The team from Rush University Medical Center in Chicago conducted a study over a six year period of 3,718 Chicago residents age 65 years and older, to assess the connection between dietary fat and dietary copper intake on mental decline.

The participants underwent cognitive testing at the beginning of the study, after three years and after six years.

One year after the study began, they filled out a questionnaire about their diets.

Although copper, zinc and iron are essential for brain development and function, an imbalance of these metals may play a role in the development of brain plaques associated with Alzheimer's disease.

Previous studies have also linked fat intake, especially that of saturated and trans fats, to Alzheimer's disease and other forms of cognitive difficulties.

Trans fatty acids, or trans fats, are created when oil reacts with hydrogen gas. They prolong the shelf-life of many manufactured food products, but also contain increased levels of a cholesterol that raises the risk of coronary artery disease and stroke.

Previous studies have indicated people who consume foods heavy with saturated or trans fats were at up to three times the risk of Alzheimer's disease.

One recent animal study found that the consumption of copper in drinking water could amplify the degenerative effects of a high-fat diet on rabbit brains.

The dietary recommended allowance of copper for adults is 0.9 milligrams per day.

Meats, such as liver, and shellfish are the foods with the highest copper levels, followed by nuts, seeds, legumes, whole grains, potatoes, chocolate and some fruits. Copper pipes may also add trace amounts of the metal to drinking water.

The researchers found that about 600 of the subjects consumed at least 1.6 milligrams of copper a day, along with foods heavy in saturated and trans fat.

According to the study by so-doing those people added the equivalent of 19 years to their ages in terms of mental decline.

Study author Martha Clare Morris of Rush University Medical Center in Chicago says the findings must be viewed with caution and more investigation into the effects of diets high in copper and saturated and trans fats were needed to assess their impact on public health.

The study was published in the journal Archives of Neurology.