Your digestive tract has the responsibility of breaking the food you eat down into smaller pieces, both mechanically and chemically, so that your intestine can absorb the nutritional molecules from the food. Doing this requires the use of digestive juices and enzymes secreted by various organs of the digestive tract.
Your digestive tract extends from your mouth to your anus, though the portion of the tract involved in actual digestion of food extends only as far as your small intestine. Your mouth, stomach, intestine, and various accessory organs secrete digestive juices -- some of which contain enzymes -- into the digestive tract. Chewing motions in the mouth and churning motions of the stomach mix food with digestive juices, which helps break it down physically and chemically.
The first digestive processes take place as you chew food. Not only are you breaking food into smaller pieces, which increases the surface area, and mixing food with juices in saliva to help moisten it for swallowing, there are also enzymes in saliva. An enzyme is a chemical that helps a reaction take place faster than it otherwise would, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry." Enzymes in saliva break down starches in your food.
The interior of your stomach is highly acidic, unlike the interiors of the other digestive compartments. The acid in stomach digestive juices helps to break down carbohydrates and fats and also helps activate stomach digestive enzymes. The enzymes secreted by your stomach, coupled with acid in stomach juices, break down proteins to a significant degree, explains Dr. Lauralee Sherwood in her book "Human Physiology." You also digest carbohydrate to some extent in the stomach.
Pancreatic and Intestinal Juices
Once food passes out of the stomach, it passes into the small intestine. The first job of the pancreas, which secretes many juices and enzymes into the intestine, is to neutralize acid from the stomach so it doesn't damage the intestine. The pancreas also secretes a variety of digestive enzymes into the intestine, explains Dr. Gary Thibodeau in his book "Anatomy and Physiology." Your intestinal juices and enzymes collectively digest the remaining carbohydrates, proteins, and fats.
Many people think that it's possible to take supplemental enzymes to assist with digestion. The fact is that there's simply no scientific evidence to support this. Most digestive enzymes function in the intestine, and when you swallow enzymes, they go immediately to the stomach, which treats them just like any other food and digests them. There is no known mechanism for augmenting digestive enzyme function through consuming supplemental enzymes.