The nutritional quality of plant protein sources (part 2)
Digestibility of plant protein
Protein quality is defined as the digestibility and quantity of essential amino acids for providing the proteins in correct ratios for human consumption. That means two requirements for a protein to be considered high quality, or complete, for humans are having adequate levels of indispensable amino acids to support human growth and development and readily digested and absorbed.
Firstly, the amino acid profile of seven popular types of plant proteins including legumes, cereals, pseudocereals, seeds, leaves, and shoots, agro-industrial by-products, was mentioned in “The nutritional quality of plant protein sources — part 1”.
Secondly, there are various methods that rank the quality of different types of protein, some of which are outdated and no longer in use, or not considered as useful as they once were thought to be. Below are standard methods to evaluate the protein quality which are currently being applied:
1 — The Protein Digestibility Corrected Amino Acid Score (PDCAAS) was developed in 1989 by a Joint FAO/WHO Expert Consultation on Protein Quality Evaluation
The Protein Digestibility Corrected Amino Acid Score (PDCAAS), which was recommended by the Food and Agriculture Organization of the United Nations (FAO), became the industry standard in 1993.
The method is based on the comparison of the concentration of the first limiting indispensable (essential) amino acid in the test protein (mg/g protein) with the concentration of that amino acid in a reference (scoring) pattern, for a given age group. The indispensable amino acid with the lowest ratio is referred to as the most limiting amino acid. The most limiting amino acid score is corrected for the fecal true digestibility of the protein. To determine fecal true protein digestibility, rats are fed a known amount of nitrogen from the test protein and then fecal nitrogen excretion is measured. This measure represents apparent protein digestibility. The fecal nitrogen excretion from the rats on a protein-free diet is then subtracted from fecal nitrogen excretion on the test protein, which accounts for non-dietary protein nitrogen excretion from bacterial cells and digestive secretions. The result is referred to as true fecal protein digestibility.
The results can be expressed as either decimal or multiplied by 100 to be expressed as a percent. A PDCAAS of < 1.00 indicates that the protein is suboptimal and PDAAS scores >1.00 are truncated to 1.00.
2 — In 2011, the FAO introduced an updated amino acid scoring system, the Digestible Indispensable Amino Acid Score (DIAAS)
DIAAS is the recommended method for dietary protein quality assessment in order to meet the needs for quality protein in mixed diets, to record the additional benefit of individual high-score protein sources in complementing less nutritious proteins, and for regulatory purposes. The DIAAS is calculated and interpreted similarly to the PDCAAS, but with a few important differences:
- First, the reference patterns for the indispensable amino acids were revised to reflect advances in the scientific knowledge regarding amino acid requirements.
- Second, a single estimate of fecal protein digestibility is no longer used. Rather, the concept of the ileal individual amino acid digestibility was incorporated. True fecal digestibility of protein, which is based on nitrogen excretion in the feces, is complicated by the considerable exchange of protein, amino acids, and urea between systemic pools and the lower gastrointestinal tract. In response to this limitation, it was recommended to measure ileal amino acid digestibility, which reflects the concentration of amino acids that reaches the ileum and would hence enter the colon, derived from ileostomy output studies conducted in animals or humans. As such, each indispensable amino acid from a given protein source will have an associated ileal digestibility value and its amino acid score will be corrected for that value.
- Finally, unlike the PDCAAS, the DIAAS method allows for scores >1.00 to acknowledge that there may be incremental health benefits associated with these higher DIAAS scores
>>>> Claims on quality:
To qualify for the nutrition claim: “source” for protein, at least 10% of the nutrient reference value (NRV) for protein must be provided per 100 g of solid product (or 5% of the NRV per 100 ml liquid). When a portion of food meets the criteria for protein quantity, then a quality measure should be applied. Quality will be dictated by where the tested protein’s DIAAS score falls within the scale. Specific brackets are recommended below:
- No protein quality claim — Score of <75%
- Good protein quality — Score ranging from 75% to 99%
- Excellent or High protein quality — Score of 100% or more
To qualify for “High” for protein, the food must contain two times the values for “source”:
3 — The Quality of Plant Proteins
In general, most animal-based protein sources, such as milk, whey, casein, eggs, and beef, have adequate protein content and PDCAAS/DIAAS at or very near 1.00. As such, they are generally considered complete protein sources for supporting indispensable amino acid requirements for human growth and development. For plant-based protein sources, if one food source is qualified as “Good-quality protein”, this food must meet the need of NRV for protein (at least 10% protein content) and the DIAAS score is over 0.75.
- Here below is the list of protein content per 100gr of some plant-food sources collected from studies found on the USDA website:
It can be seen from the list that the protein content of most uncooked, unprocessed plant foods meets the need for nutrient reference value (NRV) for protein and is even equivalent to beef. However, cooking treatment may significantly decrease the protein content of plant foods, except for nuts, and slightly affected soybean.
- Here below is the list of studies that calculate PDCAAS and DIAAS scores of milk, whey, and selected vegetable protein sources, based on the amino acid reference standard of FAO FN Paper 92 2011.
From the data on the list, we can see that there are few plant food sources own PDCAAS or DIAAS scores over 0.75 which are soybean, potato, peas, lentils, or quinoa. Among them, soybean is the only plant food source that meets the need of NRV for protein (in processed form) and has a high DIASS score to be qualified as “high” protein quality.
