Inborn Errors of Metabolism

Until now, most studies on the pathophysiological mechanisms of inborn errors of metabolism have almost exclusively focused on the measurement of concentrations of different metabolites in plasma, urine, or cerebrospinal fluid. These static measurements cannot discriminate between increased or diminished metabolite production and utilization, which limits the interpretation of these studies with respect to the functional consequences of an enzymatic defect. Understanding the changes in dynamics of affected metabolic pathways is a prerequisite for understanding the pathophysiology of a metabolic disorder. Stable isotope infusion techniques have made it possible to study metabolic fluxes in humans in vivo. In this chapter, stable isotope techniques are described to study inborn errors of intermediary glucose, fat, and protein metabolism in vivo.