To examine relationships between dietary intake, growth and body composition patterns in patients with inborn errors of intermediary protein metabolism and to determine a safe protein:energy ratio (P:E ratio) associated with optimal growth outcomes.

Retrospective longitudinal data of growth and dietary intake in patients (n = 75) with isovaleric acidemia (IVA; n = 7), methylmalonic acidemia/propionic acidemia (MMA/PA; n = 14), urea cycle defects (UCD; n = 44), classical maple syrup urine disease (MSUD; n = 10) were collected. Prospective longitudinal data of growth, dietary intake, and body composition from 21 patients: IVA (n = 5), MMA/PA (n = 6), UCD (n = 7), and MSUD (n = 3) were collected at clinic visits.

Fifty-two of 75 (66%), 49 of 74 (68%), and 44 of 65 (68%) patients had a z-score of 0 (±1) for lifetime weight, height, and body mass index, respectively. Patients with MMA/PA had the lowest median height and weight z-scores, and MSUD patients had highest median body mass index z-score at all ages. In IVA, MMA/PA, and UCD, total natural protein intake met or exceeded the Food and Agriculture Organization of the United Nations (FAO)/World Health Organization (WHO)/United Nations University (UNU) recommended safe levels. Median percentage fat mass was 17.6% in IVA, 20.7% in MMA/PA, 19.4% in UCD, and 17.8% in MSUD. There was a significant negative correlation between percentage fat mass and total protein intake in IVA, MMA/PA, and UCD (r = −0.737; P = .010). The correlation between the P:E ratio and growth variables in IVA, MMA/PA, and UCD suggest a safe P:E ratio (>1.5 to < 2.9) g protein:100 kcal/day.

Growth outcomes in inborn errors of intermediary protein metabolism are not always ideal. Most patients with IVA, MMA/PA, and UCD consume sufficient natural protein to meet FAO/WHO/UNU recommendations. A P:E ratio range of (>1.5 to < 2.9)g protein/100 kcal/day correlates with optimal growth outcomes.