The presence of vitamin D in the brain and its link to improved cognitive performance in aged subjects was originally discovered by a Tufts University study. Higher vitamin D concentrations were linked to a 25% to 33% decreased risk of dementia or mild cognitive impairment in four different brain areas. Authors did note, however, that the observational nature of the study and the lack of a diverse subject population were two of its drawbacks, and there may be further explanations for the results.