Complete Protein, Incomplete Truth: Why “Protein Quality” is a Myth

It’s time to rethink what “protein quality” really means—and who it’s actually for.

The notion that some proteins are inherently "better" than others continues to dominate nutrition conversations—and headlines. A recent trade media article, “Protein quality essential consideration for consumer health,” revives this framing, warning that plant proteins are inferior and that overlooking protein "quality" could pose a public health risk—even in well-fed countries [1].

But this viewpoint deserves a closer look. What exactly does "protein quality" mean? Is it a reflection of long-term human health? Or is this just another marketing ploy? Let’s unravel the science—and the assumptions.

The Origins of Protein Quality

The earliest measures of protein quality—such as the Protein Efficiency Ratio (PER) and Biological Value (BV)—were based on how well a protein source promoted growth in young rats [2]. These methods tracked weight gain following protein intake, using growth as a proxy for “quality.” That might be useful if you're breeding rodents. But humans mature over decades, not weeks. By the time we reach adulthood, growth is no longer the goal—so why was protein “quality” defined by it at all?

Even more advanced scoring systems like PDCAAS (Protein Digestibility-Corrected Amino Acid Score) and DIAAS (Digestible Indispensable Amino Acid Score)—designed to better reflect human needs by focusing on ileal digestibility or referencing a child’s amino acid requirements—still rest on a fundamental flaw: they rely on data from animals, not humans [3].

Because direct sampling of the human intestine is invasive and impractical, most DIAAS data is derived from growing pigs—chosen not only for anatomical similarities to humans [3, 4] but, as one researcher bluntly put it, because “they are easy to work with” [4]. Even this more generous rationale overlooks a critical point: the digestive system evolved to meet an organism’s specific nutritional needs. If those needs differ—as they do between pigs and humans—then so too does the function and behavior of the digestive system itself.

For example, pigs can synthesize their own vitamin C, while humans must obtain it entirely from food. Pigs also reach reproductive maturity in just 5–6 months, compared to more than a decade in humans. Their nutrient demands reflect a biology geared toward rapid growth and short life cycles, whereas humans are built for slow development and long-term physiological resilience.

These evolutionary differences underscore just how distinct the metabolic priorities of pigs and humans are—and cast serious doubt on the relevance of pig-based digestibility data for evaluating human nutrition. After all, if the goal is to inform long-term health outcomes in humans, why are we still outsourcing our standards to animals bred for rapid growth?

The Blind Spot in Protein Scoring

Mainstream nutrition messaging often warns of “low-quality” plant proteins—typically defined by their digestibility and completeness. But let’s be blunt: more people are dying of cancer than of protein deficiency. So what are we really optimizing for?

A protein’s ability to fuel growth doesn’t necessarily make it healthy. In fact, it may do the opposite. In lab studies, animal proteins like casein (from cow’s milk) were shown to promote cancer development, while plant proteins did not—even when calorie intake was identical. When rats were fed a diet with 20% casein, pre-cancerous lesions thrived. When casein was reduced to 5%, those lesions receded or disappeared [5].

And this wasn’t just an animal finding—it echoed in human populations, too. These results were part of a large, comprehensive body of research led by Dr. T. Colin Campbell, a renowned nutrition scientist and longtime authority on dietary protein. Known as the China-Cornell-Oxford Project, it remains one of the most expansive epidemiological nutrition studies ever conducted.

Across 65 rural Chinese counties, researchers found that animal protein intake strongly correlated with higher cancer rates, while plant-based diets were associated with dramatically lower risk [6].

Reflecting on this disconnect between growth and long-term health, Dr. Campbell writes in The China Study: “The concept of quality really means the efficiency with which food proteins are used to promote growth. This would be well and good if the greatest efficiency equaled the greatest health, but it doesn't.” [5]

So when regulatory bodies tout proteins that score high on DIAAS or PDCAAS as the “best,” we must ask: Best for what?

If an animal protein promotes rapid growth but also raises cancer risk, triggers growth-related hormones linked to cancer, and lacks the protective nutrients found in plant foods, maybe it’s time to redefine what “quality” really means.

Reframing the Protein Conversation

Imagine if we rewrote the rules. Instead of judging protein by its ability to grow lab animals—or even children—what if we judged it by how well it supports long-term human health?

Here’s what a more relevant definition of protein quality might look like:

• Lowers risk of cancer, heart disease, and diabetes
• Supports immune resilience and metabolic function
• Comes packaged with fiber, antioxidants, and phytonutrients
• Minimizes inflammatory markers and growth-promoting hormones
• Promotes satiety without harming kidney or cardiovascular function

And by this more relevant standard, plant proteins are the real winners. They may not all be “complete” in isolation, but that’s irrelevant when eating a varied diet. As long as your meals include legumes, grains, seeds, nuts, and vegetables, you'll meet your essential amino acid needs—without the baggage of saturated fat, cholesterol, or cancer-promoting mechanisms.

The Real Deficiency Isn’t Protein

In the U.S. and most developed countries, true protein deficiency is exceedingly rare. The real nutritional shortfalls are in:

• Fiber
• Potassium
• Magnesium
• Whole-food antioxidants

Yet store shelves overflow with ‘high-quality’ protein products—from meat snacks to dairy bars to protein-fortified drinks. 

This isn’t about health. It’s about marketing.

So who benefits from this obsession with “protein quality”? Certainly not public health. Certainly not you.

Final Thought: Quality That Supports Life

When it comes to protein, it’s not just about what builds bodies—it’s about what breaks them down.

We should care less about how fast a protein grows a rat and more about whether it helps a human live longer, feel better, and thrive.

The protein that helps you grow is not always the one that helps you live.

References

[1] Bambridge-Sutton, Augustus. Protein Quality Essential Consideration for Consumer Health. FoodNavigator-USA, August 21, 2025. https://www.foodnavigator-usa.com/Article/2025/08/21/protein-quality-important-for-health

[2] Shils, Maurice E., and Moshe Shike. 2006. Modern Nutrition in Health and Disease. 10th ed. Philadelphia, PA: Lippincott Williams & Wilkins.

[3] Food and Agriculture Organization (FAO). 2013. Dietary Protein Quality Evaluation in Human Nutrition: Report of an FAO Expert Consultation. Rome: FAO. https://www.fao.org/3/i3124e/i3124e.pdf

[4] Stein, Hans H. 2024. “The Pig Is an Excellent Model to Determine Amino Acid Digestibility of Human Foods and to Generate Data Needed to Meet Human Amino Acid Requirements.” Front Nutr. 2024 Aug 1;11:1434430. https://doi.org/10.3389/fnut.2024.1434430
Alternate access: https://www.researchgate.net/publication/383122087

[5] Campbell, T. Colin, and Thomas M. Campbell. 2005. The China Study: The Most Comprehensive Study of Nutrition Ever Conducted and the Startling Implications for Diet, Weight Loss and Long-Term Health. Dallas, TX: BenBella Books.

[6] Chen, Junshi, T. Colin Campbell, Junyao Li, and Richard Peto. 1990. Diet, Lifestyle and Mortality in China: A Study of the Characteristics of 65 Chinese Counties. Oxford: Oxford University Press.