A lifetime of too much copper in our diets may be contributing to Alzheimer's disease, US scientists say. 

一生的饮食中摄入过多铜元素可能引发阿尔茨海默症。

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.

Barrier

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 call beta amyloid.

One of the hallmarks of Alzheimer's disease is the formation of plaques（斑块，瓷片） of amyloid（淀粉体） 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（晦气，剧烈的打击） 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".