Men in their 70s who are carrying more fat on their frames have a sharper decline in mental function over time than their slimmer peers, according to a new study in the Archives of Neurology.

But for women, there was no link between excess fat and worsening cognitive function; in fact, there was a tendency for the heavier women to fare better, although it didn't reach statistical significance, Dr. Alka M. Kanaya of the University of California-San Francisco and her colleagues found.

While obesity has been linked to dementia, the researchers note, many studies have used body mass index (BMI) to gauge overweight, which may not be as accurate a measurement of body fat in older people.

To get a better sense of how body composition might relate to mental function in older people without dementia, Kanaya and her team looked at 3,054 healthy men and women 70 to 79 years old. They used special imaging scans to check the study participants' total fat mass, amount of belly fat under the skin and around the internal organs, and also measured body mass, waist circumference, and sagittal diameter — the distance from the back to the front of the body, which is another technique for measuring belly fat.

The study participants also completed a brief test of general cognitive function at the study's outset and again 3, 5 and 8 years later.

Over the next 7 years, the researchers found, the men with the most body fat and the most subcutaneous fat in their abdominal area showed the greatest decline in mental function. Fat surrounding the abdominal organs, also known as visceral fat, was linked to mental decline, as was BMI, waist circumference, and sagittal diameter.

But for women, there was no significant relationship between any measures of body fat and mental decline.

The researchers accounted for several factors that could be related to both body fat and cognitive function, including diabetes, high blood pressure and levels of fat-secreted hormones, and the relationship remained strong.

Future studies should confirm the findings, the researchers conclude, and also look into the gender differences in the relationship between body fat and mental decline.