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Carrying too many pounds on the body can weigh on the brain as well.

Research shows obesity impacts brain health from childhood well into adulthood, affecting everything from executive function skills — the ability to initiate, plan and carry out tasks — to substantially raising dementia risk. By middle age, the consequences of excess weight are substantial. Several studies have shown middle-aged adults who have a body mass index at or above 30, which qualifies as obesity, are more likely to get dementia than their healthy-weight peers.

"It's fairly clear that being obese in midlife is bad for the brain and much of the rest of the body, too," said Mark Espeland, a professor of gerontology and geriatric medicine at Wake Forest School of Medicine in Winston-Salem, North Carolina. "Preventing that from occurring is so very important."

But researchers are still sorting out how and why the extra pounds harm the brain, and whether the higher dementia risk is cumulative over a lifetime or if obesity affects the body differently at different stages of life. 

It is possible that cognitive challenges come first, contributing to poor eating behaviors beginning in childhood, said Alexis Wood, associate professor of pediatric nutrition at the Children's Nutrition Research Center at Baylor College of Medicine in Houston. The center is operated in partnership with the Agricultural Research Service of the U.S. Department of Agriculture.
"There is pretty robust and substantial evidence that goes across the whole of childhood, from toddlerhood to adolescence, that shows a higher weight status is associated with lower cognitive functioning, particularly in the area of executive function," she said. "Why that is, is the subject of much debate.”

Some studies trace the relationship between diet, weight and brain health back to the womb. By toddlerhood, excess weight is already associated with a child's ability to control and direct behavior; integrate new information; plan; and solve problems, Wood said. It's just not clear which comes first.

For example, a study published in the found children with higher verbal and executive function skills in preschool were less likely to become overweight later in childhood. young children who are overweight or obese are less able to control impulses than those at a healthy weight.

"If cognitive function challenges come first, the prevailing thought is that this regulates how children interact with their environment," Wood said. " Lower cognitive function in this area alters eating behavior and predisposes you to poor eating behaviors."

If weight problems come before cognitive changes, however, it could be that the excess fat increases inflammation. Over time, that can lead to "changes in connectivity and structure and function of the brain," she said.

One theory is that it's not just the extra weight that's causing the problem, but conditions and illnesses associated with obesity that collectively contribute to poor brain health.
"People who have obesity are more likely to have Type 2 diabetes, hypertension and high cholesterol," said Kristine Yaffe, professor and vice chair of psychiatry, neurology and epidemiology at the University of California's Weill Institute for Neurosciences in San Francisco. "It might be the constellation of cardiovascular risk factors that go along with obesity that we know can have a detrimental effect on the aging brain, whether that's contributing to the development of Alzheimer's disease or vascular dementia or some sort of mix of the two."

Another possibility is that the hormones secreted by fat cells, such as leptin, play a role, Yaffe said. Leptin helps regulate hunger. However, people with too many fat cells produce such high levels of leptin that the body becomes insensitive to it, producing a cycle in which the person keeps eating because they never feel full.

Yaffe's research published in showed that in older women with a healthy body weight, leptin levels were associated with a lower risk for dementia or cognitive decline. However, in women with obesity, that protection disappeared.

It also could be that people who have obesity are less active and more prone to other illnesses that cause higher levels of inflammation, which "has a big role in precipitating or exacerbating Alzheimer's disease and vascular dementia," Yaffe said.

While gaining weight, especially in midlife, increases dementia risk, the opposite does not appear to hold true: A large, federally funded, long-term study of a weight loss intervention found no associated cognitive benefit. "Frankly, we don't know why that is," said Espeland, one of the study’s authors.