Complete proteins and incomplete proteins

Another way to describe the quality of proteins is to say that they’re either complete or incomplete. A complete protein is one that contains ample amounts of all essential amino acids; an incomplete protein does not. A protein low in one specific amino acid is called a limiting protein because it can build only as much tissue as the smallest amount of the necessary amino acid. You can improve the protein quality in a food containing incomplete/ limiting proteins by eating it along with one that contains sufficient amounts
of the limited amino acids. Matching foods to create complete proteins is called complementarity.

For example, rice is low in the essential amino acid lysine, and beans are low in the essential amino acid methionine. By eating rice with beans, you improve (or complete) the proteins in both. Another example is pasta and cheese. Pasta is low in the essential amino acids lysine and isoleucine; milk products have abundant amounts of these two amino acids. Shaking Parmesan cheese onto pasta creates a higher-quality protein dish. In each case, the foods have complementary amino acids. Other examples of complementary protein dishes are peanut butter with bread, and milk with cereal. Many such combinations are a natural and customary part of the diet in parts of the world where animal proteins are scarce or very expensive. Here are some categories of foods with incomplete proteins:

• Grain foods: Barley, bread, bulgur wheat, cornmeal, kasha, and pancakes
• Legumes: Black beans, black-eyed peas, fava beans, kidney beans, lima beans, lentils, peanut butter, peanuts, peas, split peas, and white beans
• Nuts and seeds: Almonds, Brazil nuts, cashews, pecans, walnuts, pumpkin seeds, sesame seeds (tahini), and sunflower seeds In order for the foods to complement each other, you must eat them together. In other words, rice and beans at one meal, not rice for lunch and beans for dinner.