Many studies have linked low vitamin D levels to various diseases (multiple sclerosis, diabetes, lupus, bone disease, cardiovascular, kidney) to name a few.  The latest study in Neurology http://www.neurology.org/content/early/2014/08/06/WNL.0000000000000755 concludes that “vitamin D deficiency is associated with a substantially increased risk of all-cause dementia and Alzheimer disease”.  A simple blood test for 25-hydroxy vitamin D can measure the amount of D in the body. The normal range is 30-75 nanograms per milliliter. http://www.nlm.nih.gov/medlineplus/ency/article/003569.htm. The study authors feel that people with baseline vitamin D levels 50 nmol/L or higher have sufficient vitamin D in their bodies while those with deficient levels 25-50 nmol/L were at significant risk for dementia and Alzheimer disease.

Vitamin D is found in food such as:  fatty fish (salmon, halibut, herring, tuna); fortified milk, oj, and cereal; and egg yolks.  It can also be produced by the body when unprotected skin is in sunlight for a sufficient amount of time.  There are also vitamin D supplements at the pharmacy.  Check with your doctor for questions regarding your vitamin D level.