Plant Protein Quality
Powering up on plant proteins—beans, lentils, peas, soyfoods, nuts, seeds, and whole grains—has been one of the hottest food and nutrition trends over the past few years. And these trendy protein sources are continuing along the same trajectory this year, appearing on many of the 2017 food trends lists. The 2017 Innova Market Insights report predicts that plant proteins will move into a “disruptive phase” this year, with food companies leveraging functional and technical benefits of plants in new product development.
Why are plant proteins soaring? You can thank the growing body of research backing the replacement of animal proteins with plant proteins for health benefits. Research has linked plant-based diets with lower risks of heart disease, obesity, hypertension, type 2 diabetes, and certain types of cancer. In addition, studies consistently show that plant-based diets are better for the environment. Diets high in meat increase greenhouse gas emissions from food production and global land clearing, as well as rate of species extinction.
“No doubt the sustainability issue is the biggest factor driving interest,” says Virginia Messina, MPH, RD, coauthor of Vegan for Life: Everything You Need to Know to Be Healthy and Fit on a Plant-Based Diet. “Consumers increasingly recognize that concerns about global warming and finite resources demand a shift toward more plant-based diets. Many people also are concerned about the treatment of farm animals and are interested in replacing meat, dairy, and eggs with plant proteins.”
Around the world, governments are promoting less animal protein in diets of their populations. For example, the Chinese government announced efforts to curb meat consumption by 50% among its nearly 1.4 billion citizens. And the Netherlands’ new dietary guidelines issued a recommendation that people eat no more than two servings of meat per week. According to research conducted by Midan Marketing and Meatingplace, 70% of meat eaters in the United States are substituting a nonprotein meal at least once per week, and 22% say they’re doing it more often than a year ago.
A couple of decades ago, you’d be hard pressed to find tofu, veggie burgers, or plant-based milk in mainstream supermarkets, restaurants, or foodservice establishments. Now these products are everywhere. Taco Bell has a “Vegetarian Certified” menu, joining the list of many fast food/chain restaurants that now have plant-based meals, such as Wendy’s, Denny’s, Subway, Chipotle, White Castle, and Chili’s. Target has a plant-based protein section in its stores.
It seems that every day you hear about a new plant-based product hitting the market. The Impossible Burger uses heme to make their plant burger “bleed,” sizzle, and smell just like real meat. On the other end of the spectrum, Hilary’s offers delicious, culinary-inspired veggie burgers with minimally processed ingredients, such as millet, quinoa, sweet potatoes, greens, beets, and dandelions. MALK makes organic cold-pressed nut milks made with sprouted, organic nuts. Ripple is a higher-protein plant-based milk made from pea protein. Even Tyson—one of the largest meat processors in the world—recently announced that it’s embracing plant proteins. They’ve invested in Beyond Meat, a company that makes plant-based meat alternatives designed to taste like the real thing.
Enter Plant Protein Quality
With so much interest in highlighting plant proteins at the center of the plate, there’s more interest in understanding the quality of plant proteins. Yet there’s confusion among consumers, as well as nutrition professionals, on this issue. Are plant proteins “incomplete”? Do we need to combine these proteins with other protein sources to make a “complete” protein?
Indeed, a recent cross-sectional study led by Glenna Hughes, MS, a consultant and previously a research scientist for DuPont Nutrition and Health who works predominantly in the field of plant protein quality, assessed dietitians’ perceptions of plant-based protein quality via an online survey. The dietitian responses to the survey showed a high level of support for plant-based diets, but a majority of respondents weren’t familiar with protein quality determination methods that are currently recognized by global regulatory and advisory agencies.
Plant Protein Quality 101
To begin, proteins are made of chains of amino acids, some of which are made by the body, while others aren’t. Those not produced by the body are called essential amino acids, of which there are nine: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Proteins in the human body tend to exist in consistent amounts of the essential amino acids—these levels also may be found in soy products and animal foods, according to Jack Norris, RD, plant-based expert and coauthor, with Virginia Messina, of Vegan for Life. These proteins found in soy products and animal foods are considered “complete” or “high quality” compared with other plant protein sources, which may have a lower percentage of at least one amino acid, though legumes are fairly close to soy, Norris notes.
