The goal of this randomized, double-blind, placebo-controlled, 6-month supplementation study in individuals with type 2 diabetes is to investigate whether taking daily dietary supplements of vitamin D3 (1600 IU) or prebiotics (inulin, 10 gram) has beneficial effects on cardiovascular disease risk factors.

The goal of this randomized, double-blind, placebo-controlled trial, with a two-by-two factorial design in individuals with type 2 diabetes (T2D) is to investigate whether taking daily dietary supplements of vitamin D3 (1600 IU) or prebiotics (inulin, 10 gram) has beneficial effects on cardiovascular disease (CVD) risk factors.

About 200 subjects with T2D will be included in this study. All participants will provide written informed consent before randomization. Eligible participants must have no history of cancer, macrovascular or microvascular complications at trial entry, and they are required to agree to limited use of vitamin D, prebiotics, probiotics, or synbiotic supplementations, as well as to convenient access to study centers. Safety exclusions include renal failure or dialysis, cirrhosis, history of hypercalcemia, or other conditions that would preclude participation.

Eligible participants will be assigned by chance to one of four groups: (1) daily vitamin D3 (1600 IU) and prebiotics (10 grams); (2) daily vitamin D3 (1600 IU) and prebiotics placebo (10 grams); (3) daily vitamin D3 placebo and prebiotics (10 grams); or (4) daily vitamin D3 placebo (1600 IU) and prebiotics placebo (10 grams). Randomization will be computer-generated within sex and age groups.

During the run-in period, baseline questionnaires will be designed to collect data on sociodemographic factors, lifestyle habits, health status, and medical conditions, and participants will undergo physical measurements at baseline. Blood, urine, and faeces samples will also be obtained at study centers.

Participants in all groups will be required to take two capsules each day, containing either vitamin D3 or vitamin D3 placebo, as well as a strip of prebiotics powder (10 g per strip, brew with warm water) or prebiotics placebo. Participants will receive their capsules and powder at study centers each month.

Participants will complete follow-up questionnaires, including information on adherence to trial regimens, outside use of supplementations, development of major illnesses, and potential side effects of the trial agents, at 3 months and 6 months after randomization. Physical measurements and collection of biological samples will be conducted at study centers 6 months after randomization.

The primary outcomes, including glycated hemoglobin (HbA1c) and blood lipids (such as total cholesterol, triglycerides, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol), will be measured using blood samples. The data will be collected and analyzed.

Inclusion Criteria:

1. Type 2 diabetes (meeting any of the following criteria):

   Physician-diagnosed type 2 diabetes, use of oral hypoglycemic medication or insulin, fasting blood glucose ≥ 7.0 mmol/L, HbA1c ≥ 6.5% (48 nmol/L), 2-hour plasma glucose (2hPG) ≥ 11.1 mmol/L.

2. Men aged 35-70 years or women aged 35-70 years.

3. Convenient access to study centers.

4. Written informed consent obtained before any trial-related activities.

Exclusion Criteria:

1. History of macrovascular diseases (including Coronary heart disease, myocardial infarction, heart failure, stroke, transient ischemic attack, peripheral vascular disease, post coronary artery bypass surgery).
2. History of diabetic microvascular complications (including diabetic nephropathy, diabetic retinopathy, or diabetic neuropathy).
3. History of other cardiovascular diseases (including aortic stenosis, transient ischemic attack, angina pectoris, cardiac arrest, complex ventricular arrhythmias, or surgery of cardiovascular diseases).
4. History of cancer, excluding non-melanoma skin cancer or cancers with a good prognosis (such as stage 1 cervical cancer).
5. Severe liver disease (such as cirrhosis), severe kidney disease (such as renal failure or requiring dialysis), or severe gastrointestinal disease (such as a history of bowel obstruction).
6. Kidney stones, hypercalcemia, parathyroid hormone abnormality.
7. Sarcoidosis or other granulomatous diseases, such as active chronic pulmonary tuberculosis or Wegener's granulomatosis.
8. Any mental disorders, such as schizophrenia, other mental disorders, or bipolar disorder.
9. Individuals with infectious diseases such as HIV/AIDS, viral hepatitis.
10. Individuals requiring long-term hospitalization or those with other serious illnesses requiring immediate hospital treatment.
11. In the past 3 months, those who have taken any vitamin D supplements exceeding 400 IU/day, calcium supplements exceeding 600 mg/day, prebiotics exceeding 2 g/day, or probiotics exceeding 10^8 colony forming units/day.
12. History of allergy or intolerance to vitamin D, prebiotics, or starch.
13. Individuals with unhealthy habits: heavy alcohol consumption (>80 grams/day for males, >40 grams/day for females) or smoking (>40 cigarettes/day).
14. Individuals with cognitive impairment, inability to communicate properly, inability to take care of themselves, or those with mobility issues.
15. Those having participated in other clinical trials or studies in the past 3 months.
16. Those who are intent to become pregnant within the next two years, are currently pregnant, or breastfeeding.
17. Individuals with abnormal laboratory tests and clinical manifestations who are judged by the researcher to be unsuitable for participation.