Protein intake and physical activity are associated with body composition in individuals with phenylalanine hydroxylase deficiency

Abstract Objective Determine whether body composition as it relates to dietary protein in patients with phenylalanine hydroxylase (PAH) deficiency is associated with genotype, dietary factors, and lifestyle choices. Methods We examined associations between protein intake (intact and medical foods: MF) and body composition in PAH-deficient patients along with, physical activity, and genotype. Protein intakes (total, intact, and MF) were analysed from three-day food records with Nutrition Data System for Research (NDSR) in 59 children and 27 adults (N = 86, median age = 16.0 years). The severity of PAH deficiency was classified using the genotype assigned value method (AV sum). Physical activity was assessed using a study-developed question (light vs. intense activity). Body composition was measured by DXA, including android:gynoid ratio (A:G), fat-free mass index (FFMI), fat mass index (FMI), and FMI:FFMI ratio. Results High intact protein intake was associated with high FFMI (r s  = 0.75, p = 0.008) and low FMI:FFMI (r s  = − 0.59, p = 0.04) in adults. Only in children, MF protein (r s  = 0.38, p = 0.04) was directly proportional to FFMI. Median intact protein intakes of adults (25.1 vs. 9.9 g/d, p  Conclusions Although fat-free mass in adults was positively associated with intact protein intake, it should be consumed as prescribed per individual tolerance to maintain plasma Phe concentrations within treatment range. In children, total protein maximized with MF should be encouraged to promote lean mass. Nutrition counselling could be complemented with physical activity recommendations for optimal clinical outcomes.