C peptide is a biomarker for beta cell function. In other words, it's a measure of beta cell health and insulin production, and is critical to developing new disease modifying therapies for type 1 Glutathione metabolism in type 2 diabetes and its relationship with microvascular complications and glycemia PLOS One Improved glycemic control either alone, or combined with antioxidant supplementation, fails to restore blood glutathione or markers of oxidative stress in adolescents with poorly controlled type 1 diabetes ScienceDirect Glutathione & Diabetes: How the Body's Master Antioxidant