Glossary
Limiting amino acid.
Direct answer
The limiting amino acid is the indispensable amino acid present in the shortest supply relative to requirement. Protein synthesis comes to a halt as soon as this building block runs out, however plentiful the remaining amino acids may be. The picture of the shortest stave in a barrel captures the mechanism well.
The crude protein content of a ration says nothing about the limiting amino acid. Two feeds with the same protein content can differ widely in amino acid profile, and what matters in addition is the share actually absorbed in the small intestine. Protein broken down only in the hindgut barely serves the host's own synthesis.
In horses, lysine is recognised as the first limiting amino acid in pasture-based and cereal-based rations. In humans, lysine is scarce above all in cereals, whereas in pulses the sulphur-containing amino acids tend to limit instead. The principle is identical; the practical consequence depends on the staple foods of each species.
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