Having examined the impact of speed of dietary protein digestion, I want to talk about protein quality.
What is Protein Quality
Quoting directly from The Protein Book:
Protein quality refers, in a general sense, to how well or poorly the body will use a given protein. More technically, protein quality refers to how well the essential amino acid (EAA) profile of a protein matches the requirements of the body; the digestibility of the protein and bioavailability of the amino acids (AAs) also play a role (1,2).
Essentially, protein quality simply refers to how well or how poorly a given protein is used by the body once it has been digested. Clearly, any protein that escapes digestion can’t do anything in the body but that doesn’t mean that all of the protein that is digested automatically works the same in the body.
Repeating myself slightly, protein quality has to do with how well a given dietary protein is used by the body for all of the different purposes that protein is used for. And the quality of the protein has to do with factors such as the amino acid profile of the protein (amino acids are just the building blocks of individual proteins) along with the speed of digestion issue I discussed in the last series of articles. I’ll talk about amino acid profile a little bit in the next article of this series.
Recall, for example, that whey protein, because of its rapid digestion, tends to promote amino acid oxidation (burning); obviously amino acids that are burned for energy can’t be used for things like synthesizing muscle tissue or what have you.
With that said, I want to take a brief look at all of the major methods of scoring protein quality. Again, for a more detailed discussion, please pick up a copy of The Protein Book.
Measuring Protein Quality
Over the years, many different approaches have been developed to determine protein quality. More continue to be developed as well. Many older methods have fallen into disuse and I’ll only really mention them for completeness.
The chemical score of a protein refers simply to its amino acid profile rated to some standard or reference protein, each amino acid is rated on a scale indicating how much of that amino acid is present compared to the reference protein.
For example, let’s say that the reference protein being used contains 100 milligrams of the amino acid leucine. Let’s say that the protein we’re looking at contains only 75 mg of leucine; that protein would be given a chemical score of 75% for leucine. If instead it contained 125 mg of leucine, it would be given a chemical score of 125% for that amino acid.
Frankly, chemical score isn’t used very much anymore and the whole concept is based on knowing what the ideal protein for human health and function actually is. Even in that case, chemical score says nothing about digestion or how a given protein is actually used by the body.
Biological Value (BV)
BV is one of the more common methods of measuring protein quality and tends to be the one that is seen the most so I’m going to give it the most discussion. BV is simply a measure of how much of the protein actually entering the bloodstream is retained in the body (e.g. used for proteins synthesis or what have you).
That is it takes digestibility into account. I’d note that some of the protein (again, researchers are actually measuring nitrogen going in vs. out but that’s not important here) that gets into the bloodstream comes back out in the urine.
Since BV is comparing protein in vs. out, the highest possible value for BV would be 100, that would mean that 100% of the protein that got into the bloodstream is being used by the body (note again, some protein won’t be digested in the first place).
No protein has a BV of 100 and claims that whey have a BV of 140 are simply nonsense (they are based on a misreading of a specific paper); this would suggest that for every gram of protein from whey that is eaten, the body somehow stores 1.4 grams of protein. An impossibility.
BV is measured by feeding subjects a protein free diet for three days and then giving them a measured amount of protein, the amount that comes back out in the urine and poop and skin and such are then estimated and BV is calculated. This type of study is called a nitrogen balance study and, for a variety of reasons can be very inaccurate. Again, more detail can be found in The Protein Book.
I’d note that BV is typically tested at very low protein intakes, far below what the average American (and certainly any athlete would eat). Eating more protein lowers the apparent BV which has led to some humorously bad interpretations of BV. As well overall energy intake drastically affects BV. If you eat more calories, apparent BV goes up, if you eat less, apparent BV goes down.
Because of this, BV has a lot of practical problems. It’s very accurate under conditions of low protein intake but caloric has to be meticulously controlled. At the types of high protein intakes seen in most modern countries, as well as with athletes, BV doesn’t tend to say very much.
