Protein in a Plant-Based Diet
By: Matthew Madore
Tags: Plant Based, Strength, Protein, Vegan, Nutrition

Protein Quality

PDCAAS (Protein Digestibility-Corrected Amino Acid Score) & DIAAS (Digestible Indispensable Amino Acid Score)

Photo by Dan Gold on Unsplash

PDCAAS and DIAAS are the two most commonly discussed scoring systems designed to assign values to various sources of dietary protein that represent their quality. The respective scores are based upon assessments of the amino acid composition of various protein sources and how well they are digested. Both are frequently used to assert that there are major differences between plant and animal-based protein sources, implying some sort of inferiority of plant-based proteins. However, most individuals don’t possess a thorough understanding of what the scoring systems entail, how they are calculated, and what the actual implications for humans are. PDCAAS has quite a few openly acknowledged limitations, and consequently there has been a recent shift towards a preference for DIAAS, so that will be the primary focus of this discussion.

The main advantage provided by the DIAAS method is that it determines the ileal digestibility of each individual amino acid instead of just the fecal digestibility of the entire protein, providing a better assessment of the overall bioavailability of the amino acids from the protein source in question, and preventing falsely overexaggerated values resulting from bacterial assimilation of amino acids beyond the ileum of the small intestine. Furthermore, PDCAAS was primarily carried out in rats, which have different amino acid requirements from humans. Alternatively, DIAAS is carried out in pigs, which have more similar requirements. The intention of these scoring systems is to assess the specific digestibility of amino acids from complex food sources in isolation as to highlight which foods may serve best to quell protein malnutrition in individuals with limited food supplies, in certain clinical situations, with chronic diseases altering normal metabolism, and so on. For example, DIAAS scores can range from 0 to over 100, with a value over 100 indicating that if that single food was used to meet daily protein recommendations (in g/kg), it could satisfy all the individual essential amino acid requirements. While some foods may have scores under 100, mixed meals consumed throughout the day that contain multiple sources of protein and supply differing proportions of EAAs can easily propel the meal’s score over 100. Since DIAAS values are also additive, they allow for such a calculation to be made following a specific formula. Therefore, taking values for foods in isolation for anything other than instances where they are the sole source of nutrition is erroneous and suggests a misunderstanding of the scoring system¹⁸. For a thorough discussion of the issues discussed here, in addition to a few others inherent to DIAAS, one can read Craddock et al.’s recent review on the scoring system with specific attention to plant-based diets, which strongly concludes with the following:

If protein quality must be assessed, the PDCAAS remains the most suitable tool for use in Western adults adhering to a plant-based dietary pattern. As our understanding of plant-based nutrition has improved however, it has become increasingly apparent that emphasizing protein quality in developed nations is unproductive, influencing food selection with irrelevant metrics rather than more important dietary factors and truly prevalent nutritional shortcomings (i.e., fiber). Pervasive use of DIAAS and food selection based on this score is thus futile at best and harmful at worst.

Even with this type of scoring system not being particularly relevant to most people’s current situation or accurately reflecting impacts on hard endpoints, some of the differences between common plant and animal-based dietary sources of protein aren’t as substantial as many make them out to be. In addition to the minor amino acid composition differences mention previously, the Stanford paper discussed earlier discusses the topic of protein digestibility, and mentions a publication¹⁹ demonstrating that the more precise data in humans shows very little actual differences in ileal-digestibility (one of the main components of the DIAAS score) of a few common plant and animal-based sources of protein.

The more precise data collected so far in humans, assessing real (specific) oro-ileal nitrogen digestibility, has shown that the differences in the digestibility between plant and animal protein sources are only a few percent, contrary to historical findings in rats or determinations using less precise methods in humans.

Ileal Digestibility Values for Various Protein Sources

In addition, factors that may elicit slight reductions in protein digestibility of other plant-based sources of protein are covered, such as anti-nutritional factors and plant cell walls. This further elucidates the fact that despite differences in some protein sources being more notable, the scores calculated using digestibility data derived from animal feeding studies (i.e., PDCAAS and DIAAS) may not necessarily represent the true values in humans given the sources are typically fed to the animals whole and raw, where these factors are maximized. To be fair, this is exactly the type of speculation that one should hope to avoid relying on, so it shouldn’t hold much weight without more empirical data. Nonetheless, a recent study designed to determine unknown true ileal digestibility values of certain legumes in humans did reflect how some of these factors may indeed affect their scores, but not to an excessive degree. It was revealed that chickpeas, yellow peas, and hulled mung beans had values of 75, 72, and 71, respectively²⁰.

Therefore, based upon the previous description of the DIAAS scoring system, this indicates that if you were to eat an amount of these legumes that only provided the protein RDI for sedentary individuals, and nothing else, you’d likely not get adequate amounts of all the amino acids. This idea may seem a bit abstract, so for the sake of clarity some calculations approximating what this would entail are to follow. Given that this RDI currently equates to approximately 0.8 g/kg body weight, which even for heavier men and women (just to exaggerate, let’s say 70 and 100 kg respectively) would equal 56 and 80 g. That would mean if a woman/man were to consume just ~1112/1578 kcal of chickpeas, 850/1200 kcal mung beans, or ~800/1100 kcal of yellow peas (an amount well under the daily caloric requirements for such individuals, even in extreme circumstances) and absolutely no other sources of protein, they would run into problems (likely more than just those related to amino acid requirements). While the recommended intake per kg of body weight would scale up with an increased frequency of exercise, so would the individual’s total caloric requirements. Furthermore, not only are these foods that possess supposedly lower scores compared to other plant-based protein sources (I.e., soy, seitan made with wheat gluten, lentils, some other legumes, and slightly more processed sources such as powders) but in the absence of dire living conditions and a limited food supply, this is an absurd situation that would almost never occur. Alas, even in such an extreme circumstance, the degree to which these sources would be insufficient is fairly small, and this minor “gap” (for the lack of a better term) can be abolished with the inclusion of just a handful of other foods that supply the limiting essential amino acids.

In conclusion, even prior to escaping the web of mechanistic findings in which many claims pertaining to the different quality of plant and animal sources of protein are woven, it should already be fairly clear as to why the implications many individuals suggest the differences have are far overstated. Though it may have advantages over PDCAAS, DIAAS still has some of its own potential pitfalls and has yet to be adopted as the predominant method for assessing protein digestibility due to the paucity of data on certain foods, the conditions in which many are tested, and the few remaining differences in digestion and amino acid requirements between pigs and humans. Given these points, as well as the fact that many people don’t eat single foods in isolation, and that there is a complete absence of protein deficiency in plant-based populations consuming sufficient calories (many of which also surpass the minimum protein recommendations by a fair amount, as discussed in the Stanford review), these aren’t very relevant outside of the specific abnormal circumstances for which they were created. Henceforth, it is unreasonable for someone who is not limited in the amounts and/or types of foods they can consume to base their dietary choices solely on either of these scoring systems, nor for anyone to use them to suggest certain foods or dietary patterns are inferior with respect to their ability to satisfy nutrient requirements, support normal bodily function, and promote growth.