Step 1: Understanding the Concept:
The nutritional quality of a dietary protein depends on its essential amino acid profile.
The "limiting amino acid" is the essential amino acid present in the lowest quantity relative to human nutritional requirements, which limits the body's ability to synthesize new proteins.
Step 2: Detailed Explanation:
Legumes (such as peas, beans, and lentils) are excellent sources of plant-based protein.
However, they have a distinct amino acid imbalance.
Legumes are rich in the essential amino acid lysine but are deficient in the sulfur-containing amino acids, primarily methionine and cysteine.
As a result, methionine is the primary limiting amino acid in most legumes.
In contrast, cereals (such as wheat, rice, and corn) are rich in sulfur-containing amino acids like methionine but are deficient in lysine.
Because of these complementary profiles, combining cereals and legumes in the diet (such as rice and dal) provides a balanced blend of essential amino acids with a high biological value.
Therefore, methionine is the limiting amino acid in legumes.
Step 3: Final Answer:
The primary limiting amino acid in legumes is methionine.