How is protein quality measured?

Posted by Dayne Hudson in Health / Nutrition

Estimated reading time: 6mins

How is protein ranked?

One way we rank protein is by its biological value.

The biological value measures protein quality, worked out by "calculating the nitrogen used for tissue formation divided by the nitrogen absorbed from food."

In other words, biological percentage measures how efficiently the body utilises protein in your diet -- how efficiently it is absorbed.

So where do your favourite protein sources rank? Take a look at the table below:

Protein SourceBio-Availability Index
Whey Protein104
Soy Protein74
Wheat Gluten64

Table Source (Table 1)

If that table blows you away, it should.

It's why I can't wait to share it with people that say protein powders and other processed foods aren't as good and "real" food.

So a protein shake has a higher biological value than chicken? You can bet your birthday money it does.

Then we've got the Protein digestibility-corrected amino acid score (PDCAAS).

Protein digestibility-corrected amino acid score (PDCAAS).

This is a method of discovering the quality of a protein-based on both the amino acid requirements of humans and their ability to digest it.

The PDCAA scores are determined by comparing the amino acid profile of the specific food protein against a standard amino acid profile with the highest possible score being a 1.0.

This score means, after digestion of the protein, it provides per unit of protein 100% or more of the indispensable amino acids required.

So how do certain foods rank? Take a look below.

Whey Protein1.00
Soy Protein1.00
Black Beans0.75
Wheat Gluten0.25

Table Source (Table 1)

But it must be pointed out that the PDCAAS is not without its faults.

For starters, it was derived from the essential amino acid requirements of the preschool-age child, and was always capped at 100% (or 1.0 as we can see above). This is because of their high requirement for amino acids.

So no protein can score higher even if it's much better quality. And scores that ARE higher are rounded down.

But the other issue is that the large part of the small intestine, and not fecal measuring, is a better way to measure protein being used.

You see, PDCAAS measures protein in the feces, but this isn't the best way. This is because it OVERESTIMATES the nutritional value of protein because many amino acids are excreted in the feces which throws the scoring off.

It's (sort of) like measuring a business's revenue and not its profits after all expenses.

So that's why many scientists say we should be casting a critical eye on this.

Whilst it's not perfect, it's still some sort of a yardstick for us.

Then we have the Protein Efficiency Ratio (PER) which measures the amount of weight gain (in grams) in rats compared to their protein intake.

But it's not really relevant to humans.

So where does this leave us? Do we have a perfect way to measure protein?

Well, don't give up just yet.

The other method is true protein digestibility, with all of this in mind.

Let's take a look at that:

Protein SourcesTrue Digestibility (%)Digestibility Relative to Reference Protein (%)
Milk and Cheese95100
Meat and Fish94100
Whole Wheat8691
Refined Wheat96101
Polished Rice8893
Soy Flour8691
Soybean Isolate9499
Peanut Butter95100
Chinese Mixed Diet9499
Brazilian Mixed Diet7882
Guatemalan Mixed Diet7992
Indian Rice and Milk Diet8792
Mixed American Diet96101

Table Source (Table 4) : Modified from Torun B (1985) Proteins: Chemistry, metabolism, and nutritional requirements. In: Brunser O, Carraza F, Gracey M, Nichols B, and Senterre J (eds.) Clinical Nutrition of the Young Child, pp. 99–119. New York: Raven Press.

This is another way to measure protein, because not all nitrogen found in our feces comes from the protein we've eaten; some of it is within the intestines already.

That's how we get the name "true protein digestibility."

But keep in mind, for us that are healthy and at the gym, if we're sticking to our recommended amount of protein, we've nothing to worry about.

Protein digestibility is more of a concern for bodies like WHO, which is using the information to adequately address nutritional issues in third world countries.

Now, let's look at the speed of digestion for our protein sources.

Protein digestion speed

Some proteins digest faster than others. Take a look at the below:

Digestion speed of varying proteins

ProteinAbsorption (g/hour)
Raw Egg*1.4
Cooked Egg*2.9
Pea Protein3.5
Milk Protein3.5
Soy Protein3.9
Casein Isolate6.1
Whey Isolate8-10
Pork Tenderloin*10

* roughest estimates. Table adapted from here.

For example, research shows whey protein digests at around 10 grams per hour. So, consume 30 grams from a whey protein shake and after 3 hours it's more or less been digested.

And this is compared to casein protein, a slow-acting protein, which digests at 6 grams per hour (double the speed of whey).

But keep in mind, this is in a FASTED state.

When there is already food present (fats, carbs, amino acids from previously eaten protein) the effect is basically the same.

So when you're fasted and need protein quickly, whey is a good bet. But once the day has gone on and other foods are already present, a "faster" absorbing protein might not have the same "rapid" effect you might think.

The bottom line on protein measuring

Is that whilst protein measuring accuracy isn't perfect, biological value, protein digestibility-corrected amino acid score, protein efficiency ratio, and true protein digestibility give us some indication. Also, protein digestibility is also of interest when we're in a fasted state. If we stick to animal-based protein sources (or plant-based as vegans) and consume enough protein in line with our goals, we shouldn't be concerned about protein any further. However, protein powders tend to deliver more quality protein than even normal foods based on these measurements.


  1. Hoffman JR, Falvo MJ. Protein - Which is Best? J Sports Sci Med. 2004 Sep 1;3(3):118-30. PMID: 24482589; PMCID: PMC3905294.
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  10. Image from 1

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