Ji Sub Jeong/HuffPost

“If you eat a dish with black beans, you’re not getting complete protein. You have to add another kind of bean to get the same kind of protein you’d get from meat.”

This suggestion came from a generally well-informed acquaintance of mine while we were on a long car ride, making me wonder if the fumes had gone to her head.

Simmering with skepticism, I asked, “Adding any kind of beans will make it complete?”

“Yes, any kind of beans,” she replied with supreme confidence. “White beans, kidney beans, lima beans, lentils. When you combine any two beans, it’s just as good as eating animal-based protein.”

My instinct was to tell her she was wrong. But our drive through a countryside without cell towers or access to Google prevented me from doing so with absolute certainty. Now, however, I’m armed and ready to bust this myth.

It turns out my acquaintance was referring to a diet fad called “protein combining” that became popular in the 1970s. It was based on the premise that vegetarian and vegan diets provide insufficient content of essential amino acids, making it necessary to combine plant-based proteins to get the same “complete” protein you’d get from an animal. Protein combining has since been discredited by the medical community, but there are still people out there who adhere to this practice, and even more people who still believe plant-based protein is incomplete.

Concepts like “good fat vs. bad fat” and “good cholesterol vs. bad cholesterol” are somewhat well-known these days, but chatter about “incomplete protein vs. complete protein” hasn’t quite made it into the nutritional zeitgeist. You may have heard about complete protein if you’re vegan or vegetarian, but that doesn’t guarantee you fully understand what it is.

Case in point: quinoa. Quinoa is often marketed as one of the only vegetarian sources of complete protein, but that’s a misleading claim because every plant-based protein is complete. There’s no information to support the idea that quinoa is a more complete source of vegetarian protein than other plant-based foods. Nor is meat, for that matter. Let’s get to the bottom of why.

What’s a complete protein, anyway?

Just to be clear, a “complete protein” is a protein that contains all nine of the essential amino acids our bodies need to function: tryptophan (the stuff in turkey that supposedly makes us sleepy), threonine, isoleucine, leucine, lysine, methionine+cystine, phenylalinine+tyrosine, valine and histidine. Those amino acids are “essential,” but our bodies can’t make them, so they must be derived from the foods we eat.

Though many vegans and vegetarians worry about getting enough protein, concern about “complete protein” intake has more to do with the quality of our protein than the quantity.

Animal protein is not more complete than plant-based protein.

Dr. Michael Greger explains at his site NutritionFacts.org that all nutrients come from the sun or the soil. Cows, for example, get their nutrients from the sun and from plant-based foods like grass and hay. So if cows eat plants, and plants provide cows with all the nutrients they need, why would we assume steak is a more complete protein than the food that provides the steak with its nutrients? The answer: We shouldn’t.

While it’s true that some plant proteins are relatively low in certain essential amino acids, our bodies know how to make up for it.

“It turns out our body is not stupid,” Greger explains. “It maintains pools of free amino acids that can be used to do all the complementing for us. Not to mention the massive protein recycling program our body has. Some 90 grams of protein is dumped into the digestive tract every day from our own body to get broken back down and reassembled, so our body can mix and match amino acids to whatever proportions we need, whatever we eat.”

Greger told HuffPost that there’s no such thing as incomplete vegetarian protein. The only incomplete protein in the food supply is gelatin, which lacks tryptophan.

So why have we been led to believe that animal protein is more complete than vegetarian protein?

Misleading studies sparked the popularity of a bogus practice called ‘protein combining’ in the 1970s.

In 1909, the biochemist Karl Heinrich Ritthausen formed a theory that vegetarian and vegan diets provide insufficient amounts of essential amino acids, making it necessary to combine plant-based proteins to get the same “complete” protein you’d get from an animal. Another 1914 study out of Yale also suggested that plant-based protein is incomplete ― but this research was conducted on infant rodents and lacked context.

Protein combining gained popularity in 1954 with the publication of Adelle Davis’ book Let’s Eat Right to Keep Fit. The concept gained even more steam in 1971, when Frances Lappé published the best-selling book Diet for a Small Planet, which echoed the same idea. Vogue and the American Journal of Nursing even talked about protein combining in 1975. By then, America was on board.

But in 1981, Lappé changed her position on protein combining in a revised edition of her book, in which she backpedaled on the entire theory and apologized for reinforcing a myth.

The biggest pushback to the theory came in 2002, when Dr. John McDougall issued a correction to the American Heart Association for a 2001 publication that questioned the completeness of plant proteins.

McDougall asserted that earlier research about plant-based protein was misleading. “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,” he said. “Furthermore, mixing foods to make a complementary amino acid composition is unnecessary.”

He went on to say:

The reason it is important to correct this misinformation is that many people are afraid to follow healthful, pure vegetarian diets ― they worry about ‘incomplete proteins’ from plant sources. A vegetarian diet based on any single one or combination of these unprocessed starches (eg, rice, corn, potatoes, beans), with the addition of vegetables and fruits, supplies all the protein, amino acids, essential fats, minerals, and vitamins (with the exception of vitamin B12) necessary for excellent health. To wrongly suggest that people need to eat animal protein for nutrients will encourage them to add foods that are known to contribute to heart disease, diabetes, obesity, and many forms of cancer, to name just a few common problems.

Other doctors supported this hypothesis, including Dr. Andrew Weil and Dr. Joel Fuhrman, and the medical community followed.

So if all protein is complete, is all protein equal?

If protein combining isn’t necessary, is it all the same? Do 10 grams of protein from lentils have the same effect on our bodies as 10 grams of protein from steak?

Though they are both considered complete proteins, Greger told HuffPost there are differences. For example, he said, “lentil protein doesn’t raise IGF-1 levels as much as beef protein, which is one reason beef is a probable human carcinogen and legume consumption is associated with lower cancer risk. The lentils would probably also be better for our kidneys as well as longevity.”

How much protein do we really need, anyway?

Whether we’re vegan, vegetarian or omnivorous, protein intake is one of our key daily dietary concerns. But how much do we actually need per day to maintain a healthy lifestyle? According to Greger, it’s not nearly as much as we think.

“As long as we’re eating enough calories of whole plant foods, one shouldn’t have to worry at all,” he said. “We only need 0.8 to 0.9 grams of protein per healthy kilogram of body weight. In other words, one PB&J could get you a third of the way there.”