Protein is a macronutrient composed of amino acids that plays a variety of roles in the body. Proteins can act as messengers, hormones, catalysts for chemical reactions and structural elements. Protein is found in a variety of dietary sources such as beans, meat, dairy products and eggs. The human body can digest animal and plant protein very efficiently. Significant protein digestion does not begin until the macronutrient reaches the stomach.
Protein digestion begins in the stomach. Secretion of hydrochloric acid from cells of the stomach is triggered when protein enters the organ. Hydrochloric acid begins to denature, or break down, the protein. Pepsinogen, a component of protein, is broken down to pepsin, which aids in the digestion of collagen found in muscle fibers. The partially digested protein then moves through the pyloric sphincter into the small intestine for further digestion.
When the partially digested protein enters the upper small intestine, or duodenum, the release of two hormones is triggered. The hormones secretin and cholecystokinin travel through the blood to the pancreas, where they trigger the secretion of bicarbonate. Bicarbonate helps neutralize the partially digested protein further. The majority of protein digestion occurs in the small intestine.
The final phase of protein digestion takes place on the brush border of the small intestine. Here, the smaller pieces of protein that have come through the previous phases of digestion intact are broken down into their individual amino acids. The majority of absorption occurs before reaching the lower small intestine, or jejunum.
Completion of Digestion
Once absorbed, amino acids and peptides are transported to the liver via the portal vein. Metabolism occurs in the liver where the products are released into general circulation. The very small percentage that remains unabsorbed either stays in epithelial cells for the purpose of building new proteins or is excreted through feces.
- Krause's Food & Nutrition Therapy; L. Kathleen Mahan and Sylvia Escott-Stump
- Advanced Nutrition and Human Metabolism; Sareen S. Gropper, Jack L. Smith and James L. Groff
- Ablestock.com/AbleStock.com/Getty Images