The phenylalanine-free diet is needed for the treatment of phenylketonuria. Phenylketonuria is an inherited metabolic condition in which there is a deficiency of the enzyme phenylalanine hydroxylase. Lack of this enzyme means the body cannot metabolise the essential amino acid phenylalanine, which then builds up in the blood and causes mental retardation and other abnormalities. Retardation can be prevented if phenylketonuria is diagnosed in the first three weeks of infancy and dietary treatment started straightaway. There is a universal screening test in the UK (the Guthrie test). Heel-prick blood samples are taken from all babies between 6-14 days old and analysed at a regional screening centre. For infants, a phenylalanine-free formula is needed, either as a supplement before breast feeds or following a formula feed. The diet must continue during weaning and childhood, with a low protein diet. Foods such as meat, fish, eggs, milk, cheese, nuts and pulses are excluded as they contain high levels of phenylalanine. Vegetables and fruit are allowed in measured amounts only! Special low protein bread, pasta, biscuits and flour are used to supplement the diet and ensure adequate calorie intake. Whether the diet can be stopped at the end of adolescence is debatable. If stopped, it should be re-started at conception and maintained during pregnancy, as high levels of phenylalanine in the mother can affect the fetus.