Table 1.

Inclusion criteria and exclusion criteria.

Inclusion criteria	Exclusion criteria
1. All participants live permanently in Longyan City, South China (latitude and longitude 115 ° 50 ′ 56 "-117 °44 ′ 15" E, 24 ° 22 ′ 31 "-26 °2 ′ 35" N).	1. Patients without glucose homeostasis such as diabetic ketoacidosis and hypoglycemia.
2. Aged between 20 and 79 years.	2. Patients with hyperthyroidism, hypothyroidism, or parathyroid disease.
3. 25(OH) vitamin D < 30 ng/mL	3. Severe disability or mental illness and pregnant and lactating women.
4. All participants will sign a written informed consent form.	4. Alcoholic or drug abusers, a vegetarian who never eats fish or meat.
5. The diagnosis of type 2 diabetes mellitus will be confirmed according to the diagnostic criteria of the World Health Organization in 1999.	5. Chronic gastrointestinal diseases such as intestinal tuberculosis, ulcerative colitis, or irritable bowel syndrome.
6. The diagnosis of distal symmetric polyneuropathy will be diagnosed according to the Guidelines for the Prevention and Treatment of Type 2 Diabetes Mellitus in China (2020 Edition). Five neurological examinations will be performed, including ankle reflexes, pinprick pain, vibration, pressure, and temperature sensations. If a patient has one clinical symptom (numbness, pain, and paresthesia) and any of the five neurological examinations are abnormal, the patient will be diagnosed with DSPN. Additionally, patients will be diagnosed with DSPN if two of the five neurological examinations are abnormal, without any clinical symptoms, Figure 2 .	6. The participant has recently taken the following medications or supplements: immunosuppressants or glucocorticoids, such as prednisone at doses >10 mg/day; drugs such as phenytoin sodium, phenobarbital, and rifampicin that may affect the catabolism of vitamin D; vitamin D or active vitamin D, such as calcitriol; and drugs that directly affect nerves (such as chemotherapy drugs, vitamin B6 or Neurobion). 7. Patients with large areas of skin pigmentation or skin diseases. Those who engage in long-term outdoor activities or have tropical tourism plans in the next 3 months. Using sunscreen on the face and hands 8. Hyperphosphatemia and renal rickets.
	9. Blood calcium level >11 mg/dL (2.75 mmol/L), history of urinary tract stones, and sarcoidosis.
	10. Acute stress diseases, such as shortness of breath, fever and diarrhea, severe leukopenia, malignant tumor, liver dysfunction (bilirubin> 1.5 times the upper limit of normal value, alanine aminotransferase> 2 times the upper limit of normal value) or organic diseases, such as decreased renal function eGFR*<20 mL/min/1.73 m2.
	11. Neuropathy caused by other causes, such as cervical and lumbar lesions (nerve root compression, spinal stenosis, cervical and lumbar degeneration), cerebral infarction, Guillain–Barre syndrome, and serious arteriovenous vascular lesions (venous embolism, lymphangitis).
	12. Participants considered unsuitable for the study based on the investigator's judgment.

DSPN, Distal symmetric polyneuropathy; eGFR, estimated glomerular filtration rate.