Table 1. Summary of the characteristics of included studies.

 T1D = type 1 diabetes, T2D = type 2 diabetes

Author	Type of Study	Participants	Findings
Lips et al. [14]	Review		Low vitamin D is associated with insulin resistance and type 2 diabetes.
Marino and Misra [15]	Review		Low vitamin D has a role in the development of Type1 DM, suggested by the presence of vitamin D response element in the human insulin gene promoter. In addition, vitamin D regulates T-cell responses and may protect beta cells from immune attacks.
Infante et al. [16]	Review		Hypovitaminosis D is an important environmental factor in the development of T1D and may have a role in the pathogenesis and determining the risk of development in the first years of life. However, evidence on vitamin D supplementation and beta-cell function preservation in T1D is still lacking.
Wang et al. [18]	case-control study	2659 Chinese participants	They found that low serum levels of 25(OH)D3,25(OH)D2 were associated with impaired fasting glucose in patients with T2DM; they explained this negative relationship to be due to altering lipid metabolism and altering the concentration of total cholesterol, low-density lipoprotein cholesterol high-density lipoprotein which leads to impaired fasting glucose and type 2 DM.
Aljabri and Bokhari [19]	clinical trial	Eighty participants with T1D whose vitamin D levels were below 50 nmol/L	Low vitamin D is associated with insulin resistance and beta cell death, contributing to the development of T1D. Additionally, Vitamin D supplementation improved glycemic control in T1D patients.
Najjar et al. [20]	systematic review and meta-analysis, including10 studies		No major effect of genetically determined reduction of 25(OH)D concentrations by selected polymorphisms on T1D risk.
Zheng et al. [21]	meta-analysis	120618 participants of European descent	MR analysis revealed a lack of evidence for a causal link between 25(OH)D and T2D despite a high inverse observational association between the two diseases. The result was against the use of Vit D supplements to prevent T2D.
Li et al. [22]	systematic review and meta-analysis, including 20 RCTs	2703 participants.	Vitamin D supplementation successfully enhanced serum 25(OH)D and improved insulin resistance.
Jennifer et al. [23]	A systematic review, including 35 (RCTs)	43407 diabetic patients	They found that supplementation with vitamin D does not appear to enhance glycemic control or insulin resistance in the short term.
Pittas et al. [24]	Randomized control trial	2423	In people at high risk of type 2 diabetes who were not specifically chosen for vitamin D deficiency, vitamin D3 treatment at a level of 4000 IU per day did not result in a significantly reduced risk of diabetes than placebo.
Zhao et al. [25]	A meta-analysis, including four prospective cohort studies	187592 participants and 9456 incident cases	They found no connection between total vitamin D intake and the incidence of type 2 diabetes.
Mohammadian et al. [26]	Systematic review and meta-analysis	44 patients with T1D who were younger than 17 years old	HbA1C improves with vitamin D3 supplementation in all glycemic control groups in children with T1D and vitamin D insufficiency.