Deficiency in vitamin D is linked to depression, Parkinson disease and to cognitive decline according to a new article in Scientific American. According to the article, about three quarters of the population in the USA suffer from vitamin D deficiency. Here, in Michigan, the percentage may be even higher because of the lack of sunlight.
If you suffer from depression, it would be prudent to ask for a blood test, to determine whether your blood levels are within normal limits. Do not take extra supplements of Vitamin D without consulting your primary care physician, as it can be toxic in large doses.