Being overweight increases your risk for Alzheimer's disease, although no one knows how. Researchers in Sweden looked at how being overweight affects your brain.

What the researchers wanted to know: How do body mass index (a measure of overweight and obesity) and waist-to-hip ratio (a measure of how fat is distributed) relate to brain atrophy?

What they did: The researchers analyzed information on some of the women from the Population Study of Women in Göteborg, Sweden. The study began in the late 1960s with 1,462 women who were born in Göteborg in 1908, 1914, 1918, 1922, and 1930. Some of the women had a psychiatric exam in 1992 or 1993; they were also invited to have a CT scan done of their brain then, and 290 agreed. A radiologist looked for signs of brain atrophy on the CT images.

What they found: Women who had a high body mass index each time they were examined (about once a decade) were likely to have atrophy in the temporal lobe, a part of the brain involved in hearing and processing speech, among other things. That atrophy was common—almost 50 percent of the women who had CT scans had signs of atrophy in the temporal lobe. The women with temporal atrophy weren't necessarily even overweight, but their average body mass index was higher than that of women who didn't have temporal atrophy. The researchers found no link between waist-to-hip ratio and brain atrophy. Also, they looked at several parts of the brain but found an association between overweight and atrophyin this area only.

What the study means to you: As dramatic as brain atrophy sounds, it's a natural part of aging, and it happens in plenty of people who have no thinking problems.

Caveats: This doesn't prove that being overweight causes brain atrophy—just that they're associated.

Find out more: A BMI (body mass index) calculator

A list of articles from the Population Study of Women: www.sahlgrenska.gu.se. The introduction is in Swedish, but that makes it more fun.

Read the article: Gustafson, D., et al. "A 24-Year Follow-Up of Body Mass Index and Cerebral Atrophy." Neurology. Nov. 23, 2004, Vol. 63, pp. 1876-1881.

Abstract online: www.neurology.org