Limiting Amino Acid

A limiting amino acid is the indispensable amino acid supplied in the lowest proportion relative to the amount specified in a reference amino acid pattern. When the amino acid composition of a dietary protein is compared with human requirements, the amino acid with the greatest relative shortfall is called the first limiting amino acid. Its limited supply can reduce how effectively that protein source provides the full pattern of indispensable amino acids required by the body .^1,2

The limiting amino acid is not necessarily the amino acid present in the smallest absolute amount. Different indispensable amino acids are required in different proportions, so each must be compared with its corresponding value in an appropriate reference pattern. The amino acid with the lowest resulting ratio is considered limiting. If another amino acid has the next-lowest ratio, it may be described as the second limiting amino acid .^1,2

The concept is central to methods used to evaluate dietary protein quality. Amino acid scoring approaches compare the amino acid content of a protein with a reference pattern. Methods such as the Digestible Indispensable Amino Acid Score (DIAAS) also account for the digestibility of individual indispensable amino acids because the amount present in a food is not necessarily the amount that becomes available for absorption .¹

Which amino acid is limiting depends on the protein source or mixture of foods being evaluated and on the reference requirement pattern. Combining different protein sources can therefore change which amino acid is limiting because one source may supply an amino acid that is relatively scarce in another .^1,2