Obesity promotes circulation of lipopolysaccharide. In animals, chronic systemic inflammation, experimentally induced by injection with LPS, also known as “LPS challenge,” can cause microglia into the brain to switch from protecting the blood-brain barrier to damaging it.
From the article:
Nearly 50 percent of all dementias, including Alzheimer’s, begins with the breakdown of the smallest blood vessels in the brain and their protective “gatekeeper cells,” according to a Keck School of Medicine of USC study.
A key point of interest was the systemic inflammation induced by injecting the mice with an inflammation-inducing substance. Such injections resulted in the movement of microglia to the blood vessels and increased the permeability of the blood-brain barrier within a few days. Then, the microglia initially acted to protect the blood-brain barrier and limit increases in permeability, but as inflammation progressed, the microglia reversed their behavior by attacking the components of the blood-brain barrier, thus increasing the barrier’s permeability. The subsequent leakage of molecules into the brain had the potential to cause widespread inflammation in the brain and consequent damage to neurons (cells of the nerves).