Body composition in young adults with inborn errors of protein metabolism--a pilot study

The natural history of inborn errors of protein metabolism and the long-term effects of prescribed semisynthetic therapeutic diets are largely unknown. We assessed body composition, measuring body-fat mass and distribution, fat-free mass, total body protein, total body potassium, bone density and skeletal muscle mass, in young adults (age > 18 years; 6 female, 5 male) with inborn errors of protein metabolism maintained on long-term low-protein diets, compared with controls. Female patients were significantly shorter (159.4 cm vs 169.2 cm, p = 0.013) and had higher BMI (25.3 vs 22.0 kg/m2, p < 0.05), abdominal to gluteal circumference ratio (0.84 vs 0.73, p = 0.011), percentage body fat (42.3% vs 29.5%, p < 0.005) and ratio of central to peripheral body fat (1.15 vs 0.86, p < 0.05) than controls. Male patients had lower height-adjusted total body bone mineral content (0.9 vs 1.02 g/m2, p < 0.04) and skeletal muscle mass (31.1 vs 36.3 kg, p < 0.04) than controls. Compared with controls, patients'nitrogen index was significantly lower (0.91 vs 1.03, p < 0.01), consistent with lower total body protein. Potassium index was significantly higher (121.2% vs 110.4%, p < 0.03), consistent with higher body cell mass, or intracellular water. Documentation of body composition in larger patient series is important to elucidate whether these results reflect increased risks (hence opportunities for prevention) of bone disease, metabolic syndrome and cardiovascular disease in this population.