Although nutritionists once counseled vegetarians to eat complementary proteins to make sure you get proper nutrition, you don’t have to combine foods to get enough protein in a vegetarian diet.
The idea was that since plant foods are limited in one or more of the essential amino acids, you should combine a food that is limited in a particular amino acid with a food that has an abundance of that same amino acid.
The concept was to complement one plant food’s amino acid profile with another’s, fitting two foods together like puzzle pieces. That way, you’d have a “complete protein,” with adequate amounts of all the essential amino acids present and available to the body at the same time.
The idea that proteins had to be complemented was also supported by some very early lab data from studies of protein-deprived rats. The rats were fed diets that were deficient in individual essential amino acids. Without these essential amino acids, the rats couldn’t build complete proteins and became protein deficient.
Of course, those were laboratory conditions. In real life, you eat whole foods that contain an array of amino acids, including all the essential amino acids. You don’t eat specially developed, amino-acid-deficient laboratory rat chow. So these studies had little relevance for free-living human beings.
Experts even went so far as to create complex protein charts that detailed the manner in which foods should be combined, and conscientious vegetarians were careful to eat their beans with rice or corn and to add milk or cheese to their grains (macaroni and cheese was a big favorite). They ruefully acknowledged that they should have taken better notes in organic chemistry class.
In recent years, however, nutrition scientists have given the issue of protein a little more thought, and the verdict is that the practice of combining foods is unnecessary.
Your body can complement its proteins without much help from you. Your job is to do two things:
Make sure you get enough calories to meet your energy needs.
Eat a reasonable variety of foods over the course of the day.
That’s really all there is to it.
