The digestive tract processes a multitude of different food components each day through the use of a wide variety of enzymes and digestive juices. Carbohydrates, fats and proteins all pass through the digestive system at a different pace, are broken down into unique components and are absorbed into the body for use as fuel or to repair and build muscles, bones and organs.
Carbs begin to digest in the mouth, where enzymes in saliva start breaking complex molecules down into simpler sugars. Chewed-up carbs then pass through the esophagus and stomach with little additional digestion. In the small intestine, they are broken down into the simplest sugar molecules, which are then absorbed through the small intestine walls into the bloodstream and used by the body as fuel or sent for storage in the liver for use at a later time. Fiber, the indigestible cell walls found in carb plant foods such as beans, brown rice and whole wheat, passes through the digestive tract essentially undigested.
Protein is found in meat, eggs, dairy products and beans, and is used by the body to build muscle and organs. Protein molecules are quite large. Chewing helps break proteins down into smaller particles for digestion. Chemical protein digestion starts in the stomach, where enzymes start to soften the protein molecules. A number of enzymes, including substances from the pancreas, then break down protein into its component amino acids in the small intestine. Amino acids are absorbed through the wall of the small intestine into the bloodstream and distributed throughout the body to repair injuries and replace dying cells.
Fats and oils do not easily dissolve into the watery digestive juices of the intestinal tract. Bile, produced by the liver and stored in the gallbladder, can attach to molecules of both water and fat. Bile breaks up conglomerations of fat in the digestive tract into smaller emulsified particles, where lipase, a fat-digesting enzyme, can break it down. The broken down fat particles -- fatty acids and cholesterol -- are absorbed through the intestinal walls into the bloodstream, where they accumulate in the chest veins and are then carried to fat-deposit areas throughout the body to be stored and used for fuel when necessary.
As food is broken down by digestive juices, the intestines also absorb necessary vitamins for use in a variety of body functions, from fighting inflammation to repairing nerve damage. Water soluble vitamins such as B complex and C are only absorbed through the intestinal walls to the extent that the body has an immediate need for them; any excess is excreted in the urine, and new quantities of these vitamins must be consumed each day for optimal health. Fat soluble vitamins such as A, D, E and K are absorbed through the intestinal walls and stored in the liver and adipose tissue, or body fat, for use when needed.