The term "complete protein" refers to amino acids, the building blocks of protein. There are 20 different amino acids that can form a protein, and nine that the body can’t produce on its own. These are called essential amino acids—we need to eat them because we can’t make them ourselves. In order to be considered “complete,” a protein must contain all nine of these essential amino acids in roughly equal amounts.
There are plenty of reasons to eat more meat-free meals: They’re nearly always cheaper, lower in calories, and better for the environment. It’s easy to get enough protein without eating animals, but the doubters often have another concern: Are these meat-free protein sources complete?