The Protein Quality Myth

The very first question every new vegan asks is, “What about protein?!” I always answer, “All plants have protein,” which is then met with, “But they don’t have all the amino acids I need, do they?” I recently put that question to Garth Davis, MD when I interviewed him for an article about the protein needs of older adults. His surprising answer was that not only do plants have all the essential amino acids we need, but that plants are the ideal source for those amino acids because consuming certain ones in high quantities may be harmful to our health.

It turns out that all whole foods contain some amount of every single dietary amino acid humans need, the only difference is that some foods contain them in larger quantities, and others contain them in very tiny amounts. Forty-five years ago, vegetarian author Frances Lappé addressed the amino acid issue in her book, Diet For a Small Planet. In it, she suggested that vegetarians should combine different plant foods at each meal in order to get enough of each essential amino acid. The book became a bestseller, and as a result, the complicated and unnecessary practice of food combining became widespread among vegans and vegetarians. 10 years later, Lappé updated the book and tried to undo the damage. “In combating the myth that meat is the only way to get high-quality protein, I reinforced another myth,” she wrote. But it was too late – the myth of protein combining persists to this day.

While it is certainly true that we need to consume adequate protein and adequate amounts of the nine essential amino acids in our diet, the idea that we either have to consume animal proteins or pair different plant foods with one another at every meal is a simply not true. Since 1971 a great deal of research has been done on protein, and here are the key findings:

  • Healthy adult women need 46 grams of protein per day and healthy adult men need 56 grams (see 2015 US Dietary Guidelines)
  • Our bodies continually recycle and reuse amino acids in the gut, and the ability of the liver and other tissues to store amino acids over the short term makes protein combining at every meal unnecessary.
  • John McDougall, MD stated it best in a published response to the American Heart Association: “…it is impossible to design an amino acid–deficient diet based on the amounts of unprocessed starches and vegetables sufficient to meet the calorie needs of humans.” In other words, if you’re getting enough calories, then you’re getting enough protein and amino acids.

So we don’t need to worry about getting enough protein and amino acids, but what about too much? Research suggests that intakes of certain amino acids once considered “optimal” may be anything but. In his book, Proteinaholic, Dr. Davis cites a number of recent animal and human studies which have shown that by restricting methionine and leucine intakes we might lower our risk of developing certain cancers and slow the aging process. While plant-based foods have optimally low to moderate amounts of both amino acids, eggs, fish, meat and poultry all contain them in high levels. This is just one more way that our society’s “more is better” attitude toward protein might be proving harmful.

Upon considering the totality of the research on protein and amino acids, it appears that we need to shift our focus away from the myth of “complete” protein sources, and start considering “optimal” sources instead.

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