Learn The Facts: Does Excess Dietary Protein Get Stored As Fat?
Written By Dylan Klein


It’s not uncommon to hear claims that dietary protein eaten in excess of some arbitrary number will be stored as body fat.

Even those who are supposed to be reputable sources for nutrition information propagate this dogma. These claims however tend to drastically ignore context. Paging through one of my old nutrition textbooks I came across a section in the protein chapter regarding amino acids and energy metabolism [1]. To quote the book directly:


“Eating extra protein during times of glucose and energy sufficiency generally contributes to more fat storage, not muscle growth. This is because, during times of glucose and energy excess, your body redirects the flow of amino acids away from gluconeogenesis and ATP-producing pathways and instead converts them to lipids[…] The resulting lipids can subsequently be stored as body fat for later use.”

This is, more or less, supported by another textbook I own [2]:


“In times of excess energy and protein intakes coupled with adequate carbohydrate intake, the carbon skeleton of amino acids may be used to synthesize fatty acids.”

While these passages do take into account the metabolic state of the person, I still find these explanations (the first one especially) to be lacking, borderline misleading. Indeed, more recent evidence is needed when talking about amino acid conversion to fatty acids. While the metabolic pathways to convert amino acids to fatty acids do, in fact, exist in the human body, the reality of the matter is that under almost no circumstance will this ever happen.

What is to follow is the long explanation as to why this is true.

Protein digestion begins in the stomach and ends in the small intestine

While the physical breakdown of proteins does take place in the mouth, it’s not until proteins reach the stomach that any appreciable chemical breakdown occurs; this is facilitated by hydrochloric acid (HCl) and the enzyme pepsin (converted from its inactive form, pepsinogen). Once the initial protein denaturing and peptide cleaving is complete, the product polypeptides pass through the pyloric sphincter of the stomach and into the proximal small intestine. The proximal small intestine (the duodenum to be exact) is where most of the digestion of proteins and virtually all of the absorption of amino acids occurs (a very small amount do get excreted in the feces). Here even more digestive enzymes are present to break down the remaining polypeptides into their individual amino acids along with some trace amounts of di- and tri-peptides.


Once broken down completely, the free amino acids and di-/tri-peptides can then enter the cells of the small intestine where some (especially glutamine) are used for energy within the intestinal cells, with the remaining passing through into hepatic portal circulation. Those that do pass into circulation are destined for the liver.

Protein absorption claims

Before we head on over to the liver and discuss amino acid metabolism with regards to the initial claims, I would first like to touch upon another related (or variant) claim that some of you may have heard in the lay media or from an uneducated classmate, etc. It usually reads:


“The average person can only absorb 30 g's of protein at one sitting. Anything above that will be stored as fat.”

Unlike the claims in the introduction, this one offers no context whatsoever. Moreover, it’s downright moronic. While this assertion sounds like straw man argument, readily poised for the takedown, I actually got this gem of wisdom from an online article written by a Registered Dietitian. Believe me; I couldn’t make this crap up if I wanted to (note: this isn’t to slander the RD profession, this just an example which happened to involve someone who should know better).


Example

For instance, let’s take someone who eats, dare I say it, 40 g's of protein in one “sitting” (whatever that means). If we are to assume only 30g are absorbed at a time, then it’s safe to say that the extra 10g will be excreted in the feces. This, however, is just plain false. The gut’s ability to absorb protein is somewhere in the ballpark of 95% (i.e. only ~2g aren’t absorbed in our example meal). Nevertheless, based on the initial argument, how are you supposed to store 10g of excess protein as body fat if you can’t even (allegedly) absorb it in the first place?

Most people (who I can understand don’t have a degree in nutrition) don’t realize the difference between utilization and absorption and make this error.


I’m not quite sure what this RD’s excuse is, because, in order to store or metabolize a nutrient (i.e. utilize it), it must first be absorbed into the body.

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