How does plant protein intake affect total protein intake, quality, and cost, and nutrient intake? Original paper

Plant-based diets are rapidly gaining popularity for their environmental (e.g., less greenhouse-gas emissions and land use) and health benefits (e.g., reduced cardiometabolic disease and mortality risk),^[1] but plant foods have their own limitations, such as low vitamin B12 content and poor iron bioavailability.^[2] Plant protein is also considered to be of “lower quality” than animal protein because it is often less digestible and deficient in one or more indispensable amino acids (IAA), which are amino acids that can only be obtained from the diet and cannot be made by the body.^[3] Is plant protein intake associated with dietary nutrient intake and protein quality and cost?

This cross-sectional study evaluated data from three cycles of the National Health and Nutrition Examination Survey (NHANES 2013–2014, 2015–2016, and 2017–2018) to assess protein and nutrient intake, quality, and cost across quartiles of plant protein intake in 14,888 adults (19 and older). As a percentage of total protein intake, the quartiles of plant protein intake were less than 28% (Q1), 28%–32% (Q2), 32%–36% (Q3), and more than 36% (Q4).

Dietary intake data were collected by trained interviewers who conducted 24-hour dietary recalls per survey set. Protein intakes from animal and plant sources were estimated using the United States Department of Agriculture Food and Nutrient Database for Dietary Studies (FNDDS) and the Food Products Equivalents Database. The FNDDS provides nutritional composition of all foods and food groups such as “protein foods”, “grains”, or “vegetables”, and the Food Products Equivalents Database provides food group composition of each food and beverage consumed. Amino acid intake was estimated according to the FNDDS and the National Nutrient Database for Standard Reference. If amino acid contents were not specified, they were estimated from matching similar foods/ingredients. Protein quality was estimated using the protein digestibility-corrected amino acid score (PDCAAS).

The cost of protein (grams per U.S. dollar) were estimated using the 2001–2004 Center for Nutrient Policy and Promotion’s National Food Prices Database and adjusted for inflation for the appropriate NHANES year. Dietary modeling was conducted to evaluate the effects of plant protein complimentarity when amino acids from grains were partially (25% or 50%) replaced by amino acids from higher quality plant protein foods such as legumes (nonoil seeds, pulses such as beans and lentils, and oil-seed crops, such as peanuts and soybeans).^[9] Data and analyses were adjusted for the complex sample design of NHANES and sociodemographics. In other words, the researchers used appropriate survey weights that estimate the number of U.S. residents that each survey respondent represents.

The usual protein intake was 82.6 grams per day, of which about 30% was plant protein. Plant protein intake was positively associated with age, proportions of Hispanic and non-Hispanic Asian populations, economic and education status, moderate physical activity, and not smoking. It was inversely associated with proportions of male adults and non-Hispanic Black populations, vigorous physical activity, smoking, and obesity.

Plant protein intake was inversely associated with total protein intake, protein quality, and protein cost. Grains were the primary source of plant protein, despite not being considered a “protein food” (according to the FNDDS), as well as the most economical source of protein (24–26 grams per dollar) from the FNDDS food groups. Legumes were the most economical source of plant-based protein (28–38 grams per U.S. dollar). Greater plant protein intake was associated with lower intakes of vitamins B₁₂ and D, but with higher intakes of fiber, folate, iron, and zinc.

Interestingly, even with a lower total protein intake at the highest quartile of plant protein intake, IAA intake was sufficient (IAA score > 1.0), but protein quality was not adequate (PDCAAS = 0.80). It’s also worth mentioning that protein from “snacks and sweets” and “grains” doubled and tripled, respectively, when comparing people who consumed less than 28% of plant protein intake as a percentage of total protein intake to people who consumed more than 36%, while mixed dishes (that could be optimized for protein complementarity) increased by about 25%. Dietary modeling that replaced 50% of grain protein amino acids with legumes suggested an improvement of protein quality of 10%.

Top 5 sources of protein according to quartiles of plant protein intake

The results from the study should be interpreted with caution because the cross-sectional design prevents the determination of cause and effect, the dietary intake data were self-reported and only collected for one day, and there were several estimates based on many imperfect databases. For example, the amino acid database was not complete and the authors used a conservative coefficient of 0.8 for the nitrogen digestibility of most foods, which is likely an underestimation.

Globally, about 60% of dietary protein, providing about 20% of total global protein intake, comes from plant sources and wheat, which is deficient in the two IAAs lysine and threonine.^[10] In a 2021 systematic review of 141 observational and intervention studies, protein intake was lower in people following plant-based diets, but well within recommended intake levels when compared to meat eaters.^[11] In a 2021 analysis of 6,498 Canadian adults that used data from the 2015 Canadian Community Health survey and was similar to the study under review, protein content and quality (PDCAAS) decreased as plant protein intake increased.^[12] Most participants consumed 25%–50% of protein from plant foods, and grains represented the majority of plant protein consumed.

Protein quality scores can give a relative reference for individual foods, but, in reality, several foods with varying amino acid compositions are combined within a meal. Until a more accurate and inclusive measurement method for protein quality becomes feasible (see sidebar and note above), overall dietary protein intake for people who consume a greater proportion of plant protein can be improved with an understanding of complementary proteins that optimize protein digestibility.^[13][14] Because the body doesn’t exactly have protein or IAA stores, it oxidizes excess amino acids that are not used within several hours, meaning that protein complementarity must be considered on a meal basis. Generally, grains and legumes or nuts and seeds complement each other, but because grains are more commonly consumed, an increase in legume and nut and seed intake within each meal help, as modeled in the study under review.^[15][16] Moreover, legumes have a high level of protein and nutrient density per dollar.^[17] Pulses, or edible seeds from legumes such as beans, lentils and peas have been shown to improve nutrient density in the U.S.^[16]

Beyond quality, quantity, and cost, food processing and nutrition content should be considered in the context of plant protein intake. Food processing can influence protein digestibility and amino acid composition. For example, dry heat reduces digestibility, while wet heat increases digestibility. In addition, Maillard browning and heat can reduce lysine bioavailability. Digestibility of limiting IAAs in legumes ranges from 75% to 101%, while that of grains ranges from 13% to 96%, often depending on processing.^[4] A 2021 crossover randomized controlled trial suggested that a whole-grain diet promotes improved protein turnover, net protein balance, and muscle function, when compared to a macronutrient-matched refined-grain (i.e., more processed) diet.^[18] Although plant protein sources can be deficient in vitamins B₁₂, they have relatively high levels of certain minerals and contain bioactive compounds such as fiber, phytochemicals, and certain peptides (i.e., small proteins of two or more amino acids) with disease-fighting properties like the ability to affect enzymes, hormone expression, and lipid metabolism. These properties may give plant protein a different “edge” over animal protein.^[19][20][21] In a 2020 prospective cohort study that included 416,104 participants, greater plant protein intake was associated with small decreases in risk of overall and cardiovascular disease mortality.^[22] It appears that, with careful monitoring and understanding of plant protein sources and processing and amino acid complementarity, plant protein may be comparable to animal protein even when it comes to muscle protein synthesis and the protein needs of older people.^{[3][23][24][10]} However, further research is required to determine how more precise protein quality measurements and the food matrix (interaction of nutrient and non-nutrient components) may influence anabolic responses to protein intake.^[13][25][4]