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Obesity subtly diminishes memory and other features of thinking and reasoning even among seemingly healthy people, an international team of scientists reports. At least some of these impairments appear reversible through weight loss. Researchers also report one likely mechanism for those cognitive deficits: damage to the wiring that links the brain’s information-processing regions.

A number of studies in recent years have shown that individuals with diseases linked to obesity, including cardiovascular disease, hypertension and type 2 diabetes, don’t score as well on cognitive tests as less hefty individuals do. To test whether weight alone — and not disease — might be partially responsible, John Gunstad of Kent State University in Ohio and his colleagues recruited 150 obese individuals for a series of cognitive tests. These people weighed on average just under 300 pounds, although some were substantially heavier. Two-thirds would shortly undergo weight-loss surgery.

Obese individuals in the new study initially performed on the low end of the normal range for healthy individuals from the database on average, Gunstad says, although nearly one-quarter of the obese participants’ scores on memory and learning actually fell within what researchers consider the impaired range.

Tested again 12 weeks after bariatric— when most had shed some 50 pounds — the lighter but still heavy patients scored substantially better. Most now performed “within the average or greater-than-average range for all cognitive tests,” the researchers reported online in October in Surgery for Obesity and Related Diseases.

Study participants who didn’t have surgery — or lose weight — performed worse on the second test.

A  study by Gunstad’s group used a form of magnetic resonance imaging, or MRI, to probe the wiring that connects nerve cells to move information throughout the brain. The bundled fibers are sheathed in a protective layer of white insulation, giving rise to the tissue’s name: white matter.

In obese individuals — but not normal-weight or overweight people — this sheathing shows signs of damage. “It’s not as though a cable has been cut,” Gunstad says. “It’s just that its integrity is diminished,” jeopardizing the strength or clarity of signals that must traverse these cognitive highways. His group’s findings appear in the March Obesity.

Using the same MRI technique last year, Knecht’s team linked C-reactive protein — a blood marker of systemic inflammation — with white matter integrity in a group of 447 older adults. Both type 2 diabetes and obesity can chronically elevate CRP levels in the blood.

As CRP levels in blood increased, Knecht and his colleagues found, so did the likelihood that white matter’s insulation would be impaired. This suggests that low-grade inflammation, which is strongly correlated with obesity, could be an important mediator,

In that study, the researchers reported in the March 30, 2010 Neurology, higher levels of CRP also correlated with “with worse performance in executive function, including tests of psychomotor speed and attention.”

J. Gunstad et al. Improved memory function 12 weeks after bariatric surgery. Surgery for obesity and related diseases. In press, Online Oct. 4, 2010. DOI: 10.1016/j.soard.2010.09.015