Dr. Robert Wagner is a multi-national champion in Powerlifting (799 squat at 198lbs) and an expert in the strength and conditioning field. He was kind enough to pen this article to expand on last week's post by Phil Wagner about protein absorption.
Protein: It's Not Just For Muscles
Recently, Coach Steel posted an article by Phil Wagner titled, “How much protein can you really eat?” Phil discussed the myth of only being able to digest 30 grams of protein per hour. In 2009 I did a presentation on a subject similar to this and asked Jim if I could post what I had discovered in preparing for that presentation. Consider this an expansion of the post Phil provided. Let me start by restating the myth in Phil’s article that one should never eat more than 30 grams of protein in a meal because your body can’t handle digesting/using /absorbing more than that in an hour.
The first step in dispelling the myth is to look at how we digest and breakdown protein. By cooking protein we initiate its breakdown. In supplement protein powders this is carried out through various processes of using heat or chemicals to initiate protein breakdown. Heat and certain chemicals, particularly acids, cause protein to denature. When protein is denatured it means the protein molecules are becoming less tightly bound or becoming looser. Imagine raw uncooked protein as a knot in your shoe lace, denaturation loosens the knot. This is important because the looser the knot the more the chemicals in the body (enzymes) that breakdown protein can access the now loosened knot making it easier to untie. In liquid protein we have an additional process. When protein molecules are denatured (in much looser state), they bounce around and collide with one another. As this continues they actually start to cling to one another forming a semisolid gel. This is called coagulation. Coagulation is another step in the breakdown of protein. Examples of this are when you see egg white turn white or when blood clots. In both circumstances the proteins denature (caused by heat cooking the egg) and chemical factors (released into blood). As the molecules bounce around and start clinging to each other, the egg turns white and becomes semi solid and blood forms a semi solid clot. Denaturation and coagulation are steps in the digestion of protein that prepare protein for complete breakdown.
The stomach is where chemical digestion starts. Gastric juice which includes hydrochloric acid (HCl) and other digestive enzymes either initiate digestion of liquid protein by denaturing it or continue the digestion started from cooking or processing in the case of supplement protein mixes. The enzymes will break the bonds that hold long chains of amino acids (AA) together into smaller chains. This process will continue in the small intestine where enzymes break the bonds into individual AA and small chains of AA. Food stays in stomach for two to three hours typically. Liquid proteins will move through the stomach quicker since they are easier to dismantle. In the stomach the coagulated protein is mixed with the juices until it breaks down and forms a soupy liquid called chyme. As the stomach mixes and produces chyme it is gradually released by squirting into the small intestine.
The Small Intestine (SI) is roughly 20 – 30 ft. in length (when stretched out). It can take food 5-14 hrs. to travel from mouth, stomach and through the SI. The digestion process continues with the release of enzymes from the pancreas. There are multiple enzymes released in the due to multitude of proteins that enter the system. The SI also produces its own digestive enzymes. These enzymes breakdown protein molecules just before absorption takes place. The release is activated by mechanical, neural and chemical (hormonal) stimulation. One of the mechanical actions is the distention of the intestinal wall. The presence of chyme also stimulates this process hormonally. Even in this abbreviated review you can see that the digestive system is well prepared to handle quite a bit of protein. If you consider the time frames over which the digestive processes occur you can see that looking at an hour time frame really limits the mechanisms and actions the body performs when it digests protein. As you will see, while protein digestion can range from 1-10 hours, the absorption process will be working in concert with digestion as proteins are broken down into their absorbable states.
The absorption of protein occurs in the small intestine. The SI is considered to be the most important absorption organ in the body. Very little of what can be absorbed reaches the large intestine, 95% of dietary protein is pulled out and absorbed by the SI. Protein is typically absorbed in three forms. It is found in individual AA, di-peptide and tri-peptides (2 – 3 AA linked to one another) and small peptide chains (<10 AA). All of these are absorbed but in slightly different ways since they use different transporters to enter the cells of the intestinal wall. The di- and tri-peptides and small peptides, once inside the intestinal cells, are broken down further into individual AA. The fact that our intestinal cells preferably absorb these various forms might lead one to realize that whole food sources of protein may be a necessity in the diet and that one cannot live on amino acid supplements alone.
Inside the intestinal cells the free AA faces two fates. They are either carried away by the blood or they are used by the intestinal cells. As a note the intestinal cells are very metabolically active and are one of the most quickly replicating tissues in the body. About 50% of the dietary AA intake is used by the intestine and other parts of the digestive system. Because these cells replicate so quickly there is a constant need for new protein to help build these new cells. The AA are also used for immune function and in some cases may even be broken down and used for energy. Considering this, the digestive systems’ requirement for AA controls the flow of protein into the rest of the body based on the demands the digestive system has from its own cells. If there is a greater demand in the digestive organs less protein may be released to the rest of the body. In fasting situations the digestive system may actually draw AA from the rest of the body for use in the digestive system itself. One of the primary AA found to move to this area is Glutamine. Glutamine which is taken from muscle is used as energy for immune and intestinal cells in the digestive system in a fasting state. Other factors affecting the flow of AA into the body are the amount of protein ingested, the quality of protein, the protein rate of absorption (slow vs. fast) and the combination of other nutrients that are ingested with protein including fat, fiber and carbohydrates.
As you can see the factors involved in the absorption of protein are complicated. To simply say that a certain amount of protein will over extend or overwhelm the system is nonsense especially when you look at the surface area available for absorption and the processes involved in breaking down the protein molecule. The regulation and movement of AA in the body depends on not only the needs of the muscles that are being trained but also the digestive system needs. In the next post I will discuss how the other factors mentioned previously affect the flow of AA, the rates of protein absorption and how research has determined how much most protein you can safely ingest.