All plant foods contain at least some of every essential amino acid, but in general, legumes are lower in methionine, and most other plant foods are lower in lysine. So, as long as one consumes a variety of foods throughout the day—even if they’re exclusively vegan—they can generally get an adequate amount of amino acids. This nullifies the concept made popular in the 1970s, which called for “complementing proteins” by combining various plant proteins, such as beans and grains, at the same meal to provide an adequate amount of all the essential amino acids at one sitting. However, the current thinking, per the Academy’s position statement on vegetarian diets, is that the liver stores the various essential amino acids over the course of a day to ensure adequate nitrogen retention and use in healthy adults.
“Plant proteins can be complementary in amino acid profiles and thus provide adequate amounts of essential amino acids. You don’t have to eat them at the same meal; you can get the amino acids over the course of a day,” Hughes says.
To further confuse matters, there’s no widely accepted definition for the term “high-quality protein,” according to Hughes, who says there are no regulatory definitions, though the Protein Digestibility-Corrected Amino Acid Score (PDCAAS) for alternative proteins that the USDA set for standards for the school lunch program is at least 0.8 (see Table 1 for more info on PDCAAS). She also says there are no definitions by the FDA or other regulatory agencies for “complete protein.”
Can plant proteins provide the necessary amounts of amino acids for the body’s metabolic needs? Experts believe so. Hughes explains that though specific needs might be different, such as for performance athletes or those with sarcopenia, both quality and quantity of protein are important. For example, there has been a great deal of research on soy protein and muscle synthesis showing that it’s comparable to whey protein, which is considered the gold standard for athletes. Thus, a well-planned diet based on plant proteins can meet nutrient needs.
Limiting Amino Acids
Lysine is the limiting amino acid (the amino acid in shortest supply) in vegan diets. If you’re eating a plant-based diet, the amino acid lysine is more important than total protein, according to Norris, because if you eat enough lysine, chances are you’re getting enough total protein. Norris lists tofu, tempeh, soy meats, lentils, and seitan as the highest sources of lysine, followed by other legumes. Quinoa, amaranth, pistachios, and pumpkin seeds also are decent sources. Norris has collected an impressive amount of information on protein and amino acid levels of plant foods here.
Virginia Messina suggests that, for the most part, quality isn’t something RDs need to be concerned about. “As long as people are eating a variety of plant foods and meeting calorie needs, they will meet needs for amino acids,” she says. “However, for those eating a vegan diet and getting all of their protein from plants, diets should always include legumes—beans, peanuts, and soyfoods—to ensure adequate intake of the essential amino acid lysine.” She notes that most vegan experts recommend at least three servings per day of legumes.
It’s completely possible to obtain an adequate amount of protein on an exclusively plant-based diet. However, very little research has been done on nitrogen balance among those eating plant-based diets. The little research available seems to support nitrogen balance for vegans who consume at least one-third of their protein as legumes (or quinoa or seitan) within a diet that provides about 1 to 1.1 g/kg of protein, according to Norris. A 2015 study based on data from EPIC-Oxford found that vegan men met the recommended dietary allowance (RDA) for all essential amino acids, and the vegan women had lysine intakes that were at 98.7% of the RDA.
So, what about the data that suggest older adults may need more protein than the RDA? Mark Messina, PhD, expert on soy and coauthor of The Simple Soybean and Your Health, says, “Even if it turns out that the protein RDA is too low, it doesn’t mean that all Americans need to consume more. The protein intake of most Americans exceeds the current RDA. It may impact recommendations for older adults, however.” He reports that since calorie needs decline with age, older people need to choose more protein-dense foods. And since plant foods provide fiber and are low in saturated fat, they certainly can be healthful choices for helping to meet the protein needs of older adults. Legumes in particular are 30% protein on a caloric basis and are underutilized in most diets.
Mark Messina says, “The relationship of higher protein intake to sarcopenia isn’t clear, though what may be at least as important as eating more protein is making sure protein intake is distributed equally throughout the day. Older adults tend to get most of their protein in the latter part of the day.”
Older people also appear to need more protein to maximally stimulate muscle protein synthesis (MPS), Mark Messina says. He adds, “Many studies have shown that the amino acid leucine is the key to stimulating MPS. Once the leucine threshold is met, the source of protein doesn’t appear to matter. So it isn’t surprising that studies have shown that plant proteins can maximally stimulate MPS. However, it may take higher amounts of plant protein compared with whey protein, for example, since whey is extremely rich in leucine.” Plant food sources of leucine include soyfoods, pumpkin seeds, seitan, and tempeh.
