What is an essential characteristic for a complete protein source?

A complete protein source is defined by its ability to provide all nine essential amino acids that the body cannot synthesize on its own. These amino acids must be obtained through diet to support various bodily functions, including tissue repair, immune function, and hormone production. Foods that are considered complete proteins, such as animal products (meat, dairy, eggs) and certain plant sources like quinoa and soy, offer all these essential amino acids in adequate proportions, making them vital for a balanced diet.

The other options, while they may touch on aspects of nutrition, do not define what a complete protein is. For example, low-calorie foods can still lack essential amino acids, and being high in saturated fats does not contribute to the protein's completeness. Finally, while plant sources can provide complete proteins, not all plant foods do, so the source of the protein alone does not determine its completeness. Therefore, containing all essential amino acids is the core characteristic that defines a complete protein source.