What defines a "complete" protein?

A "complete" protein is defined as a protein that contains all nine essential amino acids in sufficient amounts needed for the body to function optimally. Essential amino acids are those that the body cannot synthesize on its own and must be obtained through the diet. Proteins that provide these essential amino acids are vital for various bodily functions, including tissue repair, immune function, and hormone production.

While some protein sources, such as animal products (meat, dairy, eggs), are typically recognized as complete proteins, there are also plant-based sources (like quinoa and soy) that can provide all essential amino acids, demonstrating that complete proteins are not restricted to animal sources.

The other options do not accurately define what makes a protein complete. For instance, a protein containing only plant-based amino acids does not guarantee it is complete unless it includes all essential amino acids. Similarly, a protein derived solely from animal sources could be complete, but it is not a definitive criterion for completeness. Lastly, a protein that lacks important amino acids cannot be considered complete, as it would fail to provide the essential amino acids necessary for the body. Therefore, the definition of complete protein hinges on the presence of all nine essential amino acids in adequate amounts.