People with an inherited disease called phenylketonutria must control their intake of phenylalanine from all sources including from aspartame. Although aspartame only contains a small amount of phenylalanine, labels on food containing this sweetener have to carry a warning that they contain phenylalanine.

Phenylketonuria (PKU) is a rare inherited disease that prevents the essential amino acid, phenylalanine (one of the components of aspartame) from being properly handled by the body. An essential amino acid is required for normal growth, development, and functioning of the body and must be obtained from the diet, as the body cannot make it. People with phenylketonuria (a phenylalanine metabolism disorder) have to control their intake of this amino acid.

PKU is one of the few genetic diseases that can be controlled by diet. A diet low in phenylalanine and high in tyrosine can bring about a nearly total cure.

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