Scientists found strong evidence that copper helps to promote the changes in the brain underlying Alzheimer's.
But they say there is little that can be done about it because copper is so abundant in the diet and also vital to health.
The metal, found in food and drinking water, plays important roles in nerve function, bone growth, the formation of connective tissue, and hormone secretion.
Researchers in the US conducted a series of experiments on mice given trace amounts of copper in their drinking water.
In human terms the doses were equivalent to the amount of copper people consume in a normal diet.
The study showed that copper accumulating in the brain disrupted the natural removal of toxic amyloid beta protein, which is strongly implicated in Alzheimer's. Copper also directly stimulated neurons that increased the production of amyloid beta, and caused the proteins to clog together in lumps that could not be cleared.
Mice with Alzheimer's disease had "leaky" brains that allowed the metal to enter them more freely.
The findings, reported in the journal Proceedings of the National Academy of Sciences, suggested a "one-two punch" both creating more amyloid beta and preventing its removal, said the scientists.
"It is clear that, over time, copper's cumulative effect is to impair the systems by which amyloid beta is removed from the brain," said study leader Professor Rashid Deane, from the University of Rochester Medical Centre in New York.