The low fasting serum C‐peptide level was associated with reduced sural nerve action potential amplitudes: A cross‐sectional study of Japanese type 2 diabetes

AIMS/INTRODUCTION: The association between the fasting serum C-peptide levels and diabetic polyneuropathy (DPN) in patients with type 2 diabetes remains controversial. Furthermore, the association with the electrophysiological parameters of nerve conduction studies (NCS) has not been elucidated. Therefore, we investigated whether the fasting serum C-peptide levels are associated with nerve conduction velocity (CV) and action potential amplitude (Amp) in patients with type 2 diabetes. MATERIALS AND METHODS: This retrospective cross-sectional study enrolled outpatients with type 2 diabetes who underwent the DPNCheck® at the Higashikurume Ekimae Clinic in Tokyo, Japan, between March 1, 2016, and June 30, 2025. Participants were categorized into tertiles according to their fasting serum C-peptide levels. Abnormal values of CV and Amp were defined as CV <46.5 m/s and Amp <10.5 μV. A logistic regression analysis was used to evaluate whether the fasting serum C-peptide levels were associated with CV and Amp abnormalities. RESULTS: One-hundred thirty-one participants were included and stratified into tertiles based on fasting serum C-peptide levels: Tertile 1, ≤1.9 ng/mL; Tertile 2, 2.0-2.8 ng/mL; and Tertile 3, ≥2.9 ng/mL. Tertile 1 was independently associated with an abnormal Amp (odds ratio: 5.08, 95% confidence interval: 1.41-18.4), whereas no independent association was observed with an abnormal CV. Tertile 2 was not associated with an abnormal CV or Amp. Additionally, a nonlinear relationship was observed between the fasting serum C-peptide levels and an abnormal Amp. CONCLUSIONS: Low fasting serum C-peptide levels were independently associated with abnormal Amp in patients with type 2 diabetes.