Scientist know that obesity harms cognition. But how? The blood brain barrier should keep out fat cell secretions.
Many thought the brain, though, should be insulated from those harmful effects. It contains no fat cells and sits behind the protective blood-brain barrier that usually blocks the entry of undesirable molecules.
However, recent disquieting studies in animals indicate that obesity weakens that barrier, leaving it leaky and permeable. In obese animals, substances released by fat cells can ooze past the barrier and into the brain.
How Fat May Hurt the Brain, and How Exercise May Help – NYTimes.com.