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 American 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Other nutrition experts and medical professionals who now agree that this theory is outdated include Dennis Gordon and Jeff Novick (registered dietitians), and John A. McDougall, Andrew Weil, and Charles Attwood (medical doctors).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."
Fruit and vegetable sources of protein are more often lower in one or more essential amino acids than animal sources, especially lysine, and to a lesser extent methionine and threonine. Nonetheless, although cereal protein is particularly low in lysine compared to animal protein, even the lysine in cereals is adequate for adult needs. Adults can achieve adequate protein with any one of the legume, cereal, nut, seed, or fruit food groups, even though each of these groups is low (compared to animal protein) in certain essential amino acids. Because of growth, children and infant have a greater requirement for complete protein than adults.
^ Maurer, Donna. Vegetarianism: Movement or moment? Temple University Press, 2002, p.37
^ Maurer, Donna. Vegetarianism: Movement or moment? Temple University Press, 2002, p.38.
^ " Vegetable Proteins Can Stand Alone, Dennis Gordon, M.Ed,R.D., Journal of the American Dietetic Association, (March 1996, Volume 96, Issue 3), pp. 230-231
^ Complementary Protein Myth Won't Go Away!, Jeff Novick, M.S., R.D., Healthy Times (May 2003)
^ The McDougall Plan, John A. McDougall (1983), pp. 98-100
^ Vegetarians: Pondering Protein?, DrWeil.com, Dec. 11, 2002
^ "Complete" Proteins?, Charles R. Attwood, M.D., F.A.A.P., VegSource.com (accessed Sep. 4, 2009)
^ Diet for a Small Planet (ISBN 0-345-32120-0), 1981, p. 162; emphasis in original
^ a b c Young VR, Pellett PL (1994). "Plant proteins in relation to human protein and amino acid nutrition" (PDF). AMERICAN JOURNAL OF CLINICAL NUTRITION 59 (5 Suppl): 1203S-1212S. PMID 8172124.