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Proteins are nutrients that serve a wide range of functions and are required for the structure, functioning, and regulation of the body's tissues and organs. Proteins are composed of building blocks called amino acids. Amino acids linked together form protein molecules.
Lentils are made up of more than 25% protein, which makes them an excellent meat alternative.
Plant protein, on the other hand, generally lacks one or more essential amino acids making it an incomplete protein. For example, grains are usually deficient in the amino acid lysine, and legumes are low in methionine and tryptophan. This limitation can be overcome by consuming lentils with cereals like rice and/or wheat which, in turn, provide the required amino acids. Because grains and legumes are not deficient in the same amino acids, they can complement each other as part of a well balanced diet. When consumed in tandem in this way, they contain all nine essential amino acids at adequate levels and are, hence, referred to as complementary proteins. Mutual supplementation is another term used when combining two or more incomplete protein sources to make a complete protein.
This article further explains this subject: