Animal and plant-based protein-rich foods are not metabolically equivalent, finds study

Dietary protein is needed to supply essential amino acids 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 2020-2025 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 ½ ounce 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.

A new manuscript recently published in The Journal of Nutrition investigated the physiological response to various ounce equivalents of protein food sources and found that the 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.

Robert Wolfe (University of Arkansas for Medical Sciences) and colleagues randomly assigned 56 young healthy adult participants to one of seven 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.

Overall, investigators found that animal-based protein food sources elicited greater anabolic responses than plant-based protein food sources. Whole body protein balance increased more in the beef, pork, and eggs groups than all of the groups consuming plant-based protein food sources. Protein synthesis increased more in the beef group than in the groups consuming plant protein foods, kidney beans, peanut butter, or mixed nuts, while the egg and pork groups suppressed protein breakdown more compared with mixed nuts.

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 in terms of either the anabolic response or caloric value and this should be considered as the DGAs develop approaches to establish healthy eating patterns.

Our research illustrates that animal-based protein foods, such as beef, eggs and pork, and plant-based protein foods, such as kidney beans, peanut butter, tofu and mixed nuts, cannot be considered to be equivalent, or a substitute for each other, when developing healthy dietary patterns, given their unique physiological effects. While it's well-established that animal proteins can more readily provide essential amino acids than plant protein foods, our study also indicates that eating animal protein foods such as beef, pork and eggs may lead to increased protein synthesis, which has been shown to have benefits such as improved satiety and lean muscle mass maintenance."

Robert Wolfe, PhD, Study Lead Researcher and Director, Center for Translational Research in Aging and Longevity, Professor of Geriatrics, University of Arkansas for Medical Sciences

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.

Journal reference:

Park, S., et al. (2021) Metabolic Evaluation of the Dietary Guidelines’ Ounce Equivalents of Protein Food Sources in Young Adults: A Randomized Controlled Trial. Journal of Nutrition.


  1. Matt Bryan Matt Bryan Canada says:

    Yes animal based protein is more readily absorbed by the body. But we need to consider if this is actually preferable in terms of good nutrition and health outcomes. Considering most people consume twice the amount of protein required and that rapid absorption of nutrients can trigger inflammation. Excess nitrogen and sulfur, elements obtained from amino-acids are also a source of inflammation in the body. Protein from plant sources is more than adequate for our needs and the amino acid profile generally contains lower levels of the sulfur containing amino acids methionine and cystine. It has been know for some time that cancer growth is absolutely dependent on a steady supply of methionine. So much so that methionine restriction is being explored as as cancer treatment therapy. So it needs to be understood that a good quality source of protein is not the same as a healthy source of protein. More is not always better, the body thrives on the correct balance of nutrients. This balance can be most readily obtained from plant sources of protein.

  2. EZ Carnivore EZ Carnivore United States says:

    Contrary to the other two comments, I'm of the opinion that more absorbable IS better. The reason why so many people are fat and sick is due to too not getting enough protein and essential nutrition from animal foods, and instead eat very protein poor but energy dense, high carbohydrate plant foods along with oxidized high fat seed oils in their highly processed diets. I believe most people are never satiated from eating, because their bodies are wanting more nutrition and aren't getting it from plants.

    As someone who has effortlessly lost 25 lbs eating an animal based keto diet, is now noticably less inflamed than when eating a standard American diet, and is always satiated from my meals and is rarely hungry, I can say that I'd rather eat only meat than go back to eating mostly plant food. Many people heal chronic conditions like diabetes and chronic pain on a carnivore diet, and even mental conditions like depression and bipolar disorder.

    The science that says meat is bad isn't very convincing, especially as it's mainly studies looking at people who eat meat while still consuming large amounts of plant foods and carbohydrates. True experimental animal based diet studies have never been done on humans, and only a couple of small epidemiology carnivore diet studys have recently been published. Much more investigation needs to be done, but just looking at the anecdotes is very telling about how powerful animal nutrition is. Besides, we've been eating meat over our entire evolutionary history, why is it only bad for us now?

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News Medical.
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