Exposure to the metal copper may increase the risk of Alzheimer's disease, research has suggested.
Alzheimer's damages brain function
Scientists carried out a study on rabbits, which have been shown to develop symptoms of Alzheimer's similar to those found in humans when fed a diet high in cholesterol.
They found cholesterol-fed rabbits were more likely to develop the plaques characteristic of Alzheimer's disease in their brains if they were given tap water to drink, rather than purified distilled water.
The plaques are made up of protein which misshapes and clumps together. As they build in number, they appear to destroy the ability of brain cells to communicate effectively with each other.
Further investigation revealed significant amounts of copper in the tap water which had been given to the rabbits.
The scientists then added trace amounts of copper to distilled water before giving it to rabbits to drink.
They found that rabbits that received the copper supplement for 10 weeks developed significantly more plaques - and preliminary signs of plaques - than rabbits that drank only distilled water.
Plaque formation in the brain
The copper-dosed rabbits were also less able to learn a difficult mental task than rabbits not receiving the supplement.
The researchers, from Sun Health Research Institute, Arizona, and West Virginia University, suggest that copper combined with cholesterol may prevent the brain from breaking down a crucial protein involved in the formation of plaques.
Harriet Millward, of the Alzheimer's Research Trust, told BBC News Online that previous studies had suggested a link between copper and Alzheimer's - and others had not.
"This is an interesting report, particularly since the results show an effect of copper on learning.
"However, more research is needed before we can make any conclusions about the role of copper in Alzheimer's disease.
"For example, copper could contribute to the disease by catalysing the production of 'free radicals' which cause significant damage to an Alzheimer's brain."
The research is published in the journal Proceedings of the National Academy of Sciences.