A new study published in The Journal of Nutrition concluded that “ounce equivalents” of animal- and plant-based protein-rich foods may not be metabolically equivalent after all.

Dietary protein is needed for the synthesis of the structural and functional components of living cells. Thus, food protein quantity and quality are both essential for good health.  The Dietary Guidelines for Americans (DGAs) published an “ounce equivalents” recommendation to help consumers meet protein requirements with a variety of protein food sources. For example, the DGAs present a variety of “ounce equivalents” in the protein food groups stating that 1 ounce of meat is equivalent to 1 cooked egg, ¼ cup of red kidney beans, 1 tablespoon of peanut butter, 2 ounces of tofu, and 0.5 ounces of mixed nuts. However, the DGAs do not currently address the issue of differences in protein quality associated with varied food sources. In general, animal proteins have higher protein digestibility and a better essential amino acid profile relative to dietary requirements.  These measures of protein quality indicate that animal proteins can more readily provide the daily requirement of essential amino acids than plant protein.

To investigate the physiological response to various ounce equivalents of protein food sources, Robert Wolfe (University of Arkansas for Medical Sciences) and colleagues randomly assigned 56 young healthy adult participants to 1 of 7 food intervention groups: 2 ounces of cooked beef sirloin, 2 ounces of cooked pork loin, 2 cooked eggs, ½ cup of red kidney beans, 2 tablespoons of peanut butter, 4 ounces of tofu, or 1 ounce of mixed nuts. Prior to the onset of the study, participants followed a 3-day dietary weight maintenance.  Participants’ net whole-body protein balance was assessed using a stable isotope tracer infusion protocol. The changes from baseline following consumption of the different protein food sources were compared with the baseline value for that individual. 

Consumption of ounce equivalents of animal-based protein food sources resulted in greater gain in whole-body net protein balance above baseline than the ounce equivalents of plant-based protein food sources. Overall, animal-based protein food sources elicited greater anabolic responses than plant-based protein food sources. The improvement in whole-body net protein balance was due to an increase in protein synthesis with all the animal protein sources. In addition, the egg and pork groups also suppressed protein breakdown compared with the plant protein sources. The magnitude of the whole-body net balance response was correlated with the essential amino acid content of the protein food source. The researchers concluded that “ounce equivalents” of protein food sources as expressed in the DGAs are not metabolically equivalent and this should be considered as the DGAs develop approaches to establish healthy eating patterns.

A corresponding editorial by Glenda Courtney-Martin (University of Toronto) stresses the importance and timely contribution of this study, which could guide future decisions regarding how protein foods can be better categorized by the DGAs.


Park S, Church DD, Schutzler WE, Azhar G, Il-Young K, Ferrando AA, Wolfe RR. Metabolic Evaluation of the Dietary Guidelines’ Ounce Equivalents of Protein Food Sources in Young Adults: A Randomized Controlled Trial. The Journal of Nutrition, Volume 151, Issue 5, May 2021, Pages 1190–1196, https://doi.org/10.1093/jn/nxaa401.

Courtney-Martin G.  False equivalence or fake news: is a peanut really an egg? The Journal of Nutrition, Volume 151, Issue 5, May 2021, Pages 1055–1056, https://doi.org/10.1093/jn/nxab051.

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