Net Protein Utilization (NPU)
NPU is extremely similar to BV. But while BV is comparing the amount of protein that is actually digested to the amount that is stored in the body, NPU simply compares the amount of protein eaten to the amount stored in the body. Put differently, BV takes digestion and actual absorption of protein into account; NPU doesn’t. This doesn’t make NPU very useful.
Protein Efficiency Ratio
PER is a measure of the amount of weight gain (in grams) in rats compared to their protein intake. It’s always measured in young growing rats and, frankly, has about zero relevance to human physiology.
Protein Digestibility Corrected Amino Acid Score (PDCAAS)
The PDCAAS is one of the newer methods of scoring protein quality and is the one most in common use. Like chemical score it compares the amino acid profile to some reference protein; as well it it takes into account digestion.
Somewhat interestingly, proteins that were scored as low quality (such as soy protein) achieved a much higher score via the PDCAAS. This is actually in line with research showing that quality soy proteins work just fine for supporting basic human protein needs.
PDCAAS does have a couple of problems, however. The first is that the highest score possible is set at 1.0, no protein can score above that value regardless of the apparent quality. Basically, scores higher than 1 are simply rounded back down.
Additionally, part of the premise of the PDCAAS is that the ideal pattern of amino acids for supporting human health (or athletic performance) is actually known. It’s possible that the ideal protein for supporting basic human health might change with age (for example, amino acid and protein requirements do change with age) or might be different different types of athletes. The idea that a single amino acid profile can be ideal under all circumstances is tenuous at best.
Does Protein Quality Even Matter?
Which brings me to my major commentary about the issue of protein quality: I consider it essentially irrelevant. I noted above that BV, for example, is measured at very low levels of protein intake and this tends to hold true for many of the other methods.
Protein quality is measured under conditions of low intake because the primary application of protein quality has to do with ensuring adequate nutrition for people who don’t have enough food. Which means that it stops having much relevance at high intakes.
That is to say, small differences in protein quality make an absolutely massive difference if you’re talking about someone in the third world who is eating small amounts of a single source of poor quality protein and doing so in the context of insufficient total caloric intake in the first place.
In that situation, small improvements in protein quality (by adding other foods or even a specific amino acid) may pay massive dividends in terms of improving health or survival of that group. So would feeding them in general.
The World Health Organization (WHO) is very concerned with this issue which is why they make a big deal out of protein quality; it’s relevant to the population that they are worried about.
Anybody reading this article isn’t in that situation for the most part. If you have the Internet and time to visit my site, odds are that finding food in general, or protein in specific, to eat today isn’t your major concern.
When people are consuming mixed proteins at the levels seen in the general public (typically 2-3 times the RDA level), and especially among athletes (who often eat more than that), protein quality ceases to become an issue. This is especially true when lots of calories are being eaten.
And while it’s possible that specific proteins might be more or less useful for athletic applications (e.g. providing amino acids specifically needed by those athletes), any athlete eating large amounts of protein will generally be consuming plenty of what they need anyhow.
Athletes tend to get really obsessive about the issue (and of course supplement companies pander to that) but at a protein intake of 1-1.5 grams per pound of lean body mass coming from mixed high quality sources, quality just won’t matter. There is much more discussion of this in The Protein Book.
One possible exception to this is dieting; when calories are restricted, the way the body uses protein can change and different proteins may be specifically beneficial (the dairy proteins whey, casein or simply milk are valuable in this regards for reasons outside the scope of this article).
I suppose if someone in the modern world was eating small amounts of a single source of poor quality protein, quality would matter as well. But that would be some weird self-imposed dietary pattern, not the kind that people in many parts of the world follow because that’s all that is available.
The bottom line is that, for folks in the modern world, eating fairly large amounts of high quality proteins and lots of calories, protein quality is probably not enormously relevant. Which isn’t to say that it doesn’t matter at all. But understanding that statement means talking about the amino acid profile of different dietary protein sources.
What are Amino Acids (AAs)?
