecules are still attached to bile molecules, forming structures called micelles, but the bile molecules stay behind when the fat is absorbed through the intestinal wall. The fat passes through the cells and comes out the other side as chylomicrons, which enter the lymphatic vessels. The lymphatic vessels drain into the general lymphatic system, which in turn drains into the bloodstream. The chylomicrons break apart in the blood, and the lipids they contained are transported throughout the body as needed.
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