Copper’s relationship to Alzheimer’s disease is a hotly contested issue in the field of neurology, as experts attempt to find proper treatments—and perhaps a cure—for a condition that affects 5 million Americans .

New research indicates that copper is one of the major environmental factors responsible for Alzheimer’s disease. This conflicts with research from earlier this year suggesting that the metal protects against this common form of dementia.

A new study identifies copper as the culprit in Alzheimer’s disease, while other evidence says it protects against dementia.

A study appearing in the latest issue of the Proceedings of the National Academy of Sciences says that copper accumulation in the body increases the progression of Alzheimer’s disease by preventing toxic proteins from leaving the brain.

Rashid Deane, a research professor in the University of Rochester Medical Center’s department of neurosurgery, is a leading expert on copper’s role in Alzheimer’s.

In his latest study, he and colleagues gave mice low levels of copper, similar to what people are exposed to normally in their food and environment, over a three month period. They found that copper accumulated in the blood vessels that feed blood to the brain. It also disrupted the removal of beta amyloid, an peptide linked to the plaques that form in the brains of Alzheimer’s patients.

“It is clear that, over time, copper’s cumulative effect is to impair the systems by which amyloid beta is removed from the brain,” Deane said in a press release accompanying the study. “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.”

People typically ingest copper in drinking water that runs through copper pipes, in nutritional supplements, and in certain foods, including fruits, vegetables, nuts, shellfish, and red meats. While normal levels of copper are important for nerve function, bone growth, and healthy connective tissues, the latest study shows it may have a detrimental effect on the brain.

Deane’s findings are consistent with other research —including similar studies Deane has done in the past—that says the increased rates of Alzheimer’s in developed nations is caused by the ingestion of inorganic copper.

However, research from earlier this year says just the opposite—that copper may be the key to warding off Alzheimer’s.