Drinking tap water containing trace levels of copper may increase the risk of Alzheimer's disease, say scientists.
Researchers believe copper combined with cholesterol may prevent the brain removing a protein which produces the brain-clogging plaques associated with Alzheimer's.
The tap water link emerged from a study of rabbits fed a high cholesterol diet.
It has long been known that rabbits fed a cholesterol-rich diet develop many of the features of Alzheimer's, leading researchers to use them as a model for studying the disease.
But two US scientists noticed that cholesterol-fed rabbits given distilled water to drink developed fewer amyloid beta plaques than those drinking tap water.
Further investigation revealed significant amounts of copper in the tap water of many laboratories housing rabbits used in Alzheimer's research.
To test the copper connection, the scientists added trace amounts of the metal to distilled water given to a group of cholesterol-fed rabbits.
Rabbits receiving the copper additive for 10 weeks developed significantly more plaques and precursors called senile plaque-like structures (SPs) than cholesterol-fed rabbits that drank "pure" distilled water.
The copper-dosed rabbits were also less able to learn a difficult conditioning task than animals not receiving the supplement.
The study was reported in the journal Proceedings of the National Academy of Sciences.