There is a lack of evidence on the impact on body composition of high protein intake and types of protein substitutes in PKU patients—particularly in adolescents, who are more inclined to dietary transgressions. In this observational, cross-sectional study, PKU patients were observed during prepubertal age (p) or after the pubertal spurt (P), assessing body composition and bone quality and correlating these parameters with dietary compliance and types of protein substitutes. Anthropometric and dietary data were evaluated together with bioelectrical impedance analysis (BIA), quantitative ultrasound (QUS) and branched-chain amino acids (BCAAs). A total of 36 patients (16 males, 17 prepubertal and 19 post-pubertal; mean ± SD age 11.4 ± 3.9 years) were included. A higher BMI was observed in adolescents (p-value: 0.018). The BIA revealed a significant increase in total body water (TBW) and muscle mass (MM) in P subjects either compliant (p-value: 0.001) or non-compliant with the diet (p-value: 0.001). MM content correlated with increased Phe intake (r = 0.63; p < 0.001). In the subgroup of five patients taking L-AAs and glycomacropeptides (GMPs), BCAA values tended to be lower than those taking only L-AA mixtures, with a significant trend for valine. Maintenance of body composition parameters within the normal range—for both fat and muscle mass—and levels of BCAAs can be helpful in reducing the risk of becoming overweight in adulthood. Further studies are needed to confirm these findings.
Keywords: PKU; adolescence; bioelectrical impedance analysis; branched-chain amino acids; muscle mass; quantitative ultrasound.