For the past 25 years or so there has been a debate, largely confined to the vegetarian community, about whether "protein complementarity" makes sense as a practical nutritional concept at all. There's no question that you need all the "essential amino acids" and that plants have these amino acids, which can't be manufactured by the body and are thus "essential" in the diet, in differing quantities. The question is whether this matters, because even the amino acids in the least quantity in (say) corn are still supplied in sufficient quantity to meet the minimum requirement.

Check out this flyer which I wrote years ago:
http://compassionatespirit.com/protein.htm

I did calculations some 25 years ago and came to the conclusion that at the current U. S. requirements for each of the amino acids, almost all plant foods (if consumed exclusively and alone to provide all your calories) would give you both enough protein and enough of all of the amino acids. Among vegetarian advocates Dr. John McDougall is the most prominent supporter of this position.

The more traditional position, advocated by the American Dietetic Association and others, is that protein complementarity is needed but not at the same meal. As far as I know, there is no particular evidence to support this, it is just maintained because the idea, adopted by "Diet for a Small Planet," that "protein complementarity" is necessary for plant foods, is the traditional one and it has never become hot enough an issue to be debated. I am fairly confident that if this were to happen, the whole concept as a practical issue would disappear.

I would emphasize that you DO need all of the essential amino acids and you CAN be protein deficient, just that as a practical matter protein balancing is not necessary. If you're getting enough calories, you're getting enough protein unless you're an alcoholic, eating nothing but cassava, something like that. If you're getting enough protein, your proteins are balanced unless you're eating mostly grapes or some other foods. The easiest way to be protein deficient is not to get enough calories. I managed to do this once myself, without knowing it, when I first became a vegan.

http://compassionatespirit.com/vegetarianism-and-genetics.htm

By the way (citations in my book "A Vegetarian Sourcebook"), they have done studies in which they fed people diets of corn alone, potatoes alone, wheat alone, or rice alone. These diets are not adequate for other reasons (like not enough vitamin C, for example) and so you would not want to eat such a diet on a regular basis. However, they were protein adequate and provided all the amino acids.

Keith Akers