Protein combining

Protein combining (also protein complementing) is the theory, now largely discredited, that vegetarians must eat foods such as beans and rice together, or at least on the same day, so the different amino acids in the foods combine to form a "complete" protein, containing all nine essential amino acids necessary for human growth and maintenance. In fact, all essential amino acids are present in common individual plant foods, including beans, rice, potatoes, and corn.

Source of the theory
The protein combining hypothesis was given wings in Frances Moore Lappé's 1971 bestseller Diet for a Small Planet. Her conclusion, however, was based on misinterpretation of studies performed in the early 1900's on the growth of rats. In later editions of Diet for a Small Planet, as early as 1981, Lappé herself reversed her opinion that protein combining is necessary. In the tenth anniversary edition of Diet for a Small Planet, Lappé writes:


 * "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."

The contemporary view
Some of the earliest research to refute the idea that proteins must be combined was performed in the 1950's by William Rose, but the results of that research didn't make their way into the popular press until 1983 with the debut book of John McDougall, M.D., The McDougall Plan (ISBN 0-8329-0392-2). In that volume, McDougall shows the requirements for each individual amino acid as determined by Rose and then shows how each is easily met by a single unrefined plant food by itself, such as corn, brown rice, oatmeal, wheat flour, potatoes, broccoli, and pumpkin. McDougall writes:


 * "Many people believe that animal foods contain protein that is superior in quality to the protein found in plants. This is a misconception dating back to 1914, when Osborn and Mendel studied the protein requirements of laboratory rats. (11) .... Based on these early rat experiments the amino acid pattern found in animal products was declared to be the standard by which to compare the amino acid pattern of vegetable foods.... It has since been shown that the initial premise that animal products supplied the most ideal protein pattern for humans, as it did for rats, was incorrect. Therefore, the idea that vegetable foods were deficient in certain amino acids for our needs was inappropriately based on a standard diet ideal for rats. At that early time no one knew the actual protein or amino acid requirements for humans."

More recently, McDougall has challenged the American Heart Association for perpetuating the idea that protein combining is necessary without providing any evidence that that is the case.

Actual analysis
The easiest way to demonstrate the theoretical adequacy of plant proteins is simply to look at their amino acid content vs. the requirements. The standard (details needed) for the individual amino acid (IAA) requirements for adults is the joint 1985 report by the Food and Agriculture Organisation (FAO), the World Health Organization (WHO), and the United Nations University (UNU), "Expert Consultation on Energy and Protein Requirements", referenced as Table 6 of Nitrogen and Amino Acid Requirements by Vernon R. Young and Sudhir Borgonha of MIT. These figures are reproduced in column 1 of the table below, and extrapolated to column 2. In the third column we list the amino acids in (uncombined) brown rice, according to the USDA Nutrient Standard Reference. This shows that brown rice even by itself easily exceeds the requirements for the essential amino acids listed for a 150 lb person consuming 2200 calories of rice, assuming complete bioavailability. The numbers for other common plant foods tell a similar story.

Protein quality
Proteins differ in their Biological Value. Higher quality proteins have a higher Biological Value (BV) such as whey protein at 100 BV compared to soy protein at 74 BV.