In the next section, a short overview of the relevant stable isotope technology will be given followed by a discussion of the experimental studies over the last decades of a selected group of IEM; glycogen storage disease type I (GSD I), medium-chain acyl-CoA dehydrogenase (MCAD) deficiency, propionic aciduria (PA) and methylmalonic aciduria (MMA), urea cycle defects (UCD), phenylketonuria (PKU) and combined D,L-2-hydroxyglutaric aciduria. Here, ACADM is linked to phenylketonuria.