LONDON (Reuters) - Mice fed junk food for nine months showed signs of developing the abnormal brain tangles strongly associated with Alzheimer's disease, a Swedish researcher said on Friday.

The findings, which come from a series of published papers by a researcher at Sweden's Karolinska Institutet, show how a diet rich in fat, sugar and cholesterol could increase the risk of the most common type of dementia.

"On examining the brains of these mice, we found a chemical change not unlike that found in the Alzheimer brain," Susanne Akterin, a researcher at the Karolinska Institutet's Alzheimer's Disease Research Center, who led the study, said in a statement.

"We now suspect that a high intake of fat and cholesterol in combination with genetic factors ... can adversely affect several brain substances, which can be a contributory factor in the development of Alzheimer's."

Alzheimer's disease is incurable and is the most common form of dementia among older people. It affects the regions of the brain involving thought, memory and language.

While the most advanced drugs have focused on removing clumps of beta amyloid protein that forms plaques in the brain, researchers are also now looking at therapies to address the toxic tangles caused by an abnormal build-up of the protein tau.

In her research, Akterin focused on a gene variant called apoE4, found in 15 to 20 percent of people and which is a known risk factor for Alzheimer's. The gene is involved in the transport of cholesterol.

She studied mice genetically engineered to mimic the effect of the variant gene in humans, and which were fed a diet rich in fat, sugar and cholesterol for nine months -- meals representing the nutritional content of fast food.

These mice showed chemical changes in their brains, indicating an abnormal build-up of the protein tau as well as signs that cholesterol in food reduced levels of another protein called Arc involved in memory storage, Akterin said.

"All in all, the results give some indication of how Alzheimer's can be prevented, but more research in this field needs to be done before proper advice can be passed on to the general public," she said.

(Reporting by Michael Kahn; Editing by Catherine Bosley)