From Complementary Protein Myth Won't Go Away!:
If myths like this not only abound in the general population, but also in the medical community, how can anyone ever learn how to eat healthfully? It is important to correct this misinformation because many people are afraid to follow healthful, plant-based, and/or total vegetarian (vegan) diets because they worry about “incomplete proteins” from plant sources.
How did this “incomplete protein” myth become so widespread?
No small misconception
The “incomplete protein” myth was inadvertently promoted in the 1971 book, Diet for a Small Planet, by Frances Moore Lappe. In it, the author stated that plant foods do not contain all the essential amino acids, so in order to be a healthy vegetarian, you needed to eat a combination of certain plant foods in order to get all of the essential amino acids. It was called the theory of “protein complementing.”
Frances Moore Lappe certainly meant no harm, and her mistake was somewhat understandable. She was not a nutritionist, physiologist, or medical doctor. She was a sociologist trying to end world hunger. She realized that there was a lot of waste in converting vegetable protein into animal protein, and she calculated that if people just ate the plant protein, many more people could be fed. In a later edition of her book (1991), she retracted her statement and basically said that in trying to end one myth—the unsolvable inevitability of world hunger, she created a second one—the myth of the need for “protein complementing.”
In these later editions, she corrects her earlier mistake and clearly states that all plant foods typically consumed as sources of protein contain all the essential amino acids, and that humans are virtually certain of getting enough protein from plant sources if they consume sufficient calories.
From "Complete" Proteins? by Charles R. Attwood, M.D., F.A.A.P.
Proteins are composed of amino acids, 12 of which are manufactured by the human body. Another 9, called essential amino acids, must he obtained from food. Most animal products, such as meat and dairy products, contain all of the essential amino acids and have been designated as containing complete proteins. Most proteins from vegetables also contain all 9 essential amino acids, but 1 or 2 may be low in a particular food compared with a protein from most animal sources. Beans, however, are rich sources of all essential amino acids.
The old ideas about the necessity of carefully combining vegetables at every meal to ensure the supply of essential amino acids has been totally refuted. Modern nutritionists, after observing populations of strict vegetarians who were healthier and lived longer than meat-eaters, now realize that all essential amino acids may be obtained from a variety of vegetables or grains eaten over a one-to-two-day period. This should be a great relief to you as a parent. Even the variety is not as critical as once thought.A dangerous myth
To wrongly suggest people need to eat animal protein for nutrients will encourage them to add foods that are known to contribute to the incidence of heart disease, diabetes, obesity, and many forms of cancer, to name just a few common problems.
From MYTH: Vegetarians Get Little Protein. Fact: Protein Combining is a Craze
The protein combining idea was contagious -- it appeared in every other book by every other vegetarian author published after that, and made its way into academia, encyclopedia entries, and the American mindset. Unfortunately, the idea that protein combining is necessary was absolutely wrong.
The first problem was that the protein combining theory was just that -- only a theory. There had never been any studies on humans. The idea of protein combining was thus more superstition than science. And it's not surprising that rats would grow differently than humans, since growing rats need ten times as much protein per calorie as growing humans. (Rat milk is 50% protein while human breast milk is only 5%.) Further, if plant foods were really so inferior, then how did cows, pigs, and chickens who eat nothing but grains and other plants get their protein? Wasn't it odd that we were eating farm animals for protein, and they were eating nothing but plants? Finally, plant foods were not even as "deficient" in various amino acids as Lappé had thought.
Here's what Marc Lawrence writes about protein combining, in an article called "Complete Protein Myth":Protein combining (also protein complementing) is a theory, now largely discredited, that vegetarians, particularly vegans, must eat certain complementary foods like beans and rice together in the same meal, so that plant foods with incomplete essential amino acid content combine to form a complete protein, meeting all amino acid requirements for human growth and maintenance.
The theory was initially promoted in Frances Moore Lappé's 1971 bestseller Diet for a Small Planet. The National Research Council and the American Dietetic Association (ADA) soon picked it up, cautioning vegetarians to be sure to combine their proteins. Later, the ADA reversed itself in its 1988 position paper on vegetarianism. Suzanne Havala, the primary author of the paper, recalls the research process:
There was no basis for [protein combining] that I could see.... I began calling around and talking to people and asking them what the justification was for saying that you had to complement proteins, and there was none. And what I got instead was some interesting insight from people who were knowledgeable and actually felt that there was probably no need to complement proteins. So we went ahead and made that change in the paper. [Note: The paper was approved by peer review and by a delegation vote before becoming official.] And it was a couple of years after that that Vernon Young and Peter Pellet published their paper that became the definitive contemporary guide to protein metabolism in humans. And it also confirmed that complementing proteins at meals was totally unnecessary.
Other nutrition experts and medical professionals who now agree that this theory is outdated include Dennis Gordon and Jeff Novick (registered dietitians), and John A. McDougall, Andrew Weil, and Charles Attwood (medical doctors).
In fact, the original source of the theory, Frances Moore Lappé, changed her position on protein combining. In the 1981 edition of Diet for a Small Planet, she wrote:
"In 1971 I stressed protein complementarity because I assumed that the only way to get enough protein ... was to create a protein as usable by the body as animal protein. In combating the myth that meat is the only way to get high-quality protein, I reinforced another myth. I gave the impression that in order to get enough protein without meat, considerable care was needed in choosing foods. Actually, it is much easier than I thought.
"With three important exceptions, there is little danger of protein deficiency in a plant food diet. The exceptions are diets very heavily dependent on  fruit or on  some tubers, such as sweet potatoes or cassava, or on  junk food (refined flours, sugars, and fat). Fortunately, relatively few people in the world try to survive on diets in which these foods are virtually the sole source of calories. In all other diets, if people are getting enough calories, they are virtually certain of getting enough protein."
Whatever all these people write, I still feel better on a diet containing protein rich vegan ingredients (beans, nuts, lentils etc) than on a vegan diet that doesn't. But then again - maybe I'm just hooked on plant proteins.Here’s where it gets interesting. The idea that plant based foods were deficient in certain amino acids was based on studies of the growth of young rats done in the early 1900’s. A subsequent study done in 1952, looked at human requirements for essential amino acids and found them to be very different from rats. Additionally it showed that the requirements for all the essential amino acids in humans could be met by many unprocessed plant foods, without combining, in excess of the recommended levels. The bottom line is that plant protein is “complete.” Vegetables and grains contain all essential amino acids and non-essential amino acids in varying proportions, and will supply in excess of what is necessary for your daily needs.