A new approach to the treatment of phenylketonuria.

Recent work has shown that in phenylketonuria, PKU, in which phenylalanine accumulates in the blood, the damage to the brain, which so often leads to mental retardation, is not solely due to the large quantities of phenylalanine that enter the brain. The raised levels of phenylalanine in the blood lead to a partial exclusion of various other amino acids from the brain and this exclusion in itself damages the brain. Based on this evidence, that in PKU some amino acids are partially excluded from entering the brain, proposals are made for a modified dietary treatment of this disease. In this diet the phenylalainine is not so greatly reduced as in the standard diet for PKU, whilst supplements of other amino acids are added. The rationale for this new diet is that the partial exclusion from the brain of various amino acids (methionine, tryptophan, histidine, tryosine, isoleucine, leucine and valine) by the raised level of phenylalanine in the blood, acting as a competitive inhibitor, can be largely prevented by increasing the blood levels of these excluded amino acids. Raising slightly their blood levels overcomes the excluding effect of moderately raised levels of phenylalanine in the blood. The advantages of the new diet are that not only is it more palatable than a diet very low in phenylalanine, so that it is more likely to continue to prove acceptable to older children and adolescents, as well as to PKU women who expect to become pregnant, but also that its margin of safety is greater if the patient does take unsuitable food.