What distinguishes complete proteins from incomplete proteins?

The distinction between complete and incomplete proteins lies in the presence of essential amino acids. Complete proteins contain all nine essential amino acids that the body cannot produce on its own, making them vital for various bodily functions, including muscle repair, nutrient absorption, and immune function. These complete proteins are typically found in animal products such as meat, fish, dairy, and eggs, as well as some plant sources like quinoa and soy.

Incomplete proteins, on the other hand, lack one or more of the essential amino acids. They are often found in plant-based foods such as beans, lentils, nuts, and grains. While these foods are valuable in a balanced diet, relying solely on them may lead to a deficiency in essential amino acids if not complemented by other sources.

The other options do not accurately define or differentiate between complete and incomplete proteins. For example, not all complete proteins are low in calories, as many are quite high-calorie foods. Additionally, while many incomplete proteins do come from plant sources, there are exceptions, and it is incorrect to state that all proteins are complete, as many protein sources are inherently incomplete. Understanding the classification based on amino acid content is crucial for maintaining a balanced diet that meets nutritional needs.