A lifetime of too much copper in our diets may be contributing to Alzheimer's disease, US scientists say.
However, research is divided, with other studies suggesting copper may actually protect the brain.
The latest study in Proceedings of the National Academy of Sciences showed high levels of copper left the brain struggling to get rid of a protein thought to cause the dementia.
Copper is a vital part of our diet and necessary for a healthy body.
Tap water coming through copper pipes, red meat and shellfish as well as fruit and vegetables are all sources of dietary copper.
The study on mice, by a team at the University of Rochester in New York, suggested that copper interfered with the brain's shielding - the blood brain barrier.
Mice that were fed more copper in their water had a greater build-up of the metal in the blood vessels in the brain.
The team said this interfered with the way the barrier functioned and made it harder for the brain to get rid of a protein called beta amyloid.
One of the hallmarks of Alzheimer's disease is the formation of plaques of amyloid in the dying brain.
Lead researcher Dr Rashid Deane said: "It is clear that, over time, copper's cumulative effect is to impair the systems by which amyloid beta is removed from the brain."
He told the BBC that copper also led to more protein being produced: "It's a double whammy of increased production and decreased clearance of amyloid protein.
"Copper is a very essential metal ion and you don't want a deficiency and many nutritious foods also contain copper."
However, he said taking supplements may be "going overboard a bit".
Commenting on the latest findings, Chris Exley, professor of bioinorganic chemistry at Keele University, said there was "no true consensus" on the role of copper in Alzheimer's disease.
His research on human brains reached the opposite conclusion: "In our most recent work we found evidence of lower total brain copper with ageing and Alzheimer's. We also found that lower brain copper correlated with higher deposition of beta amyloid in brain tissue.
"He said at the moment we would expect copper to be protective and beneficial in neurodegeneration, not the instigator, but we don't know.
"The exposure levels used mean that if copper is acting in the way they think it does in this study then it must be doing so in everyone."
Dr Eric Karran, from Alzheimer's Research UK, said: "While the findings present clues to how copper could contribute to features of Alzheimer's in mice, the results will need replicating in further studies. It is too early to know how normal exposure to copper could be influencing the development or progression of Alzheimer's in people. "
Dr Doug Brown, from the Alzheimer's Society, said: "Considering copper is a vital mineral for the body, people should treat these results with caution and not cut it out of their diet. More research is needed to understand the role that copper might play in the brain."