What defines a complete protein?

A complete protein is defined as one that contains all nine essential amino acids that the body cannot synthesize on its own and must obtain from dietary sources. These essential amino acids play critical roles in various bodily functions, including tissue repair, hormone production, and enzyme function.

Animal-based proteins, such as those found in meat, dairy, and eggs, are typically complete proteins, as they contain sufficient amounts of all essential amino acids. While some plant-based proteins may be incomplete on their own, certain combinations of plant foods can also provide all essential amino acids, making them complete when consumed together (for example, rice and beans).

The other choices do not accurately describe what constitutes a complete protein. A protein being low in calories or low in fat pertains to its overall nutritional composition rather than its amino acid profile. Similarly, a protein derived only from plants does not guarantee it is complete, as many plant proteins lack one or more essential amino acids. Therefore, the defining characteristic of a complete protein is its composition, specifically the presence of all essential amino acids.