LPL and hyperinsulinemic hypoglycemia, familial, 4: Additional studies of a larger cohort of people with MC4R deficiency are needed, using radioisotopes to quantify lipid species, precursors and metabolites, including measurements of flux and kinetics; direct measurement of lipoprotein lipase activity, which reflects the amount of lipoprotein lipase bound to endothelial sites (measured post-heparin administration), would also be useful.