Vitamin D deficiency is widely prevalent and contributes to insulin resistance. Vitamin D deficiency is associated with increased BMI, increased blood pressure, and decreased HDL cholesterol. Those with the lowest Vitamin D levels had significantly higher BMI than those with lesser deficiency.
Humans are able to manufacture Vitamin D from sun exposure, but people living in northern latitudes along with widespread sunscreen use limit this process. Dietary sources are scarce and optimal levels can probably only be reached by supplementation or sun exposure.