Protein combining or protein complementing is a dietary theory for protein nutrition that purports to optimize the biological value of protein intake. According to the theory, vegetarian and vegan diets may provide an insufficient amount of some essential amino acids, making protein combining with multiple foods necessary to obtain a complete protein food. The terms complete and incomplete are outdated in relation to plant protein. In fact, all plant foods contain all 20 essential amino acids including the 9 essential amino acids in varying amounts. 
 Though it is undisputed that diverse foods can be combined to make up for their respective limiting amino acids, a general consensus has emerged among nutrition scientists and writers contrary to the original vegetarian nutrition dogmas of the 1970s. Though historically, protein combining was promoted as a method of compensating for supposed deficiencies in vegetables as foods, studies on essential amino acid contents in plant proteins have shown that vegetarians and vegans typically do not need to complement plant proteins in each meal to reach the desired level of essential amino acids as long as their diets are varied and caloric requirements are met. The position of the Academy of Nutrition and Dietetics is that protein from a variety of plant foods eaten during the course of a day supplies enough of all essential amino acids when caloric requirements are met.
 Protein nutrition is complex because any proteinogenic amino acid may be the limiting factor in metabolism. Mixing livestock feeds can optimize for growth, or minimize cost while maintaining adequate growth. Similarly, human nutrition is subject to Liebig's law of the minimum: The lowest level of one of the essential amino acids will be the limiting factor in metabolism.
 Plants are thus rated as protein sources by their limiting amino acids.
 According to WHO, human need for proteins is 0.66 g per kg of bodyweight per day. A 70 kg person has an estimated protein requirement of 46.2g (70 kg x 0,66 g/kg).
 In addition, there is a specific need of essential amino acids quantities. See Essential amino acid#Recommended daily intake for a table of the values; the tables below use a percentage of the requirement.
 In the above examples, neither whole rice nor canned chickpeas have sufficient amounts of all required amino acids when used as the only source of 46.2 g of daily protein. The insufficient amino acid is called the "limiting" amino acid: lysine in rice and methionine in chickpeas. Consuming the specific quantity for long periods of time might result in deficiency of the amino acid.
 In the above example, the combination of whole rice and canned chickpeas has no limiting amino acids. This means that consuming only rice and chickpeas, in these specific quantities of 306g/day and 261g/day respectively, for long periods of time, would not result in any of the essential amino acid deficiency, at least to the extent they are metabolized at the time.
 As the data of the example show, all essential amino acids are found in a specific plant. However, one or more of them may be limiting. For this reason, vegan and vegetarian diets need to be varied in terms of plants consumed.
 The first biochemist to enter the field was Karl Heinrich Ritthausen, a student of Justus von Liebig. Thomas Burr Osborne continued what Ritthausen started and published The Vegetable Proteins in 1909. Thus Yale University was the early center of protein nutrition, where William Cumming Rose was a student. Osborne also worked to determine the essentials, and later led the Biochemistry Department at the University of Chicago.
 When Ritthausen died in 1912, Osborne praised his efforts in biochemistry:
 Osborne then joined forces with Lafayette Mendel at the Connecticut Agricultural Experiment Station to determine the essential amino acids.
 In the 1950s and 1960s, Nevin S. Scrimshaw took this knowledge to India and Guatemala. He designed meals using local vegetables to fight against the scourge of kwashiorkor. In Guatemala he used the combination of cottonseed flour with maize, while in India he combined peanut flour with wheat.
 In 1954, Adelle Davis published Let's Eat Right to Keep Fit, which described the importance of combining "incomplete" proteins to make "complete" proteins, and advised that any incomplete proteins not complemented within one hour could not be used by the body.
 In 1971, Frances Moore Lappé published Diet for a Small Planet, which explained how essential amino acids might be obtained from complementary sources in vegetarian nutrition. The book became a bestseller :
 Lappé wrote:
 In 1975, both Vogue and American Journal of Nursing carried articles describing the principles and practice of protein combining. 
For a time, 
The American National Research Council and the American Dietetic Association (ADA) cautioned vegetarians to be sure to combine their proteins.
 In 1985, the principle of protein combining was explained by J. Rigó:
 In 2011 PLOS ONE published an article investigating the specifics of protein combining for 1251 plant-based foods. The bases of reference are the amino acids indispensable to human nutrition, and the ideal proportioning of these amino acids in a meal. They explain, "complementation involves consuming two or more foods together to yield an amino acid pattern that is better than the sum of the two foods alone."  In contrast to pairings based on food groups, such as pairing a grain with a bean, the investigators reported that pairing by food group was not supported by their work: "Examining the top 100 pairings for each food, we found no consistent pattern of food group-food group pairings."
 Protein combining has drawn criticism as an unnecessary complicating factor in nutrition.
 In 1981,    Frances Moore Lappé changed her position on protein combining from a decade prior in a revised edition of  Diet for a Small Planet in which she wrote:
 Necessity of protein combining was not asserted. Rather, the increased biological value of meals where proteins are combined was noted. In a concession, Lappé removed from the second edition "charts that indicate exact proportions of complementary proteins".: 239 
 The American Dietetic Association reversed itself in its 1988 position paper on vegetarianism. Suzanne Havala, the primary author of the paper, recalls the research process:
 In 1994, Vernon Young and Peter Pellett published their paper that became the definitive contemporary guide to protein metabolism in humans. It also confirmed that complementing proteins at meals was totally unnecessary. Thus, people who avoid consuming animal protein do not need to be at all concerned about amino acid imbalances from the plant proteins that make up their usual diets.
 While many plant proteins are lower in one or more essential amino acids than animal proteins, especially lysine, and to a lesser extent methionine and threonine, eating a variety of plants can serve as a well-balanced and complete source of amino acids.
 In 2009, the American Dietetic Association wrote: 
 The American Heart Association now states:
 Some institutions use the Protein Digestibility Corrected Amino Acid Score to assess diets without consideration of protein combining and hence find the use of combinations to be a challenge to their methodology.[citation needed]