Protein Intake Per Meal: How Much Can You Digest?
One of the more common questions that many gym enthusiasts bring up is “How much protein can the body digest at one time?”
For some odd reason this purported, arbitrary amount of protein seems to land somewhere between 30-50g for all humans, regardless of body mass.
If that supposition isn’t already asinine enough, allow me to elucidate you about why the body can handle quite a bit more protein in one sitting then people seem to give it credit for.
Early phases of protein digestion
Before we dabble in protein quantities and tangible figures for you to implement, it’s worthwhile to have a rudimentary understanding of how protein is actually metabolized by the human body. Granted the entire scope of protein digestion is a field of research in and of itself, we will still go over the gist of how it works.
Firstly, as you likely already know, the ingestion process begins in the mouth which is primarily responsible for the physical breaking down of foods. After you’re incisors and molars do their dirty work, the broken bits of food/nutrients (in this instance, protein) make their way to the stomach where the digestion process kicks in.
The stomach is a rich source of gastric juice, which is composed of hydrochloric acid, sodium chloride and potassium chloride. These acids initiate the chemical breakdown (denaturation) of proteins and activate the necessary digestive enzymes to further the process of digestion. One of the key enzymes in protein digestion is pepsin, which is why some naturopaths assert that those who eat a large amount of protein should supplement with this enzyme, but that remains to be investigated for efficacy.
Late phases of protein digestion
Moving on, after the protein has been denatured sufficiently; the eluting polypeptides make their way to the duodenum, which is the anterior segment of the small intestine. The duodenum is the site of the majority of protein digestion and amino acid absorption. The plethora of digestive enzymes present in the small intestine serves to further cleave the polypeptides into isolated amino acids and minute amounts of di-/tri-peptides.
Towards the end of protein digestion, the isolated amino acids are destined to either entrance of the intestinal cells or passing through circulation towards the liver. Once in the liver, amino acids are finally subject to the proper metabolic pathways in accordance with the body’s energetic requirements (e.g. utilized for protein synthesis, used as a substrate for gluconeogenesis, etc.)
So how much protein can the human body really digest in one sitting?
With the basics of protein digestion covered, let’s take a pragmatic look at the theories and research behind how much protein can be digested at a given time. It should be noted here that this question is asking about how much can be digested, which isn’t the same as asking how much can be used for muscle protein synthesis (MPS); digestion and MPS aren’t interchangeable terms like some people seem to believe.
Back on topic, the idea that the human body has a rather random “protein cap” at the 30-50g mark just doesn’t hold up from an evolutionary nor biochemical aspect. Essentially, the supposition that your body doesn’t (read: can’t) absorb/digest more than 30g-50g of protein at once is inherently suggesting that you are just excreting any amount of protein over that mark in your feces.
So in essence, instead of your body digesting the “excess” protein, it magically bypasses the highly conserved/intricate digestive process that we covered in this article and sends the extra protein to your colon. Hmmm…we’d be pretty screwed from a physiological standpoint if that were true, not to mention people would be living on the toilet.
Moreover, there is little-to-no literature that confirms the body doesn’t absorb more than 30-50g of protein at a given moment. In fact, the literature supports that the body can indeed digest quite a large bolus; it just takes longer than a smaller dose.[1]
Basically, rather than just redirecting excess protein to your colon, the rate of digestion compensates to reduce the supply of nutrients being sent to the anterior small intestine (i.e. the stomach delays gastric processes).
For the absolute extremists who like to propose rather absurd circumstances, like eating 200g of protein at once, would the body be able to digest all of it? Well frankly, yes, but not all of that will be put to “good use” so to speak.
Protein can indeed be converted to fat, but the pathways to do so are inefficient biochemically so the significance of this conversion is trivial. Most likely, the majority of protein that isn’t used for MPS or other anabolic processes is probably subject to hepatic gluconeogenesis and subsequently stored in the form of glycogen.
So there you have it, you can eat quite a bit of protein at any given time, just don’t be too extreme about it if you want to effectively utilize the amino acids.
References:
1. Adibi, S. A., & Mercer, D. W. (1973). Protein digestion in human intestine as reflected in luminal, mucosal, and plasma amino acid concentrations after meals. Journal of Clinical Investigation, 52(7), 1586.