Now, as I’ve mentioned but not gone into any great detail in this series, amino acids are simply the building blocks of protein. Depending on which reference source you use, there are 18-22 different amino acids that occur in the human food supply. Whole food proteins are simply long chains of these amino acids bonded together.
Typically whole food proteins are extremely long chains of amino acids. As I discussed when I talked about protein digestibility these long chains are cut into smaller and smaller chunks during digestion until only single amino acids and chains of 2-3 amino acids are actually absorbed.
I’d note that individual amino acids are often sold for either health or sports performance purposes. Readers may be familiar with the amino acid L-tryptophan which is often sold as a sleep aid. L-Tryptophan converts to serotonin in the brain which is involved in sleep. Take L-tryptophan on an empty stomach and you get drowsy because of increased brain serotonin levels.
In the athletic realm, all kinds of products are available. The branched-chain amino acids (BCAA) L-leucine, L-isoleucine and L-valine have been pushed for years to athletes; recently there has been a big push for isolated leucine for a number of reasons that I’ll touch on in Part 2.
Another example is L-carnitine, an amino acid involved in fat metabolism that has been sold as a fat loss aid for years (it doesn’t work by the way). I, myself, have recommended the amino acid L-tyrosine (which converts in the brain to adrenaline and noradrenaline) as part of a stimulant cocktail to improve performance.
You may be wondering what the ‘L-‘ means above; it refers to the chemical structure of the amino acid (to be technical, it’s an organic chemistry notation that stands for levorotatory). There are also ‘D-‘ amino acids (the ‘D’ stands for dextrorotary). The human body only uses the ‘L-‘ form of amino acids; the ‘D-‘ form can actually be toxic.
Essential and Non-Essential Amino Acids
I should note that the amino acids are usually subdivided into essential amino acids and inessential or non-essential amino acids. It’s important to note that both are absolutely essential for life, the term inessential/non-essential simply means that those amino acids don’t need to be obtain from the diet; the body can make them. The essential amino acids can only come from the diet. Hence they are “essential”.
I should also note things aren’t actually quite this simple, some amino acids which are inessential under normal conditions can become essential under others; glutamine is perhaps the most well known example.
Under normal conditions, glutamine is inessential, the body can make what it needs. However, under conditions of massive stress (such as blunt force trauma, burn injuries or sepsis), the body can’t make as much glutamine as it needs; glutamine becomes conditionally essential under those conditions.
And while there are a few other odd exceptions to the essential/inessential distinction, they tend to be rare and not very relevant under most conditions, so I won’t talk about them.
Why are Amino Acids Important?
As I discussed previously, after being broken down in the gut and intestine, proteins then appear in the bloodstream as amino acids. These are then used in the body for various processes such as the synthesis of new proteins.
Your heart, liver and many other organs are made of protein, skeletal muscle contains about 20% protein (most of it is actually water), your hair and skin is made of protein, there are numerous enzymes and liver proteins made in your body every day; all are synthesized from incoming amino acids from the diet.
And recall from the discussion of protein digestion speed that the the tissues in the human body are in a constant state of turnover, which is the combination of breakdown and re-synthesis. So skeletal muscle is being broken down and remade, so is hair, skin, etc. Of course, since no process in the body processed with 100% efficiency, some of the broken down amino acids are lost.
That is, fundamentally, the basis for human protein requirements; the amino acids lost in the process of breakdown and re-synthesis have to be replaced from the diet. Otherwise, there will be a gradual loss of protein tissues in the body (as occurs in complete starvation). Lose enough body protein (about 40%) and you die.
Now, since the body is actually using specific amino acids for these various processes, it’s actually a little more accurate to say that the body has specific ‘amino acid requirements’ rather than ‘protein requirements’ per se. I’d note that there is also a general ‘nitrogen requirement’ (that can only be met with dietary protein) but I don’t want to get into that level of detail.
