For example, through a combination of steady state measurements and 13C615N4-arginine tracing experiments, we report an increase in arginine catabolism into ornithine in humans, suggestive of species-specific arginase 1 activity and nitric oxide synthesis—an observation that may impact the translatability of cardiovascular disease studies carried out in non-human primates (NHPs). Here, ARG1 is linked to cardiovascular disorder.