When you take in food, it's the job of your digestive tract to break that food down into smaller pieces through mechanical digestion, and then break larger molecules into smaller ones through chemical digestion. The intestine then absorbs the small molecules into the bloodstream to provide for cellular energy needs. This entire process needs to happen relatively quickly -- before food moves through the digestive tract toward the rectum. Digestive enzymes help digestive processes happen in a timely manner, and the digestive system relies upon a variety of them.
Amylase
While different parts of the digestive tract secrete different enzymes, amylases are found throughout the digestive tract, from mouth through intestines. Your saliva contains small amounts of an enzyme called salivary amylase, notes Dr. Lauralee Sherwood in her book "Human Physiology." This enzyme digests starch, which is the carbohydrate found in grain, starchy vegetables, and beans. As starch breaks down, it becomes sugar. If you keep a starchy food, like a cracker, in your mouth long enough, you'll taste the cracker start to become sweet -- this is the result of starch breakdown due to salivary amylase. The stomach and intestines also digest starch using amylase.
Protease
Proteases are protein-digesting enzymes. They're not found in the saliva, but the digestive juices of the stomach and intestine both contain proteases. When you consume protein, it remains unchanged in your mouth, though you do break down larger chunks of protein-containing food into smaller ones using the teeth and tongue. Once protein reaches the stomach, protease enzymes including pepsin begin to break protein into its constituent molecules, called amino acids, explains Dr. Sherwood. Your digestive tract can absorb amino acids into the bloodstream.
Lipase
Neither the mouth nor the stomach secrete lipases, which are fat-digesting enzymes. Instead, these enzymes come from the pancreas, which secretes them into the small intestine, explains Dr. Gary Thibodeau in his book "Anatomy and Physiology." In order to work on fat, lipases must mix with fat. This is logistically difficult, because lipases are water-soluble and fat is not. Bile salts from the gallbladder mix with fat and help to make it more water-soluble, allowing lipases to break fat molecules into smaller parts that the intestine can absorb.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "Anatomy and Physiology"; Gary Thibodeau, Ph.D.; 2007
