Our results demonstrated that vitamin D significantly improved muscle insulin resistance, one of the main defects leading to type 2 diabetes mellitus [32], by counteracting the diet-induced reduction in efficiency of the insulin pathway, shown by the impaired phosphorylation of IRS-1 protein as well as of the downstream key insulin signalling molecules, Akt, and the Akt substrate GSK-3β. Here, IRS1 is linked to type 2 diabetes mellitus.