Childhood obesity is a major health concern and the current epidemic is increasing in numbers and severity with co-morbid diseases such as non-alcoholic fatty liver disease (NAFLD) and type 2 diabetes mellitus (T2DM). Clinical studies have shown a high prevalence of vitamin D deficiency (VDD) among overweight children. Recent studies of VDD in humans and animals indicate that VDD contributes to the development of insulin resistance (IR). However, mechanisms leading to NAFLD and further progression to the more severe form, non-alcoholic steatohepatitis (NASH), are unclear. Some clinical data indicate that VDD might be a risk factor for developing NASH. However, other studies could not confirm this association and the association between VDD and NASH remains controversial.