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 The terms complete and incomplete are outdated in relation to plant protein.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 amino acids including the 9 essential amino acids in varying amounts.[1]
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.[2] 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.[3]
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.[4]
Plants are thus rated as protein sources by their limiting amino acids.[5]
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.[6] 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.
Rice and chickpeas as protein sources, in 46.2 g of protein (daily requirement)
Amino acid
Amount (mg) in rice protein (612g of whole rice)[7]
% WHO requirement
Amount (mg) in chickpea protein (522g of canned chickpeas)[8]
% WHO requirement
Histidine
1236
176%
1274
182%
Isoleucine
2056
146%
1984
141%
Leucine
4021
144%
3294
118%
Lysine
1854
88%
3095
147%
Methionine
1095
156%
606
87%
Phenylalanine
2509
143%
2479
141%
Threonine
1781
169%
1717
163%
Tryptophan
618
220%
444
158%
Valine
2852
156%
1942
106%
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.
Rice and chickpeas as separate protein sources, 50-50 ratio (23.1g each)
Amino acid
Amount (mg) in rice protein (~306g of whole rice)[7]
Amount in chickpea protein (~261g of canned chickpeas)[8]
Combined amount (mg)
WHO recommended intake amount (mg), 70 kg
% WHO requirement
Histidine
618
637
1255
700
179%
Isoleucine
1028
992
2020
1400
144%
Leucine
2011
1647
3658
2790
131%
Lysine
927
1548
2475
2100
118%
Methionine
548
303
851
700
122%
Phenylalanine
1254
1240
2494
1750
143%
Threonine
891
858
1749
1050
167%
Tryptophan
309
222
531
280
190%
Valine
1426
971
2397
1820
132%
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.
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.[9]
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.[10]
Popularization
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.[11]
An extension of a one-page handout that Lappé had circulated among her fellow improvisors in Berkeley, Diet for a Small Planet (1971) soon became the vegetarian text of the ecology movement, selling in the next ten years almost two million copies in three editions and six languages.[12]
Lappé wrote:
Complementary protein combinations make for delicious recipes – they are combinations that formed the basis of the world’s traditional cuisines. We use them naturally in our cooking without even being aware of it. The three most common complementary protein combinations are:
Protein combining reached the pages of a general chemistry textbook in 1982:
There is an increased possibility of protein deficiency with a strictly vegetable diet, unless the vegetable sources are carefully combined so that they complement one another.
— Stanley E. Manahan, General Applied Chemistry, second edition, page 473
In 1985, the principle of protein combining was explained by J. Rigó:
The biological value of proteins in general, hence also of grain-proteins, is fundamentally determined by the ratio between the essential amino acids to be found in cereals and the requirement of essential amino acids of the living creature, consuming protein...the most important way of raising the biological value ... [is] given by the technique of complementing.[17]
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."[18] 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."[18]
Criticism
Protein combining has drawn criticism as an unnecessary complicating factor in nutrition.
"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 [1] fruit or on [2] some tubers, such as sweet potatoes or cassava, or on [3] 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."[13]: 162
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".[13]: 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:
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.]
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.[19]
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.[19]
Plant protein can meet protein requirements when a variety of plant foods is consumed and energy needs are met. Research indicates that an assortment of plant foods eaten over the course of a day can provide all essential amino acids and ensure adequate nitrogen retention and use in healthy adults, thus, complementary proteins do not need to be consumed at the same meal.[20]
You don’t need to eat foods from animals to have enough protein in your diet. Plant proteins alone can provide enough of the essential and non-essential amino acids, as long as sources of dietary protein are varied and caloric intake is high enough to meet energy needs. Whole grains, legumes, vegetables, seeds and nuts all contain both essential and non-essential amino acids. You don’t need to consciously combine these foods (“complementary proteins”) within a given meal.[21]
^Maurer, Donna. (2002). Vegetarianism: Movement or Moment? Philadelphia: Temple University Press. ISBN1-56639-936-X. p. 37
^Rigó, J. (1985) "Nutritional Functions of Cereals", in Amino Acid Composition and Biological Value of Cereal, Radomir Lásztity & Máté Hidvégi editors, International Association for Cereal Chemistry