In a double-blind, placebo-controlled trial involving 96 participants, the researchers gave half of the participants 5,000 international units of vitamin D-3 every day for 6 months and the other half of the participants a placebo that looked identical to the vitamin D-3 capsules.

     The researchers assessed peripheral insulin sensitivity via M-value, which was obtained from a 2-hour hyperinsulinemic-euglycemic clamp at baseline and at 6 months from study enrollment, which took place from January 2013 to October 2016. The researchers also evaluated hepatic insulin sensitivity, whole-body insulin sensitivity, beta-cell-function and insulin secretion while participants took part in a 75-g 2-hour oral glucose tolerance test.

     At the end of the 6-months, high-dose vitamin D supplementation for 6 months significantly improved peripheral insulin sensitivity and beta-cell function in individuals at high risk of diabetes or with newly diagnosed type 2 diabetes. They also showed that those with the poorest insulin sensitivity at baseline benefited the most from vitamin D supplementation.

     They also showed that those with the poorest insulin sensitivity at baseline benefited the most from vitamin D supplementation. In participants who were at risk of diabetes but did not have impaired glucose sensitivity, vitamin D made no difference.