Scientists urge dietary recommendations to prioritize protein quality, not just quantity

Amid an ongoing “protein boom,” researchers call for dietary guidelines to pay more attention to protein quality — how well a food can meet human metabolic needs for essential acids and nitrogen. The team argues that current methods do not capture the scope of amino acids’ metabolic activity.

Addressing this gap, their review assesses dietary protein quality by combining metrics that consider food chemistry factors — essential amino acid composition, digestibility, food matrix interactions, and modifiable changes from cooking or processing. 

These factors impact the absorption and bioavailability of amino acids and their capacity to stimulate protein synthesis. 

The team also translates their findings into practical recommendations to improve protein quality in diets based on whole foods. They urge that this is particularly important in low- and middle-income countries where severe protein malnutrition is more common. 

In higher-income countries, increasing dietary intake of essential amino acids could also improve health and function. 

Nutrition Insight examines the review’s findings and advice with co-author Joseph Matthews, Ph.D., postdoctoral research fellow in Geriatrics and Protein Metabolism at the University of Arkansas, US. 

Matthews hopes the review will raise awareness that “what you eat is not the same as what you absorb.” Moreover, he underscores that what constitutes a “high-quality” protein differs among population groups and depends on context. 

“The amount of protein (in grams) listed on a product label is not necessarily what will be absorbed and available to the body for protein synthesis.” 

He highlights the importance of explaining the concept of protein quality and its relationship to quantity in dietary guidelines. 

“A simple message is that if your protein quantity intake is lower, your protein quality needs to be higher or should be prioritized. And vice-versa — if total protein intake is higher, the body’s essential amino acid needs can be met with a range of protein quality sources.” 

Outdated evaluation methods

The review, published in The Journal of Nutrition, warns that an overreliance on methods such as protein digestibility-corrected amino acid score (PDCAAS) or digestible indispensable amino acid score (DIAAS) limits understanding of protein quality.  

Although DIAAS is a more accurate method, neither method measures the bioavailability of amino acids nor adequately captures the impact of protein combinations on nutrient absorption or utilization. 

The researchers compared the density and digestibility of essential amino acids in various protein sources (Image credit: University of Arkansas).Matthews says that tools like DIAAS are not consumer-friendly and should not be included in dietary guidelines. 

“However, it would help to explain that not all proteins are utilizable. This should be reflected in the dietary guidelines, e.g., the proportion of protein from a given source that can actually be digested and utilized by the body.”

Highlighting alternatives to these standard scoring systems, the study introduces two novel tools with the potential to better assess protein quality. The Meal Protein Quality Score focuses on a personalized, per-meal assessment to account for a timeframe in which people can consume complementary proteins to maximize their metabolic activity. 

Secondly, the EAA-9 framework ensures dietary recommendations for each essential amino acid are met, comparing intakes against daily requirements. 

However, the authors urge refining these tools to incorporate cooking effects, food matrix interactions, and real-world dietary patterns.

Overall, they advocate developing comprehensive scoring systems that integrate digestibility, amino acid consumption, bioavailability, and the synergistic effects of combining different protein sources. 

Moreover, their study highlights the significance of amino acid density in evaluating protein quality, as diets dense in essential amino acids are more likely to support optimal health outcomes. 

Diet-specific quality protein gaps

The paper stresses that if people consume lower daily protein, they need a higher quality to meet essential amino acid requirements and reap the associated health benefits. 

Although plant-based diets generally offer a lower protein quality, the study suggests combining proteins to complement amino acid profiles.Diets that include animal-derived proteins, such as omnivorous and lacto-ovo-vegetarian diets (including dairy and eggs but excluding meat and fish), generally have a higher essential amino acid density. 

Meanwhile, diets higher in plant-derived proteins have a lower density, requiring greater total protein and energy intakes. For example, national dietary guidelines in the Netherlands recommend a 30% increase in daily protein intake for people on vegan diets. 

Similar to these findings, a New Zealand study revealed that although people on a vegan diet mostly consumed sufficient protein volumes, their diets lacked the essential amino acids leucine and lysine when accounting for digestibility. 

The study suggests that separating plant proteins from their original food matrix, such as in protein concentrates or isolates, can improve density and digestibility. However, it cautions that this would diminish the benefits of eating whole plant-based foods, which are important sources of micronutrients, fiber, polyphenols, and phytonutrients. 

Improving protein quality

Whole foods’ plant-based diets can be designed to meet protein quality standards, but they need to be well-planned and include a variety of protein sources. 

For example, Matthews details: “Combining complementary proteins (protein sources with different essential amino acid profiles that complement each other) can help improve the metabolic utilization of those proteins, such as combining legumes with grains.” 

However, he cautions that this may not be feasible in all regions due to cultural diets and the availability of certain crops or foods.

Matthews says that processing methods like soaking, fermenting, dehulling, and cooking can improve the protein quality of legumes.Matthews highlights a research gap in understanding the time course for consuming complementary plant-based proteins for the best metabolic benefit. “This would provide valuable information on how to structure vegan diets to meet protein quality needs.”

The authors also detail several methods to improve a protein’s quality, such as processing or cooking methods that reduce antinutrients, denature proteins, and change food particle size and structure. 

“Processing methods like soaking, fermenting, dehulling, and cooking can improve the protein quality of legumes,” says Matthews. The study highlights that these processing methods are common in lower- and middle-income countries. 

At the same time, the authors note that protein quality decreases when foods are exposed to prolonged storage, heat sterilization, and high surface temperatures.

Healthy aging needs

The study also cautions that older adults may absorb and utilize dietary proteins from whole foods less effectively than younger adults.

It includes several considerations for dietary protein quality in older adults, such as chewing efficiency, food particle size, and higher essential amino acid density and leucine intakes to maximize muscle protein synthesis.

“Older adults require more total protein or higher protein quality to maintain muscle mass, strength, and function,” explains Matthews. “Preventing frailty can maintain quality of life throughout a person’s advancing years, giving them more independence.”

“While protein is important, exercise is the foundation, and increasing physical activity and exercise levels should be considered the foundation of healthy aging.”