To evaluate whether the observed postnatal growth retardation in symptomatic individuals was due to the protein restricted diet or the caloric intake as part of the conservative management, we studied 46 severity-adjusted UCD individuals (mOTC-D: n = 9, 19.6%; fOTC-D: n = 11, 23.9%; ASS1-D: n = 11, 23.9%; ASL-D: n = 15, 32.6%) with sufficient information on biochemical and therapy-related longitudinal data, comprising a mean individual observation period of 3.13 years (min: 1.01 years; max: 9.94 years) within the preschool age. This evidence concerns the gene ASS1 and urea cycle disorder.