How Do You Measure Protein Quality?

The best test for assessing dietary protein’s ability to stimulate muscle protein synthesis (MPS), which is scientific speak for muscle growth, is to use stable isotope amino acid (AA) tracer methodology. Stable isotope AA tracers are able to directly measure AA bioavailability and utilization (i.e. the exact quantity of AA from a particular protein source that the body is able to use).

However, there are few studies that assess the MPS response to the ingestion of protein sources, especially plant protein sources, using stable isotope AA tracers.

Various alternative measures exist to evaluate dietary protein quality. The most widely adopted measures are the Protein Digestibility Corrected Amino Acid Score (PDCAAS) and, more recently, the Digestible Indispensable Amino Acid Score (DIAAS).

Although neither of these approaches provides precise insight into the true anabolic potential of a specific dietary source, they currently provide the best protein quality measures available.

DIAAS is suggested to be superior to PDCAAS because it treats dietary amino acids as individual nutrients and not simply as protein. Furthermore, PDCAAS analysis uses egg as a reference protein (a base score of 1.00) while DIAAS uses a theoretical protein that covers all known dietary requirements.

Protein Digestibility of Animal- vs. Plant-Based Protein

Researchers at Massey University in New Zealand define the digestibility of certain protein sources as the proportion of dietary protein-derived amino acids (AAs) that are effectively digested and absorbed, thus becoming available in a form suitable for body protein synthesis.

The entire journal article for their published research, titled Available versus digestible dietary amino acids, can be found in the British Journal of Nutrition here.

In general, it appears that plant-based protein sources may exhibit lower digestibility than animal-based proteins.

Animal-based protein sources, including dairy, eggs, and meat, are highly digestible. For example, most animal products clock in around 90 to 95% protein digestibility, so if you eat 40 grams of protein from these food sources, your body ultimately utilizes 38 grams or so.

Plant-based sources such as maize, oat, bean, pea, and potato tend to exhibit lower digestibility than do animal-based sources, with values ranging from 45% to 80%. So if you eat 40 grams of protein from these food sources, your body utilizes 18 to 32 grams.

However, purified plant protein sources such as soy protein isolate, pea protein concentrate, and wheat gluten (seitan) display a digestibility that is similar to that of animal-based protein sources (greater than 90%).

The researchers from Massey University, mentioned above, calculated PDCAAS and DIAAS scores for animal- and plant-based protein supplements, either in a concentrated or isolated form.

Both protein concentrate and isolate remove carbs and fat from a certain food source such as whey, soy, or pea. The difference between them is that protein isolate simply removes more of the carbs and fat, leaving a more “pure” protein powder behind. Protein concentrate is typically about 70 to 85% pure protein while protein isolate is 90 to 95% pure protein.

Note: the researchers define the Limiting AA as the “first limiting amino acid when compared to an ideal protein.”

Does Plant Protein’s Lower Digestibility and Essential Amino Acid (EAA) Content Inhibit Muscle Growth?

Gram for gram, animal-based protein has been proven time and again to be more effective at building muscle than most plant-based protein.

There is even research to suggest that plant-based protein sources such as soybeans, kidney beans, and legumes have “antinutritional” factors (i.e. compounds that interfere with the digestion and absorption of available protein), but more research must be conducted.

Researchers from the Human Performance Laboratory at the University of Connecticut conducted a long-term training study (9 months) in which a whey protein supplement was shown to significantly enhance gains in lean body mass over those seen in a soy protein-supplemented group by ~83%.

This study is one of the longest protein supplementation with resistance exercise trials every done and highlighted the importance of protein quality in determining exercise-induced muscle mass gains.

In another study, researchers overcame the apparent difference in the protein quality (measured by leucine content and amino acid bioavailability) of a vegan protein concentrate (a rice-derived protein named Oryzatein) vs. whey protein isolate by feeding their subjects a very large quantity of protein.

In feeding their subject groups 48 grams of whey isolate and 48 grams of rice protein concentrate, they delivered leucine doses of ~5.5 grams and ~3.8 grams, respectively, both greatly surpassing the highest levels of leucine needs and saturating the MPS response for both groups.

Thus, “equivalency” of protein in this study was not a function of the protein quality itself, but of the large per-dose quantities of protein consumed.

Collectively, these studies prove that gram for gram, animal-based protein is more effective at building muscle than plant-based protein.

However, the study that provided greater amounts of plant-based protein to compensate for its lower digestibility showed minimal differences in lean mass gain with resistance exercise when compared with animal-based protein.

So here’s the bottom line: the best way to compensate for the lower essential amino acid (EAA) content from plant-based protein is to simply eat greater quantities of plant-based proteins to support muscle mass gains. Plant-based protein supplementation, particularly with pea protein powder, can help significantly to support muscle mass gains.

Despite vegan strength athletes being at a slight disadvantage when it comes to protein digestibility than their meat-eating counterparts, we can gain muscle and strength just as easily as omnivores. It simply requires that you consume an adequate amount of high-protein plant foods like lentils, nuts, seeds, chickpeas, tofu, spirulina, hemp, tahini, and oats. I also drink one or two pea protein shakes per day, but you don't have to supplement with protein powders.

And more importantly for our health than protein, we don’t have to worry much about our fiber, cholesterol, and micronutrient consumption…unlike most meat eaters. #WINNING 😎

That's a wrap! Thank you so much for reading this article!

I’ll be publishing an article next week discussing exactly how much protein you need to consume as a vegan bodybuilder to maximize your muscle mass and strength gains.

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Your vegan trainer,

Leif

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