Why are plants called second class protein?

Plants are often referred to as "second-class proteins" because they typically lack one or more of the essential amino acids that the human body cannot produce on its own. While animal proteins are considered "complete" because they contain all essential amino acids in adequate amounts, many plant proteins are "incomplete." However, by combining different plant sources, such as legumes and grains, one can achieve a complete amino acid profile. This classification highlights the need for careful dietary planning for those relying primarily on plant-based proteins.