Plant Protein Digestibility
Vegans may require a slightly higher protein intake due to the slight decrease in digestibility of plant proteins. Protein in whole plant foods is digested slightly less well compared with isolated proteins or animal proteins, says Virginia Messina, who suggests aiming for a protein intake that’s about 10% higher than the RDA as a good idea for people who are getting most of their protein from whole plants. As mentioned earlier, many plant-based experts recommend about 1 to 1.1 g/kg of protein; it’s an appropriate level to compensate for digestibility.
Mark Messina adds that the RDA is based on a mix of high- and low-quality proteins, so for those consuming a plant-based diet it’s reasonable to recommend consuming at least 10% more protein than the RDA. “The percentage will depend on the sources of plant protein. For example, whole beans contain factors that inhibit protein digestion, whereas in proteins extracted from whole beans some of these factors are eliminated. To this point, isolated soy protein is very well digested (>95%) whereas the protein from whole beans may be digested at a rate of only 75% or 80%,” he explains.
Plant Protein Scoring
If you really want to get to the bottom of plant protein quality, you have to talk about PDCAAS, a method of evaluating protein quality based on the amino acid requirements of humans and their ability to digest them. The score is based on the ratio of the amount of the first-limiting essential amino acid in the protein source to the amino acid requirement of a 1- to 2-year-old child, corrected for protein digestibility based on true fecal nitrogen digestibility, and using the growing rat as a model for the adult human. Values at higher than 100% aren’t accepted as such but are truncated to 100%.9
Hughes explains that PDCAAS has been in use since the Food and Agriculture Organization and World Health Organization of the United Nations first published it in 1991. “It was the first method that actually introduced the concept of comparing the amino acids in protein foods with a reference pattern, correcting for digestibility, and then getting a score. The beauty of the method is that it allows you to look at combinations and how they complement one another with a score,” Hughes says. She reports that PDCAAS was first used by the FDA for nutrition labeling in 1993, and that the recently published labeling regulations in May 2016, which included revised Nutrition Facts labeling, confirmed that they will still be using PDCAAS for nutrition labeling.
Digestible Indispensable Amino Acid Score (DIAAS), a newer method of scoring protein quality, has been introduced. Hughes explains that DIAAS has a similar basis as PDCAAS; you compare the amino acid profile with a reference pattern and correct for digestibility. However, a different method is used to evaluate digestibility, and the digestibility of each amino acid is considered instead of the overall protein. “It’s more complex and more expensive and acceptance is a ways out there yet,” Hughes adds.
Some protein experts believe DIAAS is the best method for several reasons, including that the scoring system uses ileal rather than fecal estimates of protein digestibility, which some say are more accurate. But Hughes says, “DIAAS hasn’t made its way into any regulatory framework. When the FDA recently updated its nutrition labeling, their response on DIAAS was that it isn’t far enough along yet, and that we need more data generated in order to make this scoring method more widely accepted.”
So why isn’t PDCAAS and DIAAS scoring information for plant proteins easily accessible as a resource for RDs? Hughes says that protein quality scoring is frustrating, because there isn’t a great deal of information widely available to people outside of the food industry. “You can’t find these numbers in one place. Part of the problem is that there haven’t been a lot of published PDCAAS [values] for proteins, especially newer ones,” says Hughes, who notes that quinoa is a good example of a plant protein for which she wasn’t able to find a published PDCAAS value in peer-reviewed literature. “It would be good if the PDCAAS for quinoa could be published. I don’t know anyone who has taken that on board. For some of the newer plant proteins, you probably aren’t going to be able to find values,” Hughes adds. She says that for the newer pea protein ingredients, there are still no published results, though you can find values listed in company promotional literature (levels are stated as 0.8).
Hughes says you could calculate the PDCAAS value yourself, as long as you know the amino acid content (which is available in the USDA nutrient database, though it may not include all plant protein sources) and the digestibility (values are harder to find). She says that soy has the highest score (1), followed by legumes and beans (0.6 to 0.7); grains and nuts (0.4 to 0.5; almonds are even lower at 0.23); and quinoa has been estimated at 0.8.
