7.4: Protein Digestion and Absorption

Digesting Proteins

How do the proteins from foods, denatured or not, get processed into amino acids that cells can use to make new proteins? When you eat food, the body’s digestive system breaks down the protein into individual amino acids, which cells absorb and use 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. We'll use eggs, a good dietary source of protein, as our example to describe the path of proteins in digestion and absorption. One egg, whether raw, hard-boiled, scrambled, or fried, supplies about six grams of protein.

Figure \(\PageIndex{1}\): White, speckled (red), and brown chicken eggs. The egg is a good dietary source of protein. "Egg Colours" by Timothy Titus is licensed under CC BY-SA 3.0.)

From the Mouth to the Stomach

Unless you are eating it raw, the first step in egg digestion (or that of any other protein food) involves chewing or mastication. The teeth begin the mechanical breakdown of the large egg pieces into smaller pieces that can be swallowed. The salivary glands provide some saliva to aid swallowing and the passage of the partially mashed egg through the esophagus. The mashed egg pieces enter the stomach through the esophageal sphincter. The stomach releases gastric juices containing hydrochloric acid and the enzyme pepsin, which initiate the chemical breakdown of the protein (Figure \(\PageIndex{2}\)). The chemical breakdown of protein through the actions of enzymes is known as proteolysis. Stomach acids unfold proteins that retain part of their three-dimensional structure after cooking and help break down the protein aggregates formed during cooking. Pepsin, which is secreted by the cells that line the stomach, begins the proteolysis process by breaking the protein chains into smaller and smaller fragments. Egg proteins are large globular molecules whose chemical breakdown requires time and mixing. The powerful mechanical stomach contractions churn the partially digested protein into a more uniform mixture called 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 time required to break down the meal in the stomach sufficiently. Food remains in the stomach longer, making you feel full longer.

Figure \(\PageIndex{2}\): Overview of protein digestion in the stomach. Protein digestion requires the chemical actions of gastric juice and the mechanical actions of the stomach. “Protein digestion in the stomach” from An Introduction to Nutrition (v. 1.0) is licensed under CC BY-NC-SA 3.0.

From the Stomach to the Small Intestine

The stomach empties the chyme containing the broken-down egg pieces into the small intestine, where the majority of protein digestion occurs. The pancreas secretes digestive juice that contains more enzymes that further break down the protein fragments. The two major pancreatic enzymes that digest proteins are chymotrypsin and trypsin. The cells that line the small intestine release additional enzymes that finally break apart the smaller protein fragments into individual amino acids. The muscle contractions of the small intestine mix and propel the digested proteins 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 \(\PageIndex{3}\)). This movement of individual amino acids requires special transport proteins and the cellular energy molecule, adenosine triphosphate (ATP). 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. Urea is a molecule that contains two nitrogens and is highly soluble in water. This makes it a good choice for transporting excess nitrogen out of the body. Because amino acids are building blocks that the body reserves to synthesize other proteins, more than 90% of the protein ingested does not get broken down further than the amino acid monomers.

Figure \(\PageIndex{3}\): Protein digestion and absorption. In the stomach, proteins are digested into smaller pieces called polypeptides that contain more than one amino acid (shown as lines). In the small intestines, polypeptides are broken down into single amino acids (shown as circles), which are absorbed into the bloodstream. “Summary of protein digestion” by Alice Callahan is licensed under CC BY 4.0.

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. Every day, over 250 grams of protein in your body are dismantled, and 250 grams of new protein are built. To form these new proteins, amino acids from food and those from protein destruction are placed into a “pool.” Although 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; otherwise, 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 1% of total body-protein content (Figure \(\PageIndex{4}\)). Thus, the body does not store protein as it does carbohydrates (as glycogen in the muscles and liver) or lipids (as triglycerides in adipose tissue).

Figure \(\PageIndex{4}\): Amino acids in the cellular pool come from dietary protein and the destruction of cellular proteins. The amino acids in this pool must be replenished because amino acids are outsourced to make new proteins, energy, and other biological molecules. “Protein Digestion and Absorption” from An Introduction to Nutrition (v. 1.0) is licensed under CC BY-NC-SA 3.0.

Attributions

• Zimmerman, "An Introduction to Nutrition (Zimmerman)," CC BY-NC-SA 3.0. Figures were updated, and some text was changed for clarity.

References