What defines a complete protein?

• A. It contains all nine essential amino acids.
• B. It lacks essential amino acids.
• C. It is derived only from animal sources.
• D. It is high in carbohydrates.

Answer: (A)

A complete protein provides all nine essential amino acids in sufficient amounts for human needs. These are the amino acids the body can’t synthesize, so they must come from the diet. That makes completeness about the presence and balance of essential amino acids, not about where the protein comes from or its carbohydrate content. Animal proteins are typically complete, and some plant sources (like soy, quinoa, and amaranth) are complete too. If a protein lacks one or more essential amino acids, it’s incomplete.