A 2023 study from Purdue University found that two-ounce-equivalent servings of animal-based proteins, such as pork and eggs, deliver higher levels of bioavailable essential amino acids (EAA) to the bloodstream than equal servings of plant-based proteins like black beans or almonds. These findings suggest that protein quality, not just portion size, is a critical factor for muscle maintenance and healthy aging.
Why Protein Quality Matters More Than Portion Size
Current nutrition guidance often treats different protein sources as interchangeable through the “ounce-equivalent” system. However, Dr. Wayne Campbell, a professor in the Department of Nutrition Science at Purdue University, suggests the basis for labeling these foods as having “similar nutritional content” is unclear. Because the body cannot manufacture essential amino acids on its own, it must obtain them through food to support muscle and whole-body protein building.
The Purdue study highlights that two foods can count as the same “ounce-equivalent” while providing vastly different amounts of usable amino acids. For example, in the study, two ounce-equivalents of lean pork loin provided 7.36 grams of EAA, while the same portion of almonds provided only 1.85 grams. According to Dr. Gavin Connolly, the clinical trials project manager at Purdue, the study’s results indicate that animal-based proteins more effectively supply the amino acids necessary for muscle health.
How the Purdue Research Was Conducted
Researchers conducted two crossover randomized controlled trials involving 55 healthy adults—30 younger and 25 older participants. Each participant completed four testing sessions, consuming a standardized meal that included two ounce-equivalents of either unprocessed lean pork loin, scrambled whole eggs, black beans, or raw sliced almonds. Blood samples were collected at intervals up to 300 minutes post-meal to measure EAA bioavailability.
The results showed that animal-based protein meals consistently resulted in higher EAA levels in the bloodstream compared to plant-based meals. Notably, lean pork led to higher EAA bioavailability than eggs, while black beans and almonds showed no significant difference from each other. Dr. Connolly noted that these findings were consistent across both younger and older adult populations.
The Purdue study, published in the journal Nutrients in June 2023, was funded by the Pork Checkoff and the American Egg Board—Egg Nutrition Center.
What Recent Evidence Says About Protein Sources
The conversation around protein quality has evolved since the 2023 Purdue study. A 2025 systematic review published in Nutrition Reviews found that animal protein provides a small beneficial effect on muscle mass compared to plant protein, particularly in younger adults. However, the review also noted that soy protein appears more comparable to animal protein than other plant sources, and there was no clear difference between plant and animal sources regarding muscle strength or physical performance.
These newer insights suggest a nuanced approach to nutrition. While animal proteins may deliver essential amino acids more efficiently, plant-based foods remain vital sources of fiber, vitamins, and minerals. Future dietary strategies may focus on carefully designed blends of protein to optimize digestibility and muscle health while maintaining sustainability goals.
Frequently Asked Questions
Are all protein sources truly equal?
No. While the Dietary Guidelines for Americans uses an ounce-equivalent system to group proteins, research indicates that animal-based proteins like pork and eggs often provide higher levels of bioavailable essential amino acids than plant-based sources like beans and almonds.
Does age affect how the body uses protein?
According to the Purdue study, there were no significant differences in EAA bioavailability between the healthy young and older adults tested. However, older adults often have higher requirements for nutrient-dense protein to support muscle maintenance and physical function as they age.
What are the limitations of the Purdue study?
The study measured EAA bioavailability in the bloodstream but did not directly measure changes in muscle protein synthesis or long-term health outcomes. Additionally, the portion sizes tested may not reflect typical daily consumption habits.
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