Protein combining or protein complementing is a process in which individual plant products that by themselves are deficient in one or more essential amino acids are combined with other plant products in order to provide all the essential amino acids for human life. While all individual plant foods provide at least trace amounts of each of the 20 amino acids, they often have an insufficient amount of some of the nine essential amino acids, making protein combining with multiple complementary foods necessary to obtain a diet with "complete protein". Consuming sufficient amounts of all nine essential amino acids is absolutely essential for human life. As all meats contain all amino acids in sufficient quantities, protein combining is only important for those who eat few to no meat in their regular diet. For those on a vegetarian or vegan diet, therefore, protein combining—that is, eating a variety of foods that each provide sufficient amounts of some of the essential amino acids, and therefore, through a combination of sources, receiving a full slate of the nine essential amino acids—is absolutely essential. However, it is so easy to achieve in practice that most people do not need to consciously aim to reach a diet with complete protein. For instance, rice and beans, a staple combination throughout the world, combine to provide more than enough of all the essential amino acids. Protein combining was historically promoted as a method of compensating for protein deficiencies in most vegetables as foods, found in limiting percentages revealed in their respective amino acid profiles. In this dogma of the 1970s, each meal needed to be combined to form complete proteins. Though it is undisputed that diverse foods can be thoughtfully combined to make a more nutritious meal, studies on essential amino acid contents in plant proteins have shown that careful combination in each individual meal is not at all required for vegetarians and vegans to reach the desired level of essential amino acids as long as their diets are varied and daily caloric requirements are met. In other words, combination can happen over a longer course of time.
Concept 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.
If the content of a single indispensable amino acid in the diet is less than the individual's requirement, then it will limit the utilization of other amino acids and thus prevent the normal rates of synthesis even when the total nitrogen intake level is adequate. Thus the "limiting amino acid" will determine the nutritional value of the total nitrogen or protein in the diet. Protein sources, including plants, are thus rated by their limiting amino acids. In any diet, it is critical to consume enough of each essential amino acid. While this is very easy in any diet which consumes meat, a plant-based diet may theoretically miss out on one or more of the essential amino acids, consuming them only in very small quantities. However, in practice, the vast majority of satisfying vegan diets contain more than enough variety in their component foods to meet the minimum requirements of all essential amino acids.
In addition to grains, such as corn, rice, or wheat, vegetable protein also occurs in legumes, which include beans and peanuts. Grains tend to be deficient in tryptophan and lysine, whereas legumes lack methionine Thus a meal combining grains and legumes such as the Mexican peasant dish of corn tortillas and refried beans is basically complete in amino acid content.
Examples of "limiting" amino acids in plant protein 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 46.2 g (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 only chickpeas or other plant-based foods that also lack methionine for long periods of time will result in deficiency of the amino acid, and severe health problems will arise.
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 306 g/day and 261 g/day respectively, for long periods of time, would easily combine to create complete protein, as it would not result in any of the essential amino acid deficiency. 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.
Plant protein research 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:
As a result of his later work he proved that wide differences exist between different food proteins; and he was the first to direct attention to this fact, and to discuss its probable bearing on their relative value in nutrition. 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.
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