The limiting amino acid in many protein sources is:

The limiting amino acid in many protein sources is:

• A. Lysine
• B. Methionine
• C. Leucine
• D. Tryptophan

Answer: (A)

Limiting amino acid is the essential amino acid present in the smallest amount relative to what the animal needs, so it governs how much protein can be synthesized from a given protein source. Lysine is often the first limiting amino acid in many plant- and cereal-based proteins because these ingredients tend to be low in lysine compared with the animal’s requirements. When lysine is scarce, it caps growth and protein deposition even if other essential amino acids are adequate. That’s why diets are commonly balanced by prioritizing lysine adequacy, sometimes adding synthetic lysine to hit the target. Methionine can be limiting in some feeds, and leucine or tryptophan can limit in particular ingredients, but lysine is the most consistently limiting across many protein sources.

Limiting amino acid is the essential amino acid present in the smallest amount relative to what the animal needs, so it governs how much protein can be synthesized from a given protein source. Lysine is often the first limiting amino acid in many plant- and cereal-based proteins because these ingredients tend to be low in lysine compared with the animal’s requirements. When lysine is scarce, it caps growth and protein deposition even if other essential amino acids are adequate. That’s why diets are commonly balanced by prioritizing lysine adequacy, sometimes adding synthetic lysine to hit the target. Methionine can be limiting in some feeds, and leucine or tryptophan can limit in particular ingredients, but lysine is the most consistently limiting across many protein sources.