WASHINGTON (Reuters) - Women destined to develop Alzheimer's disease or another form of dementia may start losing weight at least a decade before being diagnosed with such a condition, U.S. researchers said on Monday.

Women who developed dementia began losing weight between 11 and 20 years before their diagnosis, and the weight loss accelerated in the decade before diagnosis, researchers led by Dr. David Knopman of the Mayo Clinic in Rochester, Minnesota found.

These women, at the time of their eventual diagnosis, weighed 12 pounds (5.4 kg) less on average than those who did not develop Alzheimer's or another dementia, they reported.

Knopman's team examined medical records, including detailed weight information, for 219 women who eventually were diagnosed with Alzheimer's or another type of dementia.

They tracked the same number of women who did not develop any form of dementia. They were of similar age and lived in the same place, Rochester.

The study also tracked men from the same city who developed dementia and men who did not, seeing no differences in weight.

Dementia is a brain disorder that undermines one's ability to perform daily activities. The most common form among the elderly is Alzheimer's, a slow disease that begins with mild memory problems and ends with severe brain damage.

LONG TIME DEVELOPING

Knopman said the findings suggest that certain behaviors develop in the women who are destined to develop Alzheimer's that predate the appearance of the disease by years.

"It reinforces the widely held but difficult-to-prove belief that the changes in the brain in dementia and Alzheimer's disease have a very long presymptomatic time course -- that they aren't things that develop overnight," Knopman said in a telephone interview.

Knopman said he favors a behavioral rather than biological explanation for the weight loss. Merely losing weight should not be seen as a diagnostic indicator of dementia, he added.

"As the brain processes of dementia are developing in the decade or two before the overt disease, changes in personality like apathy and loss of initiative are beginning to emerge," Knopman said.

This might make the women more likely to lose interest in meals, particularly when combined with the loss in the sense of smell that may herald the onset of cognitive impairment, the researchers wrote in the journal Neurology.

The reason the men might not be losing weight is because generally they are not the ones responsible for preparing meals in their household and instead are served by a spouse or perhaps adult children, Knopman added.