Foods are categorized as containing either complete protein or incomplete protein. Those providing complete protein contain all eight essential amino acids. Meats, poultry, fish, and other animal products are good sources of complete protein, but they are not necessarily the healthiest sources, as many of these foods also contain high amounts of saturated fat and cholesterol.
Vegetables, grains, legumes, and nuts do not contain all the essential amino acids. However, each of these food groups contains different types and ratios of the essential amino acids, and when combined, they provide complete proteins. This process of combining complementary proteins has been referred to as mutual supplementation. The traditional dishes from cuisines that rely on plant foods as their major source of protein are excellent examples of this practice: lentils and rice, pasta and beans, tortillas and beans, tofu and rice, or hummus and pita bread.
Recent studies have shown that the body does not require that all eight essential amino acids be consumed in the same meal. A diet balanced over the course ofthe day with various complementary protein sources normally supplies all the amino acids required by a healthy individual.
The Professional Chef, 7th Edition by The Culinary Institute of America