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 Oxidative Stress, Glutathione Insufficiency, and Inflammatory Pathways in Type 2 Diabetes Mellitus: Implications for Therapeutic Interventions DIABETES (wheel) SpectraCell Laboratories Supplementation with l glutathione improves oxidative status and reduces protein nitration in myenteric neurons in the jejunum in diabetic Rattus norvegicus ScienceDirect