If a faulty metabolic gene is present in both parents there is a one in four chance their child will be born with a genetic metabolic disorder.

Genetic metabolic defects result in missing or improperly constructed enzymes being present in the body. This means a step in the metabolic process, in which food is turned into energy, doesn't perform as it should.

Although metabolic conditions can be serious - resulting in slow physical development and possible neurological damage - and in some cases life threatening, they can be successfully managed with early diagnosis and treatment.

Phenylketonuria (PKU) is one metabolic disease where early treatment has been a success.

In the 50s, PKU wasn't even recognised by health professionals. As a result there were high numbers of children developing neurological problems and not growing properly without explanation. So researchers started looking into why this could be.

Anthea Hope, a nutritionist at Locksbottom Clinic, Crofton Road said: "When there is too much pheylalanine - a component of protein - it is like a damn building up and excess pheylalanine can harm brain tissue."

The Guthrie, or heel prick, test is carried out at birth to find levels of pheylalanine in the blood. If a baby is diagnosed with PKU they are put straight onto a low-protein diet for life.

Sally Brothers, head of nutrition and dietetics at University Hospital Lewisham says the low protein diet has been affective in lowering pheylalanine levels.

She said: "It is important for patients with PKU to avoid all high protein sources such as meat, cheese, nuts, egg and fish. The only treatment available is the diet.

"Families dealing with PKU have a close relationship with their dietician. Young children have their blood tested weekly and diets changed accordingly."

"Patients need a range of foods in their diet as they are so limited. Low-protein bread, biscuits and pasta are good for a balanced diet and are available on prescription as they are expensive."

In adulthood the diet can be relaxed but it is important not to abandon it.

Sally said: "When patients start school they sometimes find it hard. Although their brain development is complete, quitting the diet can lead to twitching or a lowering of IQ. Pregnant women with PKU must especially stick to their diet."