The human body uses 20 amino acids to build proteins which are classified as essential (your body cannot self produce these) and non essential. For optimal health, your body needs all the essential amino acids in the right ratios.
Animal protein sources, such as meat, fish, poultry, eggs and dairy, are similar to the protein found in your body. These are considered to be complete sources of protein because they contain all of the essential amino acids that your body needs to function effectively.
On the contrary, plant protein sources, such as beans, lentils and nuts are considered to be incomplete, as they lack one or more of the essential amino acids that your body needs.
Some sources report soy protein as complete. However, two essential amino acids are only found in small trace amounts in soy, so it is not comparable to animal protein.