Obesity is linked to an increased risk of type 2 diabetes, which is known to be associated with cognitive impairment. So Antonio Convit at the New York University School of Medicine wanted to see what impact obesity had on the physical structure of the brain. He used magnetic resonance imaging to compare the brains of 44 obese individuals with those of 19 lean people of similar age and background. What he found was shocking.
He found that obese individuals had more water in the amygdala - a part of the brain involved in eating behaviour. He also saw smaller orbitofrontal cortices in obese individuals, important for impulse control and also involved in feeding behaviour (Brain Research, in press). "It could mean that there are less neurons, or that those neurons are shrunken," says Convit.
Eric Stice at Oregon Research Institute, Eugene, thinks that the findings strengthen the "slippery slope" theory of obesity. "If you overeat, it appears to result in neural changes that increase the risk for future overeating," he says. Obesity is associated with a constant, low-level inflammation, which Convit thinks explains the change in brain size.
Brain regions key to cognition are smaller in older people who are obese compared with their leaner peers, making their brains look up to 16 years older than their true age. As brain shrinkage is linked to dementia, this adds weight to the suspicion that piling on the pounds may up a person's risk of the brain condition.
Previous studies suggested that obesity in middle age increases the risk of dementia decades later, which is accompanied by increased brain shrinkage compared with leaner people. Now brain scans of older people have revealed the areas that are hardest hit, as well as the full extent of brain size differences between obese people and those of average weight.
High insulin levels and type 2 diabetes tend to accompany being overweight and are risk factors for brain tissue loss and dementia. However, the relationship between brain size and body mass index still stood when the researchers accounted for these conditions, indicating that body fat levels may be linked directly to brain shrinkage. Increased body fat ups the chances of having clogged arteries, which can reduce blood and oxygen flow to brain cells, the resulting reduction in metabolism could cause brain cell death and the shrinking seen.