• 23 August 2009 by Nora Schultz
• Magazine issue 2722. Subscribe and get 4 free issues.
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Expanding waistlines may cause shrinking brains

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.

The brains of elderly obese people looked 16 years older than the brains of those who were lean

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.

From brain scans initially carried out for a different study, Paul Thompson from the University of California in Los Angeles and colleagues selected 94 from people in their 70s who were still "cognitively normal" five years after the scan. This was to exclude people with disorders that might have confused the results. The researchers then transformed these scans into detailed three-dimensional maps.

People with higher body mass indexes had smaller brains on average, with the frontal and temporal lobes - important for planning and memory, respectively - particularly affected (Human Brain Mapping, DOI: 10.1002/hbm.20870). While no one knows whether these people are more likely to develop dementia, a smaller brain is indicative of destructive processes that can develop into dementia.

The team also found that the brains of the 51 overweight people were 6 per cent smaller than those of their normal-weight counterparts, on average, and those of the 14 obese people were 8 per cent smaller. "The brains of overweight people looked eight years older than the brains of those who were lean, and 16 years older in obese people," says Thompson.

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. Thompson suggests that as 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 celldeath and the shrinking seen.

In an as yet unpublished study, Thompson's team has shown that exercise, which improves cardiovascular health and blood flow, protects the very brain regions that had shrunk in the current study. "The most strenuous kind of exercise can save about the same amount of brain tissue that is lost in the obese," he says. This indicates that it is blood flow that drives brain health, not the other way round. As these areas undergo the most remodelling throughout adult life, they may be more sensitive to any changes in oxygen supply and nutrients, Thompson suggests.

But Deborah Gustafson at the University of Gothenburg in Sweden, who previously found that overweight women had less brain tissue than their leaner counterparts, questions whether obesity is driving brain atrophy or vice versa. She points out that brain atrophy in the frontal and temporal lobes, which also control eating behaviour and metabolism, could cause weight gain. "There are not enough longitudinal data available for us to know which is the chicken and which is the egg."