"But what about protein complementarity?"
In 1971, a revolutionary new book came out espousing the virtues of a meatless diet. It became a million-copy bestseller and convinced many people to try vegetarianism or become vegetarians. That book was Diet for a Small Planet by Frances Moore Lappé.
In her book, Lappé put forward as her central concept the idea of "protein complementarity"--the idea that vegetarians should eat different kinds of proteins at a single meal in order to get the same quality of protein which was found in meat. Countless thousands of vegetarians thereafter referred to Lappé's charts and tables and struggled to understand the intricacies of balancing tryptophan, lysine, methionine, and all the other amino acids.
The basic idea was this: while meat contains all of the amino acids, plant foods were deficient in one or more of the eight "essential" amino acids. Therefore, balance plant foods weak in one amino acid but strong in a second amino acid, with other plant foods strong in the first but weak in the second. Simple, right? Well, simple to some people, but not so simple to others, who eventually gave up the effort and went back to a meat-based diet out of fear of missing one or another of the amino acids.
And yet the central thesis of this best-selling book, one which even today many vegetarians believe in, is false. There's no question that you need all of the amino acids. But virtually all plant foods have all of the essential amino acids; and not only are the amino acids there, they are present in more than enough quantity to meet the needs of normal adults, if you are on a calorically adequate diet.
It's true that plant foods have more of the requirements of some amino acids than of others. Rice is strong in tryptophan, methionine, and valine, and weak in isoleucine and lysine. But rice protein sufficient to provide 100% of our quantitative protein needs, also provides 265% of the adult male requirement for lysine and 266% of that for isoleucine. (It provides 400% or more of all of the others.)
The same is true for virtually all other plant foods. In fact, some plant foods which do not quite provide the requirement for total protein, such as sweet potatoes, do provide the minimum requirement for all of the essential amino acids.
Rats and People
The whole idea of "protein complementarity" got started in 1914 when Osborne and Mendel published a paper on rat nutrition. They noticed that baby rats fed a plant food diet did not grow as fast as other rats who ate the same diet plus a lysine supplement. Conclusion: these plant foods needed a lysine supplement.
Unfortunately, the nutritional requirements of rats and humans are quite different, and this was quickly demonstrated by experiments on humans. Studies in which humans have been fed wheat bread alone, or potatoes alone, or corn alone, or rice alone, have all shown that these plant foods contain not only enough protein, but enough of all of the essential amino acids, to support growth and maintenance of healthy adults. Particularly striking were the experiments involving rice: not only was the rice protein more than adequate, it was adequate when only about 2/3 of the calories were provided through the rice. This means that the actual requirement for protein for most individuals is actually less than 8% of calories as stated by the National Research Council; the NRC has padded its figures with a "safety factor" which many individuals do not need.
A few sample plant foods are shown with their "limiting amino acid" content in the accompanying table. (Limiting amino acids are the amino acids the food contains the least of in relation to human nutritional requirements.)
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