“The digestibility can be affected by the form of the protein; sometimes, the less processed the plant, the lower the digestibility of protein. That’s why plant proteins on average have digestibility values of about 70% to 90%, vs soy, casein, and egg, which have a digestibility value of 97% or above,” says Hughes, who recommends about 10% to 20% additional protein to compensate for digestibility and the limiting essential amino acids of many plant proteins. However, combining plant proteins can make a difference in scoring. Hughes says that the PDCAAS of beans and rice by themselves is 0.6, but when combined it bumps it up to nearly 0.8.
“The real concern is for vegans—in particular those who aren’t eating a varied diet. More variety is one way to get some complementary protein choices going on. Also, if people are eating on the low end of their protein needs, one practical recommendation would be to recommend high-quality plant proteins such as soy, and then to make sure they are following the recommendations per the Academy position on vegetarian diets,” Hughes says. The position states that protein needs may be somewhat higher, especially when consuming protein sources that are less well digested. Hughes recommends that some of those protein sources be at a PDCAAS of 0.8 or above.
Virginia Messina suggests that for people who are getting most of their protein from plants, they should understand the importance of including legumes in diets. She adds, “For those who have trouble meeting protein needs, perhaps because of lower calorie intake, including one or two servings of soyfoods every day can help. Meat analogs made from isolated plant proteins can be an especially good source of this nutrient for consumers who aren’t getting enough.” She stresses that as long as vegans are consuming adequate calories, eating a variety of foods, and including at least three servings per day of legumes, you can assume they are meeting their protein requirements.
For more information on plant-based proteins, check out:
Image: BEST Homemade Seitan, Sharon Palmer, MSFS, RDN
Written by Sharon Palmer, MSFS, RDN on February 28, 2017; updated April 3, 2020
- Nunes K. Five leading trends for 2017. Food Business News website. https://www.foodbusinessnews.net/articles/news_home/Consumer_Trends/2016/11/Five_leading_trends_for_2017.aspx?ID=%7BCE8CD57C-0425-4150-BBD5-3BADAB777AEA%7D&cck=1. Published November 14, 2016. Accessed November 15, 2016.
- Melina V, Craig W, Levin S. Position of the Academy of Nutrition and Dietetics: vegetarian diets. J Acad Nutr Diet. 2016;116(12):1970-1980.
- Tilman D, Clark M. Global diets link environmental sustainability and human health. Nature. 2014;515(7528):518-522.
- Milman O, Leavenworth S. China’s plan to cut meat consumption by 50% cheered by climate campaigners. The Guardian. June 20, 2016. https://www.theguardian.com/world/2016/jun/20/chinas-meat-consumption-climate-change. Accessed November 15, 2016.
- Geiling N. The Netherlands’ new dietary guidelines take meat off the menu. ThinkProgress website. https://thinkprogress.org/the-netherlands-new-dietary-guidelines-take-meat-off-the-menu-a97c40f05d84#.kmgk2tfc5. Published March 25, 2016. Accessed November 15, 2016.
- Tyson Meat Company invests in plant protein Beyond Meat. NutriFusion website. https://nutrifusion.com/tyson-meat-company-invest-plant-proteins-beyond-meat/. Accessed November 15, 2016.
- Hughes GJ, Kress KS, Armbrecht ES, Mukherjea R, Mattfeldt-Beman M. Initial investigation of dietitian perception of plant-based protein quality. Food Sci Nutr. 2014;2(4):371-379.
- Schmidt JA, Rinaldi S, Scalbert A, et al. Plasma concentrations and intakes of amino acids in male meat-eaters, fish-eaters, vegetarians and vegans: a cross-sectional analysis in the EPIC-Oxford cohort. Eur J Clin Nutr. 2016;70(3):306-312.
- Rutherfurd SM, Fanning AC, Miller BJ, Moughan PJ. Protein digestibility-corrected amino acid scores and digestible indispensable amino acid scores differentially describe protein quality in growing male rats. J Nutr. 2015;145(2):372-379.
- Food and Agriculture Organization. Protein quality evaluation. https://apps.who.int/iris/bitstream/10665/38133/1/9251030979_eng.pdf. Published 1991. Accessed November 29, 2016.