A number of dietary elements and foods have been reported to be either risk or protective factors for the development of dementia and Alzheimer disease (AD). These include fat, fatty acids, antioxidants, fish, homocysteine/methionine, vitamins and alcohol. We propose that brain diseases with aging are not be only the result of pathogenic processes, but also due to the failure of protective mechanisms, and that diet influences the success of these protective mechanisms. Both animals and humans with genetic forms of AD do not get the disease until a certain time in mid or late life. Therefore, there must be protective factors responsible for the delayed onset of disease.