Middle-aged people who are overweight or obese have lower levels of certain brain chemicals that signal good brain health and function, according to a new study using high-tech brain scans.

The findings suggest that excess body fat may speed the brain aging process, which could put people at greater risk of developing age-related diseases of the brain, such as Alzheimer’s disease, Dr. Stefan Gazdzinski of the San Francisco VA Medical Center and colleagues say.

The researchers looked at magnetic resonance imaging (MRI) brain scans from 50 healthy middle-aged men and women, measuring amounts of a variety of chemicals in the white and gray matter of the brain. Gray matter consists of the bodies of nerve cells, while white matter is made up of the connections between these cells.

Five of the study participants were obese, 15 were overweight, and the remaining 30 were normal weight.

The higher a person’s body mass index (BMI), the ratio of body height to weight, the lower the concentration of N-acetyl-aspartate (NAA), a brain chemical that serves several functions and also acts as a marker for overall brain health, in the white matter of the brain’s frontal, temporal and parietal regions. Heavier people also had less NAA in their frontal gray matter, and lower concentrations of choline-containing metabolite - substances key to the formation of cell membranes-in their frontal white matter.

The strongest relationship between BMI and brain chemistry was seen in the white matter of the frontal region, which is believed to be particularly vulnerable to aging-related damage, the researchers note.

It’s possible that being heavy accelerates brain aging, or that being overweight or obese in childhood affects brain development, the researchers note.

The data didn’t allow them to determine if the brain abnormalities might be related to body fat alone or if it suggests other health problems, nutrition, or sedentary living, they add. But if other research that does address these factors confirms the current findings, the researchers say, the results could provide important clues to changes in the brain that might precede dementia.

SOURCE: Annals of Neurology, published online April 11, 2008.