The importance of dietary protein quality in mid- to high-income countries

When protein intake is assessed, emphasis is generally placed on the amount of protein rather than quality; trends of higher average daily protein intakes in mid- to high-income nations often lead to the assumption that considerations for protein quality are not important. However, a 2024 study sheds light on why protein quality should not be overlooked.

Protein Quality: Overview and Key Definitions

What is protein quality and how is it assessed?

Protein quality reflects the indispensable (essential) amino acid profile, content, and availability and digestibility in the protein.

A protein can be considered "high-quality" or “complete” when it contains appropriate amounts of all nine essential amino acids in forms that are easily absorbed and utilized by the body.

Different methods exist for measuring protein quality, including the recent Digestible Indispensable Amino Acid Score (DIAAS) which is largely recognized as the most accurate mode of protein quality assessment.

What are the current recommendations concerning protein intake?

Currently, the Recommended Dietary Allowance (RDA) for protein is set at 0.83 g protein/kg body weight (BW)/d. However, increasing evidence suggests that protein requirements in adults may be slightly higher, at around 1.1-1.2 g/kg BW/day, particularly for older adults. Importantly, this RDA was derived from a mixed diet with high amounts of high-quality, utilizable protein. Therefore, for accurate comparison, dietary protein intakes must be measured as utilizable protein, thus taking into account protein quality.

The importance of protein quality: Key study findings

This study explored the impact of changes in protein quality on protein intake by analyzing data from the National Health and Nutrition Examination Survey (NHANES) and recent research focused on specific populations.³

Protein intake in the general adult population

Using the data of 44,018 adults from the NHANES (2001-2018), the study assessed how incremental changes in dietary protein quality would impact the percentage of the population falling below the protein EAR and RDA. The study modulated protein quality values in 0.1 DIAAS score increments for total dietary protein DIAAS scores ranging between 1.0 (100%) and 0.6 (60%).

The results revealed key issues: 

• Even when the overall protein quality was assumed to be high/complete (overall DIAAS of 1.0 or 100%), 22.4% of all adults aged 19-50 were below the RDA and 11% did not even meet the EAR:
  • The likelihood of being below the RDA was greater in women (32%) and increased with age (37% in adults aged 71+). 
• When assuming overall moderate protein quality (DIAAS of 0.8 or 80%), an estimated 44% of adults aged 19-50 were below the RDA and 25% failed to meet the EAR;
• Data revealed important effects of decreasing protein quality on the prevalence of adults falling below the EAR and RDA. For example, as DIAAS scores decreased from 1.0 to 0.6, the percentage of adults aged 71+ below the RDA went from 37% to 88%. 

Overall, even in the United States, a substantial portion of the general adult population fails to meet their protein requirements, regardless of protein quality. This study emphasizes the importance of considering protein quality, as lower quality exacerbates the prevalence of inadequacies.

Protein intake in key population groups

The implications of changes in protein quality were further explored to identify key study findings for specific subpopulations for whom protein intake is particularly important.

Individuals undergoing weight loss: 

• Utilizable protein intakes above the RDA have been shown to help prevent loss of lean body mass during periods of weight loss or weight loss maintenance; 
• When protein quality is lower, a compensatory increase in the quantity of dietary protein was required to achieve adequate utilizable protein, which resulted in higher total daily energy intake coming from the additional protein and an imbalanced macronutrient distribution. 

Older adults:

• Physiological changes related to aging may disrupt net protein balance, notably through impaired muscle protein synthesis, which can lead to loss of muscle mass;
• Many experts are now advocating for a higher protein RDA for older adults, with recommendations in utilizable protein intakes ≤1.5g/kg BW/day being proposed. Reaching this intake with lower-quality protein may disrupt macronutrient balance;
• Many studies have revealed insufficient protein intake in older adults, especially when protein quality is factored in. 

Endurance sports and resistance exercises: 

• Depending on the type of exercise, intensity, and duration, optimal protein intake amounts will vary; 
• Consuming sufficient protein to maintain lean body mass can lead to a disproportionately high percentage of total energy intake from protein if the protein quality is low, resulting in an imbalanced macronutrient distribution; 
• Athletes in endurance sports and resistance exercises with higher protein needs will benefit from making more intentional and informed protein choices to prevent these macronutrient imbalances. 

Veganism & vegetarianism:

• Given the lower protein quality of plant-based proteins and the relatively high energy-to-protein ratio of plant-based foods, deliberate choice of efficient and utilizable protein becomes imperative in meeting daily needs; 
• A study by the European Prospective Investigation into Cancer and Nutrition-Oxford Study (EPIC-Oxford cohort study) of 30,251 participants found that once adjusted for dietary protein quality (DIAAS), even at a DIAAS of 90%, levels of utilizable protein consumption was close to or below the RDA in both vegetarians and vegans; 
• Given that the mean protein intake among vegetarians and vegans is only slightly higher than the RDA, if active awareness of protein quality is not practised, there is a risk of not meeting RDA;
• In athletes, those following a vegetarian diet displayed lower lean body mass compared to their omnivorous counterparts, highlighting how low-quality protein may contribute to protein deficiency despite aiming for sufficient utilizable protein intake.

Overall, the authors emphasize the importance of considering protein quality, even in mid- to high-income countries, and explore how increasing protein quantity alone does not offset the potential unintended consequences of low-quality protein. Furthermore, they emphasize the importance of protein quality in achieving targeted utilizable protein intake while maintaining an optimal macronutrient balance.