The United States Food and Drug Administration currently measures protein quality using the Protein Digestibility-Corrected Amino Acid Score, which tends to rank animal-sourced proteins higher than plant proteins because of their high digestibility and levels of all essential amino acids. The authors suggest that evaluating a protein’s quality on its amino acid profile is not necessary, because, to quote, “when a variety of plant protein sources are consumed in sufficient quantities, as would be true of almost any dietary pattern that includes appropriate variety and quantity to meet other nutrient requirements, needs for essential amino acids can be met without any animal protein intake”.
Furthermore, they argue, consuming proteins currently defined as “high quality” does not necessarily improve overall dietary quality or health outcomes. They note that some sources claim that consuming a plant-based diet is associated with better health outcomes compared to meat-based diets, while some sources of animal protein are associated with higher risk of chronic diseases such as cardiovascular disease and cancer.
The paper sets out a new way of assessing protein quality based on points for both health and environmental impacts. As an illustration, the paper uses carbon footprint as a proxy for environmental impacts, but the authors note that the points-based principle could be applied with a number of different impact categories and relative weightings of each category.
Prevailing definitions of protein quality are predicated on considerations of biochemistry and metabolism rather than the net effects on human health or the environment of specific food sources of protein. In the vernacular, higher “quality” equates to desirability. This implication is compounded by sequential, societal trends in which first dietary fat and then dietary carbohydrate were vilified during recent decades, leaving dietary protein under an implied halo. The popular concept that protein is “good” and that the more the better, coupled with a protein quality definition that favors meat, fosters the impression that eating more meat, as well as eggs and dairy, is desirable and preferable. This message, however, is directly opposed to current Dietary Guidelines for Americans, which encourage consumption of more plant foods and less meat, and at odds with the literature on the environmental impacts of foods, from carbon emissions to water utilisation, which decisively favour plant protein sources. Thus, the message conveyed by the current definitions of protein quality is at odds with imperatives of public and planetary health alike. We review the relevant literature in this context and make the case that the definition of protein quality is both misleading and antiquated. We propose a modernized definition that incorporates the quality of health and environmental outcomes associated with specific food sources of protein. We demonstrate how such an approach can be adapted into a metric and applied to the food supply.
Katz, D.L., Doughty, K.N., Geagan, K., Jenkins, D.A. and Gardner, C.D., 2019. Perspective: The Public Health Case for Modernizing the Definition of Protein Quality. Advances in Nutrition, nmz023.
Read the full paper here. See also the Foodsource resource What are the nutritional issues around meat and dairy?