As a final note, I want to mention that the tissues in the human body that use proteins all use them in varying proportions and amounts. That is, the amino acid profile of say, your liver, or a specific enzyme may not be the same as skeletal muscle, hair or bone. Basically, the tissue you’re focusing on will determine what the ideal amino acid profile ‘might be’. I’ll come back to this.
Back to Protein Quality
Now, as I discussed above, one of the determinations of protein quality has to do with how well or poorly a given protein fulfills the amino acid requirements of the body and the above discussion basically explains why. Every day your body loses some amino acids which have to be replaced. One determinant of a protein’s quality is how well it matches the body’s need for those specific amino acids.
Let me mention again that most of the work on protein quality deals with the issue of general health, especially in those people who are not getting sufficient protein, protein from high quality sources, and who aren’t eating much in the first place. That is, the research is aimed at folks in third world countries.
The goal is to find ways of improving overall health and bodily function in people who are starving to death. And the focus is basically on keeping them healthy overall, that is meeting the amino acid requirements of the whole body in terms of keeping the basic stuff functioning well (or at least passably). Issues such as optimizing athletic performance or increasing muscle mass are not the focus.
Not only does this mean it has questionable relevance to those of us lucky enough to live in a modern world where protein and food is generally very available, it also means that it isn’t aimed at athletes or individuals involved in training (which tends to be the group I focus on). It’s conceivable (and, of course, supplement companies pander to this idea) that athletes or individuals in hard training might have specific requirements for amino acids.
That is to say, it’s conceivable that someone involved in a strength/power sport (powerlifting, bodybuilding, etc.) might require a different amino acid profile to support the growth of skeletal muscle; an endurance athlete might conceivably need a specific amino acid profile to support the synthesis of mitochondria (the powerhouse of the cell) or enzymes involved in energy production. This topic is drastically under studied.
But, simply (and of course this is discussed in great detail in The Protein Book), amino acid requirements can be sub-divided into the amino acid requirements needed to support basic health and bodily function (what most research deals with) and the amino acid requirements (if any) to optimize athletic performance.
Meeting Basic Bodily Amino Acid Requirements
Now, for reasons I’m not going to get to, the amino acid requirements for 2-5 year old children are actually used to examine whether or not a specific protein is sufficient. That is, any dietary protein which has an amino acid profile that meets or exceeds the requirements for 2-5 year old children is considered sufficient to support the basic needs of adults.
I’d note that, in keeping with the section on essential/inessential amino acid discussion above, the real focus is on whether or not a given protein can meet the essential amino acid requirements of the body. Assuming sufficient protein is being consumed in the first place, the inessential amino acid profile isn’t that relevant.
And as I show in Table 2 on Page 56 of The Protein Book (which I’m not going to reproduce here), basically all high quality proteins, and this even includes soy protein, can meet the basic amino acid needs of an adult human being.
Human milk, cows milk, eggs, beef, whey and soy all contain amino acids far in excess of the requirements for 2-5 year old children; by extension this means that they can readily meet the requirements for adults.
This is in keeping with the discussion of the PDCAAS from above showing that proteins such as soy (which were typically thought of as low quality) are more than sufficient to meet adult human essential amino acid requirements. Assuming adequate dietary protein is being eaten in the first place (and this is basically never an issue in the modern world), all proteins easily meet human protein requirements.
Which doesn’t make them all identical or equivalent mind you; there may be reasons (such as the presence or absence of other nutrients such as iron, zinc, or calcium, or the fatty acid profile) to choose one protein over another. But from the standpoint of amino acid profile, there isn’t much of a functional difference between proteins (I’d note, rather tangentially, that recent work has suggested that fish protein per se seems to have benefits on insulin sensitivity, possibly due to the high taurine content).
- A Complete Guide to Dietary Protein
- Dietary Protein Sources – Protein Quality Part 2
- Dietary Protein Sources – Speed of Digestion
- Dietary Protein Sources – Other Factors
- Nutrient Intake, Nutrient Storage and Nutrient Oxidation