Women who are obese in middle age are twice as likely to develop dementia later in life than those of normal weight, a US study reports today.

Figures based on 27-year records involving 10,276 members of the Kaiser Permanente medical care programme in California add mental disorder to the health risks facing the extremely overweight, which include heart disease and diabetes.

Measurements based on body mass show a "non-significant" 30% rise in risk for obese men, researchers from the company say in the medical journal BMJ Online.

However, measurements of high skinfold thickness on the shoulder and upper arm are also a predictor of extra dementia risk, 60%-70% in both men and women. There were fewer obese and overweight men in the study, so the power to detect an effect might have been smaller, the researchers say.

This might also depend on differences in where fat is stored in men and women.

It is thought to be the first study to look at skinfold thickness and dementia, although one has previously suggested a link with Parkinson's disease.

Incidence of dementia is expected to increase as people live longer, but failure to contain the present "epidemic" of obesity may make that rise steeper, the authors say.

"If these results can be confirmed elsewhere, perhaps treatment of obesity might reduce the risk of dementia."