**1. Introduction**

In the recent decades, the population in the industrialized Western countries has become remarkable sedentaryandhavehada considerable increase inthe intakeofwhathasbeencalled "fast food," meals that are rich in fat and carbohydrates and contain elevate levels of cholester‐ olaswell.The elevatedconsumptionoffastfoodhashadastrongimpactonpublichealth,which has importantrepercussions in several levels including an economic impact due to the elevated cost of a chronic use of specialized health services and a detrimental effect in both, life quality and expectancy for the patients. Among the adverse health effects of this type of diet, we can mention obesity, vascular diseases, and metabolic syndrome, and it has been recently pro‐ posed that it can increase the risk of developing Alzheimer's disease (AD), which is the most common type of dementia in elderly people. It is considered that a particular type of diet could accelerate the progress of the disease for a not yet well-known mechanism [1]. It is a revolu‐ tionary idea, since we have had for several years the conception that brain is actually protect‐ ed by the blood–brain barrier (BBB); however, experimental evidence suggests that the consumption of diets rich in fat can disrupt the permeability of BBB, making it vulnerable to systemic molecules that could trigger degenerative processes [1, 2].

In the current chapter, we will review the state of the art related to the impact of diets rich in fat or cholesterol on the brain, and how the alterations induced in other organs can impact brain functioning and could increase the susceptibility to develop dementia. The bibliographic revision was carried out running an exhaustive search on the research articles related to the topic employing the database of the US National Library of Medicine, National Institutes of Health, PubMed.gov. Firstly, reviewing the most recent papers and those with the most relevant information. Thereafter, we carefully followed the references cited by the reviewed articles in order to study the grounding data on the subject and which direction it followed until our days in order to document the accuracy and evolution of the data.
