What is a 'second class protein'?

A 'second class protein' refers to proteins that are not considered complete, meaning they do not contain all nine essential amino acids in sufficient quantities for human needs. These proteins are typically derived from plant sources, such as legumes, grains, and vegetables. While they can be valuable components of a diet, they often need to be combined with other protein sources to provide a balanced amino acid profile. Examples include beans, rice, and nuts.