How protein turns into amino acids to fuel essential functions

Author: Steve Fratini, PhD, August 3, 2023

When we consume foods that contain protein, our bodies break the proteins down into simpler entities (i.e., amino acids).  Our bodies then use the amino acids to build other proteins that are needed for various bodily functions.

The following is a summary of the process of protein digestion, amino acid extraction, and then reassembly into proteins needed for the human body:

1. Mechanical digestion begins in the mouth, where food is chewed and broken down into smaller pieces.  This helps to expose more of the protein molecules to the digestive enzymes.

2. Chemical digestion begins in the stomach, where the protein molecules are broken down by enzymes called proteases.  These enzymes cleave the peptide bonds that hold the amino acids together.  The peptide bonds are like the links that connect the various charms in a charm bracelet, with each charm in the bracelet representing an amino acid.

3. The chyme (partially digested food) then moves into the small intestine, where most of the protein digestion occurs.  The pancreas secretes enzymes into the small intestine that further break down the protein fragments into smaller peptides and amino acids.  Typically, the protein in the food that we eat is a mixture of essential (i.e., indispensable) and non-essential amino acids; our bodies can use but cannot make the former.

4. The amino acids are then absorbed into the bloodstream through the walls of the small intestine.

5. Next, the amino acids are transported to the liver, where they are either used to build new proteins or are broken down for energy.

6. And, finally, if the body needs new proteins, the amino acids are reassembled into specific proteins that are needed by the body.

The process of protein digestion and reassembly is a complex one, but it is essential for the human body to function properly.  Proteins are needed for a variety of functions, including building and repairing tissues, making enzymes, and transporting nutrients.  See the “What happens if you don’t eat enough protein?” article for a more detailed discussion of this topic.   Understanding the process of protein digestion helps us to better understand how to obtain enough protein to stay healthy.

Here are some additional details about the process:

• The enzymes that break down proteins are called proteases.  There are many different types of proteases, each of which breaks down proteins in a slightly different way.  As we age, enzyme deficiencies can make it harder to digest protein.

• The liver is the body's main protein factory.  It is responsible for producing many of the proteins that the body needs, including enzymes, hormones, and antibodies.

• The body can also recycle amino acids to make new proteins.  This process is called protein turnover.  Muscle tissue for example can be scavenged for indispensable amino acids that are need to make higher priority proteins such as those needed for the repair of damaged tissue.

For a more detailed explanation, see Section 5.4 of An Introduction to Nutrition [1], the YouTube video Protein Digestion and Absorption [2] or the Kahn Academy video Overview of Amino Acid Metabolism [3].

References

[1] Byerley, An Introduction to Nutrition, LibreTexts, https://med.libretexts.org/Courses/American_Public_University/APUS%3A_An_Introduction_to_Nutrition_(Byerley)/APUS%3A_An_Introduction_to_Nutrition_1st_Edition.

[2] Hasudungan, A., Protein Digestion and Absorption, YouTube video, https://youtu.be/HL7-fYBtJ1Y.

[3] Overview of Amino Acid Metabolism, Kahn Academy video, https://www.khanacademy.org/test-prep/mcat/biomolecules/fat-and-protein-metabolism/v/overview-of-amino-acid-metabolism.