Introduction Diabetic peripheral neuropathy (DPN) is among the most common chronic complications of diabetes mellitus and represents a major cause of morbidity, impaired quality of life, sleep disturbance, chronic pain, and functional disability.(Kurz et al., 2026) The pathogenesis of DPN is multifactorial and involves oxidative stress, chronic inflammation, endothelial dysfunction, mitochondrial injury, and microvascular ischemia, leading to progressive nerve damage.(Yang et al., 2025) Despite the availability of symptomatic therapies such as gabapentin, pregabalin, and duloxetine, many patients continue to experience persistent neuropathic symptoms, poor sleep quality, and impaired daily functioning. Existing pharmacological therapies are often associated with incomplete response, adverse effects, and high economic burden.(Kaye et al., 2025) Recently, vitamin D has attracted attention beyond its classical role in calcium and bone metabolism. Experimental and clinical evidence suggest that vitamin D possesses neuroprotective, anti-inflammatory, immunomodulatory, and antioxidative properties. Vitamin D receptors are widely distributed in neuronal tissues, Schwann cells, and immune cells, suggesting a potential role in nerve repair and modulation of neuropathic pain pathways.(Liu et al., 2025) Several observational studies have demonstrated a high prevalence of vitamin D deficiency among patients with diabetic neuropathy, and low serum vitamin D levels have been associated with increased neuropathic pain severity and poorer quality of life. However, available interventional studies remain limited and have produced inconsistent findings regarding the therapeutic benefit of vitamin D supplementation in DPN.(Ou et al., 2021) Therefore, this study aims to evaluate whether vitamin D supplementation can improve neuropathic pain, sleep quality, and health-related quality of life among patients with diabetic peripheral neuropathy.

Inclusion criteria

Participants must fulfill all the following:

1. Age between 18 and 75 years 2. Diagnosed with type 2 diabetes mellitus according to ADA criteria 3. Clinically diagnosed diabetic peripheral neuropathy 4. Neuropathic symptoms for at least 3 months 5. Vitamin D insufficiency or deficiency:

• Serum 25(OH)D < 30 ng/mL 6. Stable antidiabetic treatment for at least 3 months Exclusion criteria

  1. Type 1 diabetes mellitus
  2. Current vitamin D supplementation within the previous 3 months
  3. Severe renal impairment (eGFR < 30 mL/min/1.73m²)
  4. Severe hepatic impairment
  5. Hypercalcemia
  6. Malabsorption syndromes
  7. Pregnancy or lactation
  8. Peripheral neuropathy due to causes other than diabetes:

• Vitamin B12 deficiency

• Hypothyroidism

• Alcoholic neuropathy

• Chemotherapy-induced neuropathy 9. Active malignancy 10. Severe psychiatric illness