Soy protein is a high-quality protein with a protein digestibility-corrected amino acid score (PDCAAS) of 1.00, which is close to some of the proteins from animal sources, such as meat and dairy. Soy proteins contain well-balanced essential amino acids except for sulfur-containing ones like methionine — Journal of Agriculture and Food Research
— The amount of dietary protein contributed by a specific plant protein source
While the PDCAAS/DIAAS of most plant proteins may be less than 1.00, the individualized protein scoring system is only one way to evaluate the potential contributions of a protein to the diet. In the case of plant versus animal proteins, simply consuming more of the plant protein can help to provide higher indispensable amino acid intakes. Given that many whole food sources of plant protein are less calorie-dense than animal sources of protein, greater overall food intake is needed to meet energy requirements which, in turn, helps meet indispensable amino acid requirements.
— Dietary protein variety is the key in order to meet indispensable amino acid requirements
While the PDCAAS of an individual protein is critical when evaluating the quality of a sole-source protein, it becomes less significant when the diet contains proteins from many sources. For example, lysine is often limiting in grain proteins, but such proteins are good sources of sulfur-containing amino acids. On the other hand, legumes are often rich sources of lysine but are limiting in sulfur-containing amino acids. Consumption of these two protein sources over the course of the day allows them to “complement” one another, helping to meet requirements for both types of indispensable amino acids. Thus, the quality of protein in the diet may be quite high if the plan is to consume a variety of plant proteins with different amino acid profiles.
A classic example would be a combination of pea and rice proteins. Protein blends of pea and rice ranging 40–90% pea protein can achieve a PDCAAS of 1.00, using the 2011 FAO amino acid reference pattern for adults.
In addition to this, a flexitarian diet, in which persons consume primarily plant-based but with small amounts of meat and dairy, represents another strategy for helping to meet indispensable amino acid requirements. It is a casual approach to vegetarianism and allows meat and other animal products to be consumed in moderation.
— Using plant-based protein isolates and concentrates
In addition, it has now become much easier for consumers to boost intake of plant proteins via the availability of multiple plant-based protein isolates and concentrates (soy, pea, canola, potato, fava, etc.) in the food industry. It was once difficult for individuals to take in relatively large amounts of protein from whole plant foods because they typically have a low percentage of protein. However, plant protein isolates and concentrates, which often contain 80% or more protein by weight, make it possible to consume 10–20 g or more of plant-based protein per one serving of a ready-to-drink shake or powder mix.
— Is it needed to combine complementary protein sources at the same meal? …. NO
Young and Pellet addressed this issue. They noted that the common limiting amino acid in grains, lysine, has a significant pool in the skeletal muscle. After a protein-rich meal, they estimated that 60% of the adult daily requirement for lysine could be stored in this pool within 3 h. If a person were to consume a lysine-poor meal within 3 h of a lysine-rich meal, there would still be adequate intracellular lysine available to promote protein synthesis. Thus, it is not necessary to consume complementary protein sources at the same meal if the gap between meals is relatively short, around 3 h; the complementary amino acids will be metabolically available for protein synthesis.
— An often-neglected aspect of plant proteins is their high content of some important dispensable/conditionally essential amino acids.
The PDCAAS method of evaluating protein quality focuses only on indispensable amino acids. However, the knowledge base around the health-related effects of individual amino acids, both indispensable and conditionally indispensable has grown dramatically. For example, soy protein, while not as high as whey in leucine, is nearly three times higher in arginine, 2–3 times higher in glutamine, and has double the glycine content. Other plant proteins can be high in these amino acids as well.
In detail, arginine is necessary for the body’s synthesis of nitric oxide (vasodilator) and creatine, for urea cycle function, regulating hormone secretion, and for immune function. Glutamine is a primary fuel source for rapidly proliferating cells such as those in the immune system and gastrointestinal tract and functions in the synthesis of arginine, ornithine, and several other compounds. Glycine is critical for collagen synthesis, comprising up to 1/3 of the amino acids in collagen and some studies suggest that its biosynthesis in humans may not be adequate to meet requirements. Although amino acids such as arginine, glutamine, and glycine might not be classified all the time as indispensable amino acids, they perform many critical functions and plant proteins can be significant sources. Thus, the content of these dispensable/conditionally indispensable amino acids deserves to be taken into consideration when evaluating the value of plant proteins in the diet.
In conclusion, the trend toward increasing plant protein intake is now growing in popularity, and becoming a part of the natural landscape due to the knowledge of the nutritional benefits of protein and sustainability concerns about the food supply are raised. However, plant proteins differ in nutritional quality, and those who choose to largely emphasize plant versus animal proteins need to be aware of these differences when planning an appropriate diet, especially in more vulnerable populations. A consumer who accepts a healthy plant-based diet needs to increase the amount, variety, and forms of plant proteins in the diet.
References:
(1) Plant Proteins: Assessing Their Nutritional Quality and Effects on Health and Physical Function
(4) Protein digestibility-corrected amino acid scores (PDCAAS) for soy protein isolates and concentrate: criteria for evaluation
(6) Evaluation of nutritional quality of a novel pea protein
(8) Composition and Protein Nutritional Quality of Quinoa
(9) Composition and protein profile analysis of rice protein ingredients
#Medical Disclaimer: My article is only for information purposes based on reliable sources. The medical and/or nutritional information on my article is not intended to be a substitute for professional medical advice, diagnosis, or treatment.
