Humans require eight essential amino acids in their diet. These are methionine, tryptophan, phenylalanine, valine, isoleucine, leucine, lysine and threonine. These essential amino acids are required in the diet because they cannot be synthesised and are needed to build proteins. Animal protein is considered complete in that it supplies all the essential amino acids in the right ratios and amounts for human nutrition. However, vegetable proteins generally are low in particular amino acids and this means they are incomplete proteins. Grains for example are low in lysine, while legumes are low in methionine and tryptophan. However, by combining grains and legumes in the diet, a complete protein is made and this can supply all the amino acids required. Beans (legumes) on toast (grain) therefore supplies complete protein. Soy is an exception, in that although is is a little low in methionine and tryptophan, is it almost as complete as animal protein.
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