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What Are Good Sources of Protein? – Digestibility

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In the introduction to this series of articles, I briefly described a number of different aspects of dietary protein that go into answering the question what is a good source of protein.  I’d mention again that ‘good’ in this sense can only be defined in a context-specific way. The protein that might be a good source under one set of conditions may not be a good source under another.  That will make more sense as I go through the series.

Today I want to talk about the issue of protein digestibility; to keep the length down I’ll save speed of digestion for Part 3 of the series.  Once again I’ll note that much of what will appear in this and subsequent articles in this series is being excerpted or paraphrased from The Protein Book, my complete look at the issue of dietary protein.

One final note: While The Protein Book is fully referenced, with over 500 research studies cited, I will not be citing references on this series of articles unless absolutely absolutely necessary.


A Primer on Protein Digestion

While the breakdown of protein begins in the mouth through the mechanical act of chewing, almost no actual digestion occurs there.  Rather, chewed protein hits the stomach where digestion and breakdown occurs via hydrochloric acid and the enzyme pepsinogen.

The majority of protein digestion occurs in the small intestine where protein is broken down into smaller and smaller amino acid (AA, the building blocks of protein) chains via a variety of protein digesting enzymes.  You can think of proteins as being a long chain of the AAs, the enzymes basically act like scissors, cutting the chains into smaller and smaller bits.

Prior to absorption into the bloodstream, whole proteins have been broken down to provide single AAs along with two and three AA chains (called di- and tri-peptides); further breakdown occurs in the intestinal cells themselves, finally releasing individual amino acids into the bloodstream.

Generally speaking, AA chains larger than three in length will not be absorbed to any appreciable degree.  I’d note that occasionally very small amounts of longer amino acid chains can slip through and this is especially the case in situations like leaky gut syndrome where the normal functioning of the gut has been compromised.

This is actually a very bad thing as the body tends to launch immune/allergic responses to the presence of undigested protein in the bloodstream; which is a big part of why the gut is set up to not allow larger protein chains into the bloodstream under normal circumstances.

Related to this is a recurrent idea, usually in sports nutrition, of supplements containing protein based hormones such as Growth Hormone (GH), Insulin-Like Growth Factor 1 (IGF-1) or others being orally consumed.  This can’t work due to the way human digestion of protein works, such peptide hormones will simply be digested in the gut and lose their biological availability.

Let me put this a different way: major pharmaceutical companies have been trying to make an oral insulin (another protein based hormone) for diabetic treatment and have basically given up on it; it took weirdly functioning drugs and there were huge problems with implementation.  If the big drug companies haven’t figured out how to do it, neither has the protein powder company claiming it in their ads.


So What is Digestibility?

Now, the above makes it sound like all ingested protein gets into the bloodstream after digestion but this is far from the case.  No process in the human body works at 100% efficiency and this is one of them.  For various reasons, a proportion of all ingested nutrients will escape digestion, continuing through the intestine to eventually end up in your poop.  Fat is typically absorbed with up to 97% efficiency and carbs can vary quite a bit depending on what you’re talking about.  But what about protein?

Researchers define protein digestibility as the amount of protein absorbed into the body relative to the amount that was consumed.  A quick note: researchers are actually measuring nitrogen absorption and excretion, rather than protein or amino acids per se, but I don’t want to get into the technical details of that here.

So, for example, they might feed someone 50 grams of protein and then see how much comes out the other end.  Let’s say that 5 grams of protein show up in the poop.  That means that 45 grams of the 50 grams ingested were actually absorbed and that protein would have a digestibility of 90% (45 grams absorbed/50 grams ingested = 0.90 * 100 = 90%).

If 50 grams of protein were fed and 25 grams showed up in the poop, that protein would have a digestibility of only 50% (25 grams absorbed/50 grams ingested = 0.50 * 100 = 50%).  Get it?

I want to note that a lot of very silly claims are often made about protein digestibility.  Companies selling protein powders argue that the digestibility of their product is impossibly high, vegetarians usually ignore the research on this topic to claim that vegetarian proteins have higher digestibility than animal source proteins, on and on it goes.  The research on this is extremely clear and I’ve reproduced the chart from The Protein Book on the digestibility of common foods below.


Food Source Protein Digestibility (%)
Egg 97
Milk and Cheese 97
Mixed US Diet 96
Peanut Butter 95
Meat and Fish 94
Whole Wheat 86
Oatmeal 86
Soybeans 78
Rice 76

Source: National Research Council. Recommended Dietary Allowances, 10th ed. National Academy Press, 1989.


Looking at the chart above, two major things stand out.  The first is that, contrary to the occasional vegetarian claim, vegetable source proteins have a significantly lower digestibility than animal source proteins.

This actually has relevance for an issue beyond the scope of this article: protein requirements.  Because they provide less available protein from consumption, a larger amount of vegetable proteins have to be consumed to meet human (or athletic) requirements.

The second is that commonly available animal-source food source proteins have extremely high digestibilities, 94-97%.  This means that for every 100 grams of protein consumed, 94-97 grams are being digested and assimilated by the gut.

Given that this likely represents the very high end of digestibility for humans (no process in humans is ever 100%).  The odds of a given commercial product being significantly above this is unlikely.  As well, even if it were the overall real-world impact would be small.

That is, let’s say a given over-priced commercial protein powder achieved a true 99% digestibility.  For every 100 grams consumed, you absorb 99 grams of protein.  That’s only 2-5 more grams than a much cheaper whole-food protein.  And given that you’ll likely be paying 2-3 times as much for the ‘magic protein powder’, this seems a pretty silly path to pursue.

Which isn’t to say that the protein powder might not have other advantages in a certain circumstance.  For example, perhaps the protein powder digests more quickly than the food; this might be valuable under certain circumstances (or negative in others).  Which is as good a bridge as I can give to the topic I’m going to discuss in Part 3 of this series: Digestion Speed.

Go to What Are Good Sources of Protein – Speed of Digestion Part 1

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