Complete proteins play a vital role in a vegan or vegetarian diet by providing the necessary amino acids that the body cannot synthesize on its own. Since many plant-based protein sources are incomplete—lacking one or more essential amino acids—it's important for individuals following these diets to combine various sources, such as legumes with grains. Foods like quinoa, soy, and chia seeds are examples of complete proteins that can help meet the body's needs. Ensuring adequate intake of complete proteins is crucial for muscle maintenance, repair, and overall health.