Phenylketonuria, or PKU, is another well-known example of how our genes and diet interact. People with this genetic disorder do not produce the necessary enzyme for breaking down phenylalanine – an essential amino acid present in dairy products, meat, fish, nuts and pulses. Phenylalanine is usually converted into another amino acid, called tyrosine. However, in individuals who lack the enzyme, it is instead metabolised into phenylpyruvic acid. If too much of this acid builds up in the body it can lead to disordered brain development and, in severe cases, mental retardation and seizures. This means that people with PKU need to stick to a diet which avoids phenylalanine-rich foods.