In the era of Phenylketonuria (PKU) newborn screening, early diagnosis in the neonatal period and prompt treatment institution has protected patients from developing severe and irreversible mental retardation. The main objective of the treatment is to prevent a chronic elevation of blood Phe concentrations, which together with reduced tyrosine concentrations may increase the risk of neurologic damage. In order to achieve this purpose, the mainstay of treatment is a special diet characterized by a natural protein restriction, supplemented with protein substitutes and special low protein foods.

The requirement to optimize growth and body composition, usually result in dietary prescriptions that are high in carbohydrate (>60% of energy intake), to promote anabolism, considering the synthetic properties of this special diet. Some studies have described a high risk of developing overweight and obesity. Although there is a tendency for a higher incidence in females, it seems that the prevalence in PKU patients follows the same trend as the general population. However, there are limited studies published so far and no longitudinal studies are available describing current practice and its impact on the prevalence of overweight and obesity; neither its consequences in terms of metabolic syndrome or cardiometabolic markers.

Recently, sapropterin dihydrochloride, which is the synthetic form of Phenylalanine Hydroxylase cofactor, is available in Portugal for patients with PKU. In practice, the sapropterin treated patients increase their natural protein intake, minimizing the synthetic characteristics of the diet. While there is a need for patient re-education about the practicalities of meeting their nutritional needs, scientific evidence about the nutritional status impact of diet liberalization is inadequate.

This study aims to test the following hypothesis:

1. Global nutritional status is not significantly affected in patients with PKU under exclusive dietary treatment.
2. There is a trend for increased rates of overweight and obesity in patients with PKU from 2009 and we consider this will continue to increase.
3. The start of sapropterin treatment allows a higher natural protein intake in patients with PKU that significantly targets nutritional status in at least one of its components (anthropometry, body composition or biochemistry).