Sarcopenia in Pediatric Intoxication Type Inborn Errors of Metabolism: A Frequent and Underrecognized Condition.

Sarcopenia is increasingly recognized in pediatric chronic diseases, yet its prevalence and determinants in children with intoxication-type inborn errors of metabolism (IEM) have never been investigated. This study aimed to evaluate sarcopenia in this population and to investigate associated metabolic alterations. We conducted a retrospective single-center study including 45 pediatric patients (0-18 years) with maple syrup urine disease (MSUD, 11 pts.), organic acidurias (OA, 22 pts.), or urea cycle defects (UCD, 12 pts.) considered for liver transplantation. Sarcopenia was defined as total psoas muscle area z-score ≤ -2 on CT scan. Anthropometric, dietary, and laboratory parameters were analyzed. Forty percent of patients exhibited sarcopenia, most frequently among OA (54.5%) and UCD (33.3%), and only occasionally in MSUD (18.2%). Sarcopenic children showed lower weight, height, and were more likely to require enteral nutritional support. Plasma levels of essential amino acids, particularly branched-chain amino acids (leucine, isoleucine, valine), histidine, and glutamine, were significantly reduced in sarcopenic patients. Leucine emerged as an independent predictor of sarcopenia (p = 0.016). FGF21 levels were elevated in sarcopenic OA and UCD patients, whereas MSUD patients with higher branched-chain amino acids levels showed lower FGF21, suggesting a role beyond mitochondrial stress signaling. Sarcopenia is common in pediatric patients with severe intoxication-type IEM and is closely linked to essential amino acid deficiencies and altered FGF21 signaling. These monogenic diseases provide unique pathophysiological models for better understanding of sarcopenia. Our findings highlight the need for targeted nutritional and metabolic strategies to preserve muscle mass in these vulnerable patients.