Regardless of the degree of stress or neonatal weight, critically ill neonates have activity levels of these glutathione synthetic enzymes comparable to those of adults. Furthermore, these neonates demonstrate unimpeded ability to transport amino acids, including cysteine derivatives, across cellular membranes. Hence, neonates demonstrate the capacity to synthesize glutathione given the appropriate substrates are available. Despite the potential ability to synthesize glutathione, low glutathione concentrations and high ratios of GSSG:GSH have been demonstrated from the blood, lung, and intestine of critically ill neonates in multiple studies. Consequently, a likely limiting factor in the production of glutathione for critically ill neonates is the availability of cysteine in their diet.