Conclusions: Current analysis verified bone density reduction in HPA patients on diet. Restricted diet seemed to be associated to worse BMD. Treating with both calcium integration and physical activity we could optimize bone formation in order to prevent osteopenia and/or osteoporosis risk. In a non-intervention dietetic study of 20 patients (1-15 years, 12 f., 8 m.) with clinically classic phenylketonuria (PKU), food intake and phenylalanine (phe) blood levels were documented daily during 2 periods of 1 week each in 15 patients, and during 1 week only in 5 patients. The study periods were selected according to the current phe blood levels, with phe being within target range during one period, and above-target during the other one. By using a kinetic single-compartment model, the zero-order net protein synthesis was found diminished at elevated phe levels in 12 of the 15 patients, indicating sub-clinical catabolic states as a frequent cause of 'diet failure'. In contrast, the first-order constant of metabolic (plus renal) phe disposal was found almost identical during both periods (R=0.9782).