Protein quality can be defined in a number of different ways. Generally the higher the quality protein, the less of it that is required to supply daily protein requirement. Animal proteins are considered higher quality than plant proteins, and this relates to the fact that plant proteins are often deficient in certain essential amino acids. For example, beans are low in the essential amino acid methionine, while grains are low in the essential amino acid lysine. Eating only beans or grains as a source of protein will therefore result in an amino acid deficiency. In contrast, animal proteins contain all the essential amino acids and can therefore be eaten in isolation with no fear of an amino acid deficiency. However, combining plant proteins, or eating them with animal proteins, supplies all the essential amino acids required and therefore in a balanced diet protein needs can be easily met. Protein quality therefore only really matters if limited sources of protein are consumed, and they are mainly plant based.
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