What distinguishes complete proteins from incomplete proteins?

Complete proteins are distinguished by their ability to provide all the essential amino acids that the human body cannot synthesize on its own. These essential amino acids must be obtained from the diet, and complete proteins, typically found in animal sources such as meat, fish, eggs, and dairy products, contain them in adequate proportions required for various bodily functions.

Unlike complete proteins, incomplete proteins, which are primarily found in plant sources such as beans, legumes, and grains, do not provide all the essential amino acids or may lack sufficient quantities of one or more of them. While combining different incomplete protein sources can yield a complete amino acid profile, complete proteins are inherently balanced and suitable for supporting various physiological processes, including muscle repair, enzyme function, and hormone production.

In contrast, the other options relate to different attributes that do not accurately describe the distinction between complete and incomplete proteins. For instance, the presence or absence of carbohydrates, caloric content, or the source of the protein (animal vs. plant) does not directly pertain to whether the protein is complete or incomplete in terms of amino acid composition.