Sequential, societal trends in which first dietary fat and then dietary carbohydrate were vilified during recent decades have left dietary protein under an implied halo. The resulting infatuation is naïve, over-simplified, misguided and misleading. Other than that — it’s perfect.
Everything you think you know about protein, in other words, if you think of it as most people do, is wrong.
Colleagues and I just published a paper in Advances in Nutrition, calling for the definition of protein quality to be modernized, an action we contend is long overdue. That paper, from which my opening line here is adapted, occasions this column — so let’s start there.
The quality of protein has historically been defined in strictly biochemical and physiologic terms. We need not bog down in the details to represent the gist: the metric has reflected the proportional concentration of 9 essential amino acids and their digestibility from specific food sources.
When all the relevant math is done, the resulting measure is called the protein digestibility-corrected amino acid score, or PDCAAS.
This makes sense in two fundamental ways. First, only the protein we can digest does the body any good in meeting its daily needs for construction of enzymes, cells, hormones, and miscellaneous replacement molecules.
Second, you can think of the various “essential” (meaning we need them, but cannot make them ourselves — hence, truly essential components of diet) amino acids as differential supplies at a construction site. You cannot build a house from an excess of lumber but an absence of nails; or a surplus of nails, but an absence of siding; or ample siding, but no roofing shingles; or … you get the idea. To build a complete structure, you need a suitably balanced distribution of component parts. So, too, in the daily construction (i.e., anabolism) detail of our own growth (children) and maintenance (adults).
So, yes — the PDCAAS does make sense. But it’s obsolete just the same.
The PDCAAS is translated directly into the vernacular as: protein quality. The word “quality” implies something better, preferred, desirable. So, without any mental gymnastics, “high quality protein” imparts its halo to the foods in which it resides, leading to the idea that these foods are ... better, preferred, desirable.
There’s the rub — for the foods that top the chart as sources of “high quality protein” are, for the most part, foods actually associated with adverse effects on the health of people and planet alike, while foods lower down the list are associated with countervailing benefits. The problem is more acute than that, since some foods known as sources of the highest quality protein are actively discouraged in the most recent Dietary Guidelines Advisory Committee Report (and, albeit less decisively, in the watered-down Dietary Guidelines for Americans, too) — while foods ranking lower as sources of “quality” protein are expressly recommended for both health (globally) and sustainability.
So, sources of the so-called highest quality protein are not the highest quality sources of protein — if by “quality” we mean foods that are … better, preferred, desirable. Foods that qualify for such accolades should promote health, reduce disease risk and respect the demands of sustainability. If not, then the word “quality” is being misused. My colleagues and I contend, in the 10 rather dense pages of our manuscript, that “quality” is being misused, because it is misleading the public — in other words, you. The prevailing view is that eating sources of higher quality protein is beneficial, when in general it is the opposite.
We make the case for modernizing the definition so it reflects the imperatives not of biochemistry but of understanding and public health. We propose that quality should refer to the food source of protein not just the protein in a food.
We present a metric that still considers the distribution of essential amino acids, and their digestibility, but then also considers, and weights appropriately, the net effects of the food on overall health and the environmental footprint of the food as well. By such a measure, beans and lentils shoot up to the top of the rankings, where they belong, and beef, for example, falls down. Beef and other meats don’t fall anywhere near the bottom, of course, because they are indeed concentrated protein sources. But they are foods we should be eating less, not more, for the health of ourselves and our planet alike, and our proposed definition of “quality” situates them accordingly.
This, then, leads to the other prevailing misconceptions about protein.
In a recent paper in Nutrition Reviews, colleague and co-author of our current paper, Christopher Gardner from Stanford University, demonstrates that all essential amino acids are present in all plant foods, belying the prevailing notion that only meat and eggs have “complete” protein.
In his extensive work on this topic, Prof. Gardner further demonstrates that even strictly vegan diets, if reasonably balanced, readily meet and exceed all amino acid requirements, even for high-performance athletes. Much the same case is made by French colleagues in a 2017 paper in Nutrients. Complete dietary protein does not require animal foods, and does not require any specific food combinations. Wholesome foods in any balanced, sensible assembly — even a strictly vegan assembly- will readily provide it.
There is the notion that the more protein, the better — and this, too, is false. Excess calories from protein are excess calories, and turn into stored body fat as do excess calories of any other origin; this is established fact. Protein is also misrepresented in the popular narrative about insulin and carbohydrate. Protein stimulates the release of insulin from the pancreas — it is a fallacy that only carbohydrate does so. Insulin release is greater when protein and carbohydrate are ingested together than when either is ingested alone, and thus an excess of both contributes to insulin resistance, and type 2 diabetes.
Finally, there is the “manly” notion that being big and strong requires meat consumption. This is only weakly belied by human, vegan bodybuilders; it is far more robustly denied by the much mightier muscles of other animals.
The greatest muscles in the terrestrial portion of the Animal Kingdom belong to elephants, strict herbivores. They grow muscles that make the strongest of all humans puny — out of grass and leaves. So, too, do their closest rivals in might — rhinos, and hippos.
Yes, of course, other very strong — albeit less strong — animals are carnivores. What this tells us is that the food source does not decide what can and cannot be turned into muscle; what decides is biology. While humans have failed at the art of alchemy, biology has succeeded magnificently. Biology can turn leaves into elephant, and hay into horse — just as it can turn wildebeest into lion. Biology decides.
Human biology, anatomy, and physiology is all emphatically omnivorous. We can make “human” out of chorizo, and we can make it out of chickpeas. We have choices.
To make good choices, we need good information — including definitions that foster understanding, and lead us in the right direction. The highest quality sources of dietary protein are those foods that foster health, and best defend the bounty of years in lives, and life in years. They are the foods that best ensure the sustainability of nutritional adequacy for the generations to follow us, that best respect the demands we are placing on the finite resources of our planet.
For these imperatives of public and planetary health to be honored, we must renounce the many misconceptions of dietary protein that prevail — and modernize the definition of protein quality.