Copper could be a major environmental culprit in the progression of Alzheimer's disease, research suggests.
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, and about a tenth of what is allowed under US water quality standards.
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.
"This impairment is one of the key factors that cause the protein to accumulate in the brain and form the plaques that are the hallmark of Alzheimer's disease."
Since copper was so essential to the body, the findings had to be treated with caution, he added.
"Copper is an essential metal and it is clear that these effects are due to exposure over a long period of time," said Prof Deane. "The key will be striking the right balance between too little and too much copper consumption.
"Right now we cannot say what the right level will be, but diet may ultimately play an important role in regulating this process."
Previous studies also suggested a link between aluminium and Alzheimer's, but the results have been unconclusive.
Dr Doug Brown, director of research and development at the Alzheimer's Society, said: "It's well known that clusters of amyloid beta are a major indicator of Alzheimer's disease however we don't know why they accumulate in this way.
"This study has found interesting new routes by which they may build up however this is early research and it's not yet clear how this might affect the disease process in people. 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."
Alzheimer's disease is the most common form of dementia, affecting about 62% of the UK's 800,000 sufferers. The number of people with dementia in the UK is expected to top one million by 2021.
Dr Eric Karran, from Alzheimer's Research UK, said the charity was currently funding a new study tracking copper in the brains of Alzheimer's patients.
He said: "We still need to understand more about how amyloid contributes to brain health both in Alzheimer's and in normal ageing. 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.
"It is important to remember that copper is a vital part of our diet and plays roles in many essential biological processes in the body. It will be necessary to look in more detail at how copper intake throughout life could influence brain function as we age and during Alzheimer's, but this requires further investment in research."
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