What is the purpose of adding cysteine to amino acids in TPN?

The inclusion of cysteine in total parenteral nutrition (TPN) primarily serves to enhance the solubility of calcium and phosphorus. When calcium and phosphorus are present together in a solution, they can form insoluble precipitates, which complicates the delivery of these essential minerals. Cysteine acts as a sulfhydryl-containing amino acid that helps to stabilize the solution by forming complexes with calcium and phosphorus, thereby preventing precipitation and improving the solubility of these nutrients. This is particularly important in TPN formulations, as maintaining the stability and bioavailability of nutrients is crucial for the patient's health, especially in those who are unable to take nutrition orally.

While cysteine is indeed an amino acid and may contribute marginally to protein synthesis, that is not its primary role in the context of TPN. Additionally, increasing energy through cysteine would not be relevant since amino acids are not directly associated with generating energy in the form of calories in TPN. Enhancing taste is also not a benefit of cysteine in TPN, as the primary goal of TPN is to provide nutrition without concern for taste, which is only relevant in oral feeding.