Hyperphenylalaninemia is the most common congenital metabolic disease affecting protein metabolism. 

In 98% of cases, it is caused by the deficiency of the enzyme phenylalanine hydroxylase. The term phenylketonuria (PKU) refers to the forms of hyperphenylalaninemia that require diet therapy. 

It is an autosomal recessive disease that affects the hepatic enzyme phenylalanine hydroxylase, which allows phenylalanine to be converted into tyrosine, a precursor for the synthesis of thyroid hormones, melanin and catecholamines. On the other hand, when phenylalanine hydroxylase enzyme is deficient, phenylalanine accumulates and there are risks of serious health consequences such as maturation deficits, myelination deficiency and direct toxic action in the central nervous system. 

The patient is clinically normal at birth. The beginning of autonomous life and the intake of phenylalanine through the diet highlights the enzymatic defect producing the accumulation of the amino acid. Hyperphenylalaninemia is a serious condition. If not treated early, in its most severe forms, it can lead to irreversible psychomotor and mental retardation. If diagnosed at birth, unfavorable clinical evolution can be prevented with a reduced and controlled diet of phenylalanine. 


There have been recent advances in drug therapy, but dietary treatment remains essential for some patients, with the aim of keeping plasma phenylalanine levels within safe margins. 

The diet for these patients must be carefully calibrated to ingest a low amount of phenylalanine, preferring foods with plant-based proteins that contain a reduced amount of the amino acid. The intake of carbohydrates may be higher than the recommendations (LARN), while the intake of saturated fatty acids, cholesterol, polyunsaturated fatty acids tends to be lower and as a result, these patients have low levels of omega 3 fatty acids, such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). 

Children with phenylketonuria in diet therapy are at risk of developing deficiencies in those nutrients of animal products, such as essential amino acids and some micronutrients such as vitamin A, C, E, selenium, coenzyme Q10, vitamin B2, B6 and B12, folate , iron, zinc, polyunsaturated fatty acids, carnitine and taurine.