Table 1 PICO for study inclusion

 Participants with MetS diagnosed by NCEP-ATP III (1), or IDF (2) criteria, aged ≥ 18 years, and with inadequate vitamin D status

Vitamin D supplementation in the form of vitamin D3 (cholecalciferol) or vitamin D2 (ergocalciferol) at any dose, given orally, daily, weekly, or monthly

RCTs with intervention compared to placebo

Fasting glucose levels (mg/dL), triglyceride levels (mg/dL), waist circumference (cm), mean blood pressure (mmHg), and HDL- C (mg/dL) (two or more outcomes will be considered in subgroup analyses)

 Participants with MetS aged under 18 years

Vitamin D supplements with other vitamin and chemical element supplements; vitamin D supplementation in fortified foods as the amount of vitamin cannot be defined accurately; studies that used active vitamin D supplementation (1.25-dihydroxy vitamin D)