Protein Digestion and Protein Absorption

Last update November 11, 2021, article reviewed & updated multiple times since April 18, 2001.

What You Need to Know Protein digestion begins in the stomach, chiefly with the action of the hydrochloric acid that is produced there, and by the enzyme called pepsin. Once in the blood, the amino acids are carried by both the red blood cells and by the liquid part of the blood, called the plasma. The destruction of proteins in the body gives rise to two classes of waste products: nitrogenous, those containing nitrogen, and non-nitrogenous, those that don’t contain nitrogen.

Continuing Our Discussion of Protein, Protein Digestion and Protein Absorption…

Last time, we began our discussion of protein with such subjects as what protein is and why it is so important. We discussed amino acids, which are the building blocks of proteins, and the fact that some amino acids can be synthesized, while others (called essential amino acids) cannot be synthesized. Today we’ll be talking about the digestion of protein and the waste products produced by the breakdown of protein.

The protein parts of every cell in the body are being destroyed continually. As a result, our bodies need to replace these protein structures constantly. This requires that we eat protein every day. Fortunately, those of us who follow a low carbohydrate lifestyle, don’t have any problems getting enough needed protein.

Protein Digestion

As I have suggested before, life is a system of cooperating enzyme reactions, and once again, enzymes are the prime movers in protein digestion just as they were in carbohydrate digestion. The enzymes for protein digestion are collectively called proteinases (protein-ACES) or proteases (pro-tea-ACES). Proteins are broken apart by the protein-digesting enzymes in a process called hydrolysis.

Protein digestion begins in the stomach, chiefly with the action of the hydrochloric acid that is produced there, and by the enzyme called pepsin (PEP-sin). Some seven or more factors influence how fast the enzymes act on the protein. These factors include the concentration of the enzyme, that is, how much of it is present; the amount of protein food needing action; the acidity of the food and of the stomach; the temperature of the food; time; and the presence of any digestion inhibitors, such as antacids. Cooking and chewing help, but protein digestion does not begin in the mouth, as carbohydrate metabolism does. The hydrochloric acid in the stomach is required to break the protein bonds. The protein-containing foods are broken apart, separating out the protein, then the proteins are broken into their constituent parts, the amino acids.

Protein digestion continues in the upper portion of the small intestine under the action of the pancreatic protein enzymes, trypsin (TRIP-sin) and chymotrypsin (KI-mo-trip-sin). The amino acids are absorbed by the blood capillaries of the small intestines, carried through the liver, and then go into the blood of the general circulation. Recall from our discussion of carbohydrate digestion that absorption is done by means of selectively permeable membranes of the small intestine walls, which are arranged in folds called villi.

Amino Acids Put To Use

Once in the blood, the amino acids are carried by both the red blood cells and by the liquid part of the blood, called the plasma. The amino acids are thereby distributed to all the body tissues, where the various body cells take what they need to repair and reform the protein structures they need.

The blood contains amino acids at all times. Fasting does not clear them, and a high protein diet does not materially increase them. The body has a constant need for protein amino acids, and it keeps a fairly uniform balance.

Taking The Protein From The Muscles

The body’s skeletal muscles act as an emergency source of protein if insufficient amounts are eaten. The body can break down its own muscle tissue, and transport the amino acids gathered from that muscle destruction to the more vital organs, if necessary. (As an aside, recall that we know that people on very low-fat diets are also, frequently and by default, on low protein diets. This is because most of the rich sources of protein in foods are also in sources of dietary fat. These dieters lose their muscle mass because their bodies cannibalize their own muscles as a source of the proteins that they need, but are not eating.)

Problems Arising From Incomplete or Improper Protein Digestion

Sometimes, instead of being properly broken down into amino acids, small amounts of whole or partial proteins are absorbed into the blood. The body wants amino acids, not whole proteins, and whole proteins are viewed by the system as an enemy. This is where we get the phrase foreign protein. The presence of protein instead of amino acids may lead to food allergies, to a shock reaction called anaphylaxis (anna-phil-AXIS), to other symptoms typical of an allergy, such as sneezing, breathing difficulties, skin rashes, headaches, nausea, or even, in severe cases, death. And these problems result from just a very small amount of the food protein, which doesn’t belong there.

Sometimes protein substances containing nitrogen may reach the large intestine. This may be undigested or partly digested food residues, unabsorbed amino acids, unused protein enzymes, or the protein of dead bacteria. These protein substances will likely be attacked by microorganisms (bacteria) that live in the intestinal tract, and be decomposed by the process called putrefaction (pew-tra-FAC-tion). This often results in diarrhea.

Waste Products of Protein Metabolism

The destruction of proteins in the body gives rise to two classes of waste products: nitrogenous (ny-TRA-gin-us), those containing nitrogen, and non-nitrogenous (non-ny-TRA-gin-us), those that don’t contain nitrogen. The non-nitrogenous types of waste products are carbon dioxide and water. Nitrogenous waste products only relate to proteins since only proteins contain nitrogen.

The nitrogenous waste products are known as urea (yur-RE-ah), uric acid (yur-ick acid), creatinine (cree-AT-tin-neen), and hippuric acid (hip-PURE-ick acid). Urea is the major nitrogenous waste product, making up some 80% of it. Urea is formed in the liver, and is excreted by the kidneys in the urine along with the other types of protein waste products.

The Science of Low-Carb & Keto Diets Presented by CarbSmart.com This article is number 8 of 24 articles describing The Science of Low-Carb & Keto Diets. Visit this link or click the above image for all of the articles in the series.	Article History Last Update November 10, 2021 Updated January 31, 2018 Updated June 24, 2010 Updated August 28, 2003 Original Article August 18, 2001
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About Dr. Beth Gruber Dr. Gruber is a graduate of the Southern California University of Health Sciences and has been in private chiropractic practice in Long Beach, California since 1964. She also received both a Bachelor’s Degree and a Master’s Degree from California State University at Long Beach. She has written on health-related subjects for over 30 years, for several different publications. She lives in Southern California with her husband of 33 years. Both she and her husband follow and live the low-carb lifestyle full time.

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