a lentils and a bowlProtein is often a big worry for people contemplating a vegan diet. It's important after all. We need it for muscles, bones, hormones, digestive enzymes, absorption of nutrients and to rebuild and replenish all kinds of cells. So knowing a little about sources of protein and recommendations for vegans can be helpful and reassuring.
Protein and Amino Acids
Proteins in foods and in the human body are composed of 20 amino acids. We humans can synthesize eleven of these as long as we get sufficient nitrogen from our diets. (Nitrogen is a component of every amino acid.)
The other nine amino acids—called essential amino acids (EAAS)—have to come from food since we can't make them. So our need for protein is actually a need for these nine essential amino acids plus enough nitrogen to manufacture the other eleven.
Every plant protein contains all nine of the EAAs. They aren't missing any of them, but (with the exception of soy protein) they are always a little low in one or two EAAs. As a result, the amino acid patterns in plants are a less precise match to human protein needs.
At one time, nutrition experts thought the answer to this was to combine proteins from different plant foods. Since grains and beans have complementary strengths and weaknesses in their EAA patterns, eating them together produced a "complete" amino acid pattern—that is, a pattern that mimics patterns found in human body proteins. And that wasn't really a big deal, because historically, it's the way most people have eaten: black beans and rice, lentil soup with bread, pinto beans with corn tortillas, hummus with pita bread.
Careful protein combining has turned out to be unnecessary, though. This is because the body maintains a reserve pool of amino acids from various sources for later use. Basically, your body can do its own "complementing" with the amino acids in a meal (1). The evidence also suggests that protein consumed at one meal can combine with protein consumed later in the day.
Protein combining is now considered an old-fashioned idea that isn't relevant in vegan or vegetarian diets.