Nutritional Status and Physical Activity Levels in Adult Patients with Phenylketonuria

Background/Objectives: Phenylketonuria (PKU) is a rare autosomal recessive disorder caused by phenylalanine hydroxylase (PAH) deficiency, impairing the conversion of phenylalanine (Phe) to tyrosine. Although early diagnosis and intervention yield excellent outcomes, dietary adherence often declines in adulthood, potentially leading to poor metabolic control and adverse nutritional consequences. This study aimed to evaluate physical activity levels, nutritional status, metabolic control, and anthropometric outcomes in adults with classic PKU, which have not been sufficiently researched in the current literature. Methods: This cross-sectional study included 100 adults with classical PKU (cPKU; baseline phenylalanine levels ≥ 1200 µmol/L) under regular follow-up at the Division of Metabolism, Hacettepe İhsan Doğramacı Childrens' Hospital. Sociodemographic traits and dietary behaviors were evaluated through structured interviews carried out by a dietitian. Dietary intake was assessed by using a 24 h dietary recall method, and nutrient analyses were performed with the Bebis 7.2 software program. Using the short version of the International Physical Activity Questionnaire (IPAQ), physical activity levels were specified, and participants were categorized according to established scoring criteria. Results: A hundred adults with classical PKU took part in the study, including 47 males and 53 females, with a mean age of 23.84 ± 5.41 years; 5% of participants were underweight, 40% had normal weight, 39% were overweight, and 16% were listed as obese. The intake of mean daily energy is 2443.8 ± 384.6 kcal for men and 1822.5 ± 312.7 kcal for women. Carbohydrates contributed approximately 61% of total daily energy intake in both genders, whereas protein accounted for 12-13% and fat for approximately 26-27% of total energy intake; 17% of participants were physically inactive, 40% were minimally active, and 43% met criteria for sufficient physical activity according to IPAQ-based classification. Energy intake, the use of Phe-free protein substitutes, and BMI were significantly higher in the sufficiently active group compared to the low-active group in men, while no significant differences were observed between physical activity groups among women. Conclusions: Adults with classical PKU showed a high prevalence of overweight and obesity, together with differences in dietary intake and physical activity patterns. Physical activity levels were associated with several nutritional and metabolic characteristics; however, further long-term research is required to fully understand these connections.