A July 2019 study by the Université Laval in Quebec found, “…improvements in glucose metabolism following vitamin D supplementation in those at high risk of diabetes, or with newly diagnosed diabetes …”
Dr Claudia Gagnon, "Type 2 diabetes and prediabetes are a growing public health concern and although our results are promising, further studies are required to confirm our findings, to identify whether some people may benefit more from this intervention, and to evaluate the safety of high-dose vitamin D supplementation in the long term. Until then I would suggest that current vitamin D supplementation recommendations be followed."