Table 2.

Current concerns and recommendations for future RCT’s designing.

No.	Concerns Related to Existing Clinical Study Data	Recommendations for Future RCT’s Designing
1.	Follow-up too short	Follow-up > 12 months
2.	Too high, too low, non-physiological, and not frequent enough doses of vitamin D	Sufficiently large doses, consistent with the recommendations for a given population
3.	Lack of reported exclusion criteria or a wide variety of exclusion criteria inconsistent between studies	Medications, current medical diseases, participant’s past medical history, which could potentially influence glucose or vitamin D metabolism, and mental illnesses routinely included in the exclusion criteria beside any relevant others
4.	Different endpoints and glycemic parameters measured	Ideal primary endpoint: diabetes mellitus type 2 developmentThe measured parameters should be consistent with prediabetes criteria established by scientific societies
5.	Vitamin D levels in participants not assessed or participants with normal levels of vitamin D included in study groups together with vitamin D deficient participants	Only participants with hypovitaminosis D divided into levels of this deficiency should be considered in order to get reliable results
6.	Study groups not homogenous regarding: diabetes status	Only patients with diabetes or with prediabetes in one study group
7.	Wide age-range and different ethnicity	Homogenous groups analyzed together
8.	Additional interventions in one study	Vitamin D supplementation should be the only intervention; neither lifestyle change nor co-supplementation should be included in study designs, not allowing for separating the effects of individual interventions
9.	Not reporting or assessing sun exposure, the seasons, and geographical location in which the study was performed	Inclusion of sun exposure, seasons, and geographical locations data with respect to vitamin D and participants