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6.2: Digestion and Absorption of Proteins

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  • Learning Objectives

    • Discuss how proteins are digested and absorbed by our bodies.

    When you eat food, the body’s digestive system breaks down the protein into the individual amino acids, which are absorbed and used by cells to build other proteins and a few other macromolecules, such as DNA. Let’s follow the specific path that proteins take down the gastrointestinal tract and into the circulatory system.

    From the Mouth to the Stomach

    The first step in protein digestion involves chewing. The teeth begin the mechanical breakdown of food into smaller pieces that can be swallowed. The salivary glands provide some saliva to aid swallowing and the passage of the smaller pieces of food through the esophagus and then on to the stomach through the esophageal sphincter. The stomach releases gastric juices containing hydrochloric acid and pepsin (an enzyme) which initiate the chemical breakdown of protein. Hydrochloric acid (HCl) in the stomach begins protein digestion by denaturing the protein; denaturation results in the loss of the protein's function. HCl also converts inactive pepsinogen to its active form, pepsin. Pepsin begins breaking the peptide bonds between amino acids.

    The powerful mechanical stomach contractions churn the partially digested protein into chyme. Protein digestion in the stomach takes a longer time than carbohydrate digestion, but a shorter time than fat digestion. Eating a high-protein meal increases the amount of time required to sufficiently break down the meal in the stomach. Food remains in the stomach longer, making you feel full longer.

    From the Stomach to the Small Intestine

    The stomach empties the chyme containing the partially digested protein into the small intestine, where the majority of protein digestion occurs. The pancreas secretes digestive juice that contains proteases (enzymes) that break down polypeptides into smaller peptides or single amino acids. The two major pancreatic enzymes that digest proteins are trypsin and chymotrypsin. Amino acids, dipeptides, and tripeptides are absorbed into the cells of the intestinal wall. The cells that line the small intestine release peptidases (enzymes) that break down dipeptides and tripeptides into single amino acids. The muscle contractions of the small intestine mix and propel the amino acids to the absorption sites. In the lower parts of the small intestine, the amino acids are transported from the intestinal lumen through the intestinal cells to the blood (Figure \(\PageIndex{1}\)1). Once the amino acids are in the blood, they are transported to the liver. As with other macronutrients, the liver is the checkpoint for amino acid distribution and any further breakdown of amino acids, which is very minimal. Recall that amino acids contain nitrogen, so further breakdown of amino acids releases nitrogen-containing ammonia. Because ammonia is toxic, the liver transforms it into urea, which is then transported to the kidney and excreted in the urine. Because amino acids are building blocks that the body reserves in order to synthesize other proteins, more than 90 percent of the protein ingested does not get broken down further than the amino acid monomers.

    Diagram of protein absorption. Once proteins are digested into dipeptides, tripeptides, and amino acids, they are absorbed into the intestinal cells and then enter the bloodstream as amino acids.
    Figure \(\PageIndex{1}\): Once proteins are digested into dipeptides, tripeptides, and amino acids, they are absorbed into the intestinal cells and then enter the bloodstream as amino acids. (CC BY-NC-SA 2.0; by Nutritional Doublethink via Flickr)

    Amino Acids Are Recycled

    Just as some plastics can be recycled to make new products, amino acids are recycled to make new proteins. All cells in the body continually break down proteins and build new ones, a process referred to as protein turnover. To form these new proteins, amino acids from food and those from protein destruction are placed into a “pool.” Though it is not a literal pool, when an amino acid is required to build another protein it can be acquired from the additional amino acids that exist within the body. Amino acids are used not only to build proteins, but also to build other biological molecules containing nitrogen, such as DNA and RNA, and to some extent to produce energy. It is critical to maintain amino acid levels within this cellular pool by consuming high-quality proteins in the diet, or the amino acids needed for building new proteins will be obtained by increasing protein destruction from other tissues within the body, especially muscle. This amino acid pool is less than one percent of total body-protein content. Thus, the body does not store protein as it does with carbohydrates (as glycogen in the muscles and liver) and lipids (as triglycerides in adipose tissue).

    Key Takeaways

    • Mechanical digestion of protein begins in the mouth and continues in the stomach and small intestine.
    • Chemical digestion of protein begins in the stomach and ends in the small intestine.
    • The body recycles amino acids to make more proteins.
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