Protein combining

Protein combining (also protein complementing) is a dietary strategy for protein nutrition by using complementary sources to optimize biological value. Originally applied to livestock feed for animal nutrition, since 1971 it has become a chemical critique of food values of vegetarian dishes.

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.

The first biochemist to enter the field was Karl Heinrich Ritthausen, a student of Justus von Liebig. When Ritthausen died in 1912, Thomas Burr 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 continued what Ritthausen started and published The Vegetable Proteins in 1909. He then joined forces with Lafayette Mendel at the Connecticut Agricultural Experiment Station to determine the essential amino acids. Thus Yale University was the early center of protein nutrition, where William Cumming Rose was a student. He also worked to determine the essentials, and later led the Biochemistry Department at the University of Chicago.

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.

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. Nonetheless, eating a variety of plants can serve as a well-balanced and complete source of amino acids.

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.

Popularization
In 1971, as America was making its lunar landings, Frances Moore Lappé published a book 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.

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:
 * Grains (rice, corn, wheat, barley, etc.) + legumes (peas, beans, lentils)
 * Grains and milk products
 * Seeds (Sesame or sunflower) +legumes

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 1981, Lappé relaxed 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 [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."

Criticism
Protein combining has drawn criticism as an unnecessary complicating factor in vegetarian nutrition. For instance, Andrew Weil wrote that "you don’t have to worry that you won’t get enough usable protein if you don’t put together some magical combination of foods at each meal."

Charles R. Attwood wrote, "The old ideas about the necessity of carefully combining vegetables at every meal to ensure the supply of essential amino acids has been totally refuted."

In Healthy Times Jeff Novick wrote that the necessity of protein combining is a "myth that won’t go away". 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."

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.] And it was a couple of years after that that Vernon Young and Peter Pellett 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.

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.