Protein combining (also protein complementing) is a dietary strategy for protein nutrition by using complementary sources to optimize biological value and increase the protein quality. Originally applied to livestock feed for animal nutrition, since 1971 it has become a chemical critique of food values of vegetarian dishes.
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.
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.
The concept that plant protein was inferior to animal protein arose from studies performed on rodents more than a century ago. Osborne and Mendel found that infant rats don’t grow as well on plants. But infant rats don’t grow as well on human breast milk either. Rat milk has ten times more protein than human milk, because rats grow about ten times faster than human infants.
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 1970, H. N. Munro described how the mammalian body maintains pools of free amino acids, particularly lysine, that it can use to do all the complementing for the organism.
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 :
- 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.
- 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:
- Grains (rice, corn, wheat, barley, etc.) + legumes (peas, beans, lentils)
- Grains and milk products
- Seeds (Sesame or sunflower) +legumes:238
Dr. Michael Greger explains:
- It is true that some plant proteins are relatively low in certain essential amino acids. So, about 40 years ago, the myth of “protein combining” came into vogue—literally, the February ‘75 issue of Vogue magazine. The concept was that we needed to eat “complementary proteins” together, for example, rice and beans, to make up for their relative shortfalls.
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.
As late as 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.
Protein combining has drawn criticism as an unnecessary complicating factor in nutrition.
In 1981, Lappé changed her position on protein combining in a revised edition of Diet for a Small Planet, in which 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.":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".: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. 
The concept that plant proteins are incomplete or inferior, and that humans have to combine proteins at meals, have thus all been dismissed by the nutrition community as myths. 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 2002, Dr. John McDougall wrote a correction to the American Heart Association for a 2001 publication that questioned the completeness of plant proteins, and further asserted that "it is impossible to design an amino acid–deficient diet based on the amounts of unprocessed starches and vegetables sufficient to meet the calorie needs of humans."
In Healthy Times Jeff Novick wrote that the necessity of protein combining is a "myth that won’t go away".
In 2005, Dr. Joel Fuhrman wrote:
- ...plant foods have plenty of protein and you do not have to be a nutritional scientist or dietitian to figure out what to eat and you don’t need to mix and match foods to achieve protein completeness. Any combination of natural foods will supply you with adequate protein, including all eight essential amino acids as well as unessential amino acids.
Dr. T. Colin Campbell wrote in 2006:
- We now know that through enormously complex metabolic systems, the human body can derive all the essential amino acids from the natural variety of plant proteins that we encounter every day. It doesn’t require eating higher quantities of plant protein or meticulously planning every meal.
In 2009, the American Dietetic Association wrote:
- 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.
The American Heart Association now states:
- 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.
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.
- Osborne, Thomas Burr (1913) "In Memoriam: Heinrich Ritthausen", Biochemical Bulletin 2:338, published by the Columbia University Biochemical Association
- Osborne, T B; Mendel, L B (1912). "Amino Acids in Nutrition and Growth" (PDF). J. Biol. Chem. 17: 27. Retrieved 12 January 2017.
- Davis, T A; Nguyen, H V; Garcia-Bravo, R; Fiorotto, M L; Jackson, E M; Lewis, D S; Lee, D R; Reeds, R J (Jul 1994). "Amino acid composition of human milk is not unique". J Nutr. 124 (7): 1126–32. Retrieved 13 January 2017.
- Sengupta, P (Jun 2013). "The Laboratory Rat: Relating Its Age With Human's". Int J Prev Med. 4 (6): 624–30. Retrieved 13 January 2017.
- Scrimshaw obituary from Massachusetts Institute of Technology
- Munro, H N (1970). Mammalian Protein Metabolism. Cambridge, Massachusetts: Elsevier Inc. pp. 299–386. Retrieved 13 January 2017.
- Davis, Adelle (1954). Let's Eat Right to Keep Fit. Harcourt, Brace. ISBN 4-87187-961-5.
- Warren Belasco (1989) Appetite for Change: how the counterculture took on the food industry 1966 — 1988, page 56, Pantheon Books ISBN 0394543998
- Lappé, Frances Moore (1981) Diet for a Small Planet, ISBN 0-345-32120-0
- Greger, Miichael. "The Protein-Combining Myth". NutritionFacts.org. Retrieved 12 January 2017.
- Judith S. Stern (1975) "How to stay well on a vegetarian diet and save money too!", Vogue 165(2):150,1
- Eleanor R. Williams (1975) Making Vegetarian Diets Nutritious, American Journal of Nursing 75(12):2168–73 from JSTOR
- Maurer, Donna. (2002). Vegetarianism: Movement or Moment? Philadelphia: Temple University Press. ISBN 1-56639-936-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
- Young VR, Pellett PL (1994). "Plant proteins in relation to human protein and amino acid nutrition". The American Journal of Clinical Nutrition. 59 (5 Suppl): 1203S–1212S. PMID 8172124.
- Attwood, Charles R. "Complete" Proteins?, VegSource.com
- McDougall, J (25 Jun 2002). "Plant foods have a complete amino acid composition". Circulation. 105 (25): :e197. Retrieved 13 January 2017.
- Vegetarians: Pondering Protein?, DrWeil.com, Dec. 11, 2002
- Complementary Protein Myth Won't Go Away!, Jeff Novick, M.S., R.D., Healthy Times (May 2003)
- Fuhrman, Joel (2005). Eat to Live. Little Brown. p. 137.
- Campbell, T. Colin; Campbell, Thomas M. (2006). The China Study. BenBella Books. p. 31. ISBN 1935251007.
- "Position of the American Dietetic Association: Vegetarian Diets" (PDF). Journal of the AMERICAN DIETETIC ASSOCIATION. 109 (7): 1267–1268. July 2009. Retrieved 13 January 2017.
- "Vegetarian, Vegan Diet & Heart Health". Go Red For Women. American Heart Association. Retrieved 13 January 2017.
- When Your Friends Ask: "Where Do You Get Your Protein", McDougall Newsletter
- The Protein-Combining Myth, NutritionFacts.org
- The Myth of Complementary Protein, Forks Over Knives