A complete protein source contains all nine essential amino acids that the body cannot synthesize. These essential amino acids are crucial for numerous physiological functions, including muscle repair and growth. Examples of complete protein sources include animal products like meat, fish, dairy, and some plant-based sources, such as quinoa and soy. Consuming a variety of protein sources ensures that individuals meet their dietary needs, particularly for those engaging in regular physical activity or resistance training, as protein is vital for recovery and performance.