What characteristic defines a complete protein?

A complete protein is defined by its ability to provide all nine essential amino acids that the human body cannot synthesize on its own. These essential amino acids are crucial for various bodily functions, including building and repairing tissues, producing enzymes and hormones, and supporting immune function.

The presence of all nine essential amino acids is what distinguishes complete proteins from incomplete proteins, which may lack one or more of these amino acids. While many animal-based proteins, such as meat, dairy, and eggs, are complete proteins, some plant-based sources, like quinoa and soy, also qualify as complete proteins. Therefore, stating that a complete protein contains all nine essential amino acids accurately captures its defining characteristic.

The other options do not fully encompass the definition of complete proteins. For instance, while essential amino acids are vital, a complete protein is not limited to containing only these; rather, it must include all of them in their entirety. Additionally, the caloric content of a protein source does not determine its completeness, nor is it accurate to state that complete proteins can only be derived from animal sources, as certain plant proteins can also be classified as complete.