Research focusing on the relationship between peripheral BDNF concentration and AD has yielded mixed results, with some studies reporting decreased levels in AD patients [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53] and others finding no difference in BDNF concentrations [54] or even increased concentrations [19,50,51,52,53,54,55]. This evidence concerns the gene BDNF and Alzheimer disease.