Complete and Incomplete Protein
The body can produce many amino acids to make peptides and proteins. However, nine amino acids are essential; they must be sourced from the diet as the human body can not make them. The nine essential amino acids are histidine, leucine, isoleucine, methionine, lysine, tryptophan, valine, threonine, and phenylalanine.
Humans do not synthesize these nine amino acids, which are dietarily essential or indispensable nutrients. These nine amino acids are called essential amino acids and are food sources of protein.
Food sources containing all nine essential amino acids in sufficient quantities are known as complete proteins. Complete proteins are found in animal products and a few plant sources.
Incomplete proteins are plant-based sources of proteins that do not contain all nine essential amino acids or do not contain sufficient amounts of essential amino acids to meet the body's daily requirements. Although "incomplete," they are no less valuable than complete proteins as protein needs are met throughout the day with meals and snacks.
Vegans and vegetarians are advised to eat a wide variety of protein-rich and fortified foods to ensure the consumption of all nine essential amino acids daily from complete and incomplete proteins.
Popular pairings of incomplete proteins​
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lentils and rice
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oats and nuts
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brown rice and black beans
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hummus and bread or crackers
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lentils or beans with pasta
A complete protein, high-quality protein
Food proteins that provide all the essential amino acids in the proportions needed by the body are called high-quality proteins or complete proteins. Examples of complete proteins include animal proteins such as meat, poultry, fish, eggs, milk, and other dairy products.
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Incomplete protein
Incomplete proteins are low in one or more essential amino acids. Plant proteins, including dried beans and peas, grains, vegetables, nuts, and seeds, are incomplete. The essential amino acid in the lowest concentration in protein is referred to as a limiting amino acid because it limits the protein's usefulness in the body.
Complementary proteins
Although plant proteins are incomplete, it does not mean they are low quality. When certain plant foods, such as peanut butter and whole-wheat bread, are eaten over a day, the limiting amino acid in each of these proteins is supplied by the other food. Such combinations are called complementary proteins, and they yield complete protein.
This is the case when grains, such as whole grain bread, are consumed with legumes, such as peanut butter, or when rice (a grain) is eaten with beans (a legume). The combination of foods provides complementary proteins, yielding complete protein. For example, beans supply plenty of the essential amino acids lysine and isoleucine, both lacking in grains.
You also get complete protein by taking dairy with legumes, grains, nuts, and seeds. Some plant proteins, such as soybeans or quinoa, are complete proteins.