What defines a complete protein?

A complete protein is defined by its ability to provide all nine essential amino acids that the body cannot synthesize on its own and must obtain from food. These essential amino acids play vital roles in various bodily functions, such as muscle repair, hormone production, and supporting immune health. Animal-based proteins, such as meat, fish, eggs, and dairy, are typically considered complete proteins because they contain adequate amounts of all essential amino acids. While some plant proteins can also be complete, many are not; therefore, relying solely on plant sources may require combining different types to ensure all essential amino acids are consumed.

The other options do not accurately capture the definition of complete protein. For instance, a complete protein can contain fat and it's unrelated to protein source categorization, such as whether it is derived from plants or animals. Digestibility is also a separate matter; while completeness refers to the amino acid profile, digestibility pertains to how well the body can absorb and utilize the protein consumed.