What Are Good Sources of Protein? – Digestibility

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



21 thoughts on “What Are Good Sources of Protein? – Digestibility

  1. No. I’ll discuss BV when I talk about quality in a few days.

    BV does take into account digestibility but there is more to the BC score than that.

    This piece is solely about how much protein is actually digested within the gut and absorbed into the bloodstream. BV is more involved than that.


  2. Interesting, the milk and cheese is so high. But I guess when you think about it milk is evolved to be eaten and digested whereas meat and vegetable proteins were not necessarily.

  3. Re: vegetable digestibility and fiber. Yeah probably.

    MAP and 100% digestibility? Total bullshit.

    And meat is still 94% digestibility, humans evolved eating that too which is why we do such a good job with it. Not just milk and cheese.


  4. So, when looking at the actual protein part of a generic protein powder like whey, it’s going to have around 97% digestibility?

    What sort of genetic variation is there in the length of the small intestine in humans? This will also effect protein digestibility for an individual, no?

  5. Is the digestibility the same value when you eat the protein along with other foods?
    Or is the above table only for use, when you eat the protein alone?

  6. Lol: Digestibility will still hold for mixed meals.

    DJMark: I’ve seen nothing (not that comes to mind anyhow) examining differences in humans (gut length won’t vary that much between folks) in terms of digestion but, outside of the situation of injury, surgery (e.g. resection of the intestine) or severe disease, I wouldn’t expect it to vary massively between individuals. There has been some recent work on calorie/nutrient absorption based on the microflora of the gut but I’ve yet to see numbers put to it in terms of how much this can actually differ between individuals. When I’ve got more useful data to present, I’ll write an article about it.

  7. Does having too much of ‘less digestive’ proteins have a strain on one’s kidneys and eventually leads to their failure in the long run? In today’s fast moving life we subject children to many flavoured protein+vitamin powders (so called ‘health drinks’ for growing children). Are these one of the reasons that kids of today are being subject to diseases like diabetes, kidney stones and even failures at a young age. I have come across a few cases around me, wherein young people below 30 years have have kidney failures and are undergoing dialysis.

  8. If a protein has low digestibility, by definition it won’t ever get to the kidneys in the first place. Digestibility has to do with whether or not it gets absorbed from the gut in the first place. If it doesn’t make it into the body, the kidney never has to deal with it.

    If there’s a reason that certain diseases are occurring with greater incidence in kids I’d post that it’s probably to do far more with increasing childhood obesity and inactivity.

  9. Lyle what are your thoughts on the old addage that you can only digest x number of grams of protein in one sitting and that the rest will be excreted?

  10. hey lyle.
    thanks for a lot of the articles you have i did find a lot about nutrition, rep range, mass buliding and all that interesting, but i have some small personal “Problems” (im from europe so my written english is pretty much garbage.. some of it)

    i have a
    i have some sort of digestion problem and im not sure if it is allegies, intolerance or too much meat, i mean you must consider that people with bad digestion (the doctor says IBS and the fanatic vegan diatist/freind of my father saids its something the medical world do not say exist) i really don´t know what to do since i just have thrown milk products out, that helped on some all-over-body-scrathing-and-feeling-bad so now i do rice protein since soy also seem to give lots of gas !!!…. now i still have some gut stuff and begin on all thords of good bacteria, general vitamins, vitamin B for stress, vitamin D for skin, flax seeds and fish oils and even some Psyllium fiber to get better, and it have all helped but i simplty still don´t feel good in my gut and it does !. (and hell as a student its expensive shit)

    !!!!!!! so on that one i guess i´m just a really “sensetive” young male wich is pretty frustating when you wan´t to just eat and lift and get big and shredded !! and just want your gut/digestion system to be a fucking peace of metal – like it seems a lot of others peoples are. !!!!!!!!!!!

    my point is just to consider that maybe “just food intolerence” is not really cool when your bulking, and in general lifting wheights and by that eat a lot, and therefore scare people in the gym by GASSING sneaky and uncontrolled all the time (especially when doing squatting). and its not really cool when your bloating wich feels really bad on both your general condition and self esteem (should be linked comment to another article about allegies and intolrence – i know).

    so is it the tuna, the gluten, the protein, the whole ting.. i´m not shure and i will try found out 🙂 this is NOT meant as a WAR ON LYLE-thing just that my logical thinking (i really felt bad about giving up whey protein because i was thinking the others was junk) says that if i get better results on rice propely because i did not digest whey and soy well, then for the vegans (i am a meat eater myself and find it hard to think that i should givet up meat) who maybefeel that their body do not diggest meat well get better results without it.

    im not so scientific, so i do not have anything to back anything up, and if i had (and the doctors and general health world did) i would propebly not have theese problems.

  11. Recently had a bout with ischemic colitis… looking for gentle proteins to eat that won’t aggravate the condition.

  12. FYI ??
    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 = 0.25 * 100 = 25%??). Get it?

  13. Just wondering something about how calories relates to protein digestibility.
    Say you eat 10 g of protein with a digestibility of 90% – so 1 g of protein is pooped out. Would the 4 calories in the 1 g of protein just pass through the body or would it be used?

  14. I love your website, but that chart in this article is 22 years old. Do you have one with more recent data?

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