What defines a complete protein?

A complete protein is characterized by its ability to provide all nine essential amino acids that the body cannot synthesize on its own. These essential amino acids are crucial for various bodily functions, including tissue repair, enzyme production, and hormone regulation. Foods that are considered complete proteins, such as meat, dairy, eggs, and some plant sources like quinoa and soy, contain these amino acids in sufficient amounts to meet dietary needs.

While digestibility is important for overall protein effectiveness, it does not define a complete protein. Additionally, complete proteins are not limited to animal sources, as certain plant-based proteins can also meet the criteria. Caloric content is not a defining factor for the completeness of a protein either; rather, it focuses solely on the presence of all essential amino acids. Thus, the defining characteristic of a complete protein is its comprehensive amino acid profile, making option A the correct choice.