The reviewed research followed 1,658 adults who were free from dementia, cardiovascular disease and stroke for an average of 5.6 years (Littlejohns TJ, et al. 2014). Vitamin D levels (25-hydroxy vitamin D) were measured from blood samples.
It was found that vitamin D deficiency is associated with a substantially increased risk for both dementia and Alzheimer's disease. For most people 2000 IU daily of vitamin D 3 is enough to maintain a good level of vitamin D. That should be easy to manage, and if you also get out in the sun a little bit without sunscreen, you should be even better off.
Littlejohns TJ, Henley WE, Lang IA, Annweiler C, Beauchet O, Chaves PH, Fried L, Kestenbaum BR, Kuller LH, Langa KM, Lopez OL, Kos K, Soni M, Llewellyn DJ. Vitamin D and the risk of dementia and Alzheimer disease. Neurology. 2014 Sep 2;83(10):920-8. doi: 10.1212/WNL.0000000000000755. Epub 2014 Aug 6.