In the context of dietary protein quality and essential amino acids, what is meant by a "limiting amino acid" in a protein source?

• A. An amino acid that is present in excess and must be converted to glucose via gluconeogenesis
• B. An amino acid that cannot be absorbed in the small intestine and is therefore excreted
• C. The essential amino acid present in the lowest proportion relative to human requirements, thereby limiting the extent of protein synthesis from that protein source
• D. A nonessential amino acid that becomes essential only during fasting

Answer: (C)