Scientists on Monday reported new evidence that copper can lead to the plaque buildup in the brain that causes Alzheimer's disease, fueling fresh debate over the mineral's role in dementia.
The scientific community is divided on the question of whether copper -- found in red meat, vegetables, dairy products as well as pipes that carry drinking water in much of the developed world -- causes or prevents Alzheimer's disease.
For the latest study in the Proceedings of the National Academy of Sciences, researchers looked at how copper in the capillaries may cause a breakdown in the blood-brain barrier, leading to a buildup of the protein amyloid beta, or plaques that are a hallmark of Alzheimer's.
According to lead author Rashid Deane, a research professor at the University of Rochester Medical Center, experiments using mice and human cells showed that low levels of copper delivered via drinking water accumulated in the capillary walls that feed blood to the brain.
"These are very low levels of copper, equivalent to what people would consume in a normal diet," said Deane.
The copper caused oxidation which interfered with another protein, called lipoprotein receptor-related protein 1 (LRP1), that would normally clear amyloid beta from the brain, his study said.
Not only did copper appear to prevent the clearance of plaque that is believed to be a prime culprit in Alzheimer's, it also stimulated neurons to produce more amyloid beta.
Researchers described their findings in a press release as a "one-two punch" that "provides strong evidence that copper is a key player in Alzheimer's disease."
"Copper is an essential metal and it is clear that these effects are due to exposure over a long period of time," said Deane in a statement.
"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."
However, other experts who have studied copper and Alzheimer's questioned the paper's findings.
"Research including our own shows the opposite, that copper prevents amyloid from forming the type of structures seen in the plaques," said Christopher Exley, professor in Bioinorganic Chemistry at Keele University in Staffordshire.
Exley and colleagues recently published their latest paper on the topic in the British journal Nature in February.
"As a group we would be thinking, based on everything that we know -- and our research has been done with human brains and brain tissues -- that if anything, copper would be protective against Alzheimer's."
Exley said a "number of things" in the PNAS paper raised red flags, such as the way they measured the copper amounts and the fact that they used animal models which do not always translate directly to humans.
"You do need a significant amount of tissue to produce results that you have a high level of confidence in. A mouse capillary -- these are very, very, very small things," Exley told AFP.
"The amount of copper which they are talking about as being possibly proactive is normal," he added.
"If you took this paper at absolute face value, it is telling everybody that we are all suffering from the effects that this paper is documenting right now because we are all exposed to this amount of copper."
Another outside researcher, George Brewer, emeritus professor of internal medicine at the University of Michigan medical school, said the "authors miss an important point about copper toxicity to the brain."
"They don't differentiate copper delivered in drinking water, as they delivered it in their study, from copper in food," Brewer said in an email to AFP.
"We have always had copper in food, so it couldn't possibly be the cause of this new AD epidemic," he said.
"If they had added this trace amount of copper to food, rather than putting it in drinking water, it would have had no effect."