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A Complete Guide to Dietary Protein

Although athletes have long known the benefits of a higher protein intake, research is finally catching up.  The idea is also becoming more prevalent in the general public as people realize that higher protein intakes are better for dieting, fat loss and appetite control.  But this raises just as much confusion as people wonder about the different dietary protein sources in terms of which is better (or best) than another.

Many websites offer simple answers to that question.  But as often as not their answer depends on what type of protein supplements they happen to be selling.  As is always the case, the answer is far more complicated.  A large number of variables including digestibility, the speed of digestion, quality, amino acid profile and others all factor into what the best dietary protein sources are.  In this guide I will look at all of them.

Many websites offer simple answers to that question, generally revolving around whatever protein they happen to sell; the answer, as always, is far more complicated than that. A large number of variables go into the declaration of what a good source of protein is and, as always, what is good in one context may not be good in another.  I’d note that this topic was of sufficient interest to me that I wrote an entire book about the topic.

Important Factors for Grading the Different Dietary Protein Sources

As I mentioned above, there are a large number of factors which go into determining the value of any given dietary protein. They include.

Digestibility and Digestion Speed

Before a protein can be used by the body, it has to be digested and absorbed into the bloodstream for use by the body.  Proteins vary in their digestibility and, logically, a protein that is poorly digested will be a poor source simply because less of what’s being eaten is being made available to the body.

Related to digestibility is the speed of digestion and there has been interest since about the late 90’s in how a given protein’s digestion speed affects how it is used by the body.

Protein Quality/Amino Acid Profile

In one sense, the topic of protein quality could be used as an overall look at many of the other topics I’m going to discuss.  In general, protein quality is a measure of how well or poorly a given protein is used by the body.  I’d note that how you define the word “use” here depends also on context.

Are we talking about a protein’s ability to sustain life, build muscle, improve performance, improve health?  Some measures of protein quality take into account digestion while others do not (which is why I’ll discuss digestion separately), the amino acid profile of the protein tends to be one of the biggest determinants of quality.

Dietary protein is made up of amino acids.  There are 18-22 different amino acids depending on who you talk to/what you read.  Different sources of dietary protein contain different proportions of amino acids and, to some degree, this will determine that protein’s quality in the body.

Presence of Absence of Other Nutrients

While often ignored, the presence or absence of other nutrients in a given protein source also impacts on how good of a protein it may be.  For example, some protein sources contain high levels of iron, B12 and zinc while others do not; the presence of absence of the omega-3 fatty acids (fish oils) may also be relevant.  Calcium is also a consideration.

Other Potential Factors

There are a number of other potential factors surrounding protein that might determine which is a good or bad source under a given context.  For example, proteins may show different effects on appetite, or blood sugar control, or what have you.

There is also the issue of cost and availability along with the amount of protein in a given amount of whole food proteins.  I’ll cover those as a catch-all final category in this series before summing up and looking at a variety of whole food proteins and how they rank on each category.

I’ll start here by looking at the issue of digestibility

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.

What is Protein 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.

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