The human body stores extra fat in several well-defined places, and as the body ages, these fat deposits tend to expand. For women, it's hips and thighs. For men, it's shoulders and the abdomen, the large area between chest and pelvis commonly known as the belly. Simple observation shows that men are more likely than women to have big bellies, particularly as they grow older.
In 2008, data from a Kaiser Permanente study of nearly 7,000 volunteers, age 40 to 45, showed that people with big bellies are more likely than those with flat tummies to develop Alzheimer's later in life. One theory to explain this is that belly fat sends damaging molecules through the bloodstream to the brain, but the true link is still a true mystery. Or as one nutrition researcher elegantly puts it, "An exciting area that needs to be explored."
Healthy bodies come in all sizes and shapes. Holding to a brain-healthy shape definitely doesn't mean dieting to skeleton size. Doing that deprives your brain (and the rest of your body) of essential nutrients. In short, a sensible diet leads to a sensible body and a sensible brain.