The plant enzyme phenylalanine ammonia lyase (PAL) will survive in the gut for long enough to deplete the phenylalanine derived from food protein and so reduce the rise in blood phenylalanine that otherwise occurs after a protein meal. This effect has been demonstrated in healthy adults and phenylketonuric (PKU) patients. When the enzyme was given to an untreated PKU patient for 12 consecutive days (three doses a day after food) his blood phenylalanine levels were reduced on average by a quarter. This enzyme preparation may have a place in the treatment of PKU.