BACKGROUND: Extrahepatic portal vein obstruction (EHPVO) in children is often associated with growth restriction, which improves after the restoration of portal venous flow with a meso-Rex bypass, but the physiologic mechanism is unknown. The purpose of this study was to investigate the mechanism of growth delay in children with EHPVO by detailing the metabolic and nutritional profile before and after meso-Rex bypass. METHODS: Twenty consecutive children with EHPVO were prospectively studied before and 1 year after meso-Rex bypass. Caloric balance was determined by investigating caloric intake via a calorie count, total energy expenditure via a doubly labeled water isotope assay and stool caloric loss by bomb calorimetry. Laboratory markers of nutrition and growth hormone resistance were tested. RESULTS: Fifteen of the 20 children underwent successful meso-Rex bypass at a median age of 4.3 years. Prealbumin level was abnormally low (14.6 +/- 3.0 mg/dL) at surgery but improved (17.0 +/- 4.3 mg/dL) 1 year later (P = 0.026). Mean insulin-like growth factor 1 (IGF-1) level at baseline was 1.57 standard deviations below normal. IGF-1 levels increased from 88.3 +/- 38.9 to 117.3 +/- 54.5 ng/mL in the year after surgery (P = 0.047). Caloric intake divided by basal metabolic rate (1.90 +/- 0.61), total energy expenditure (97.2 +/- 15.0% of expected), and stool caloric losses (3.7 +/- 1.8% of caloric intake) were all normal at baseline. CONCLUSIONS: Children with EHPVO suffer from malnutrition and growth hormone resistance, which may explain their well-established finding of growth restriction. Prealbumin and IGF-1 levels improve after a successful meso-Rex bypass.