But the researchers behind the study were quick to stress that high intake of copper alone was not linked to changes in cognitive function, and that the effect appears only in combination with a diet high in saturated and trans-fats.
Indeed, outside of the high saturated and trans fat group, people with a high intake of copper were more likely to have healthier lifestyles and better mental function, report the researchers in the American Medical Association's Archives of Neurology (Vol. 63, pp. 1085-1088).
The researchers, led by Martha Clare Morris from the Rush Institute for Healthy Aging, recruited 3718 participants with an average age of 74 to the Chicago Health and Aging Project (CHAP). Dietary assessment was gathered by a food frequency questionnaire (FFQ), and cognitive function was assessed at year three and six using four tests; the East Boston Tests of Immediate Memory and Delayed Recall, the Mini-Mental State Examination, and the Symbol Digit Modalities Test.
After an average of five and a half years of follow-up the researchers found that cognitive function scores declined by an average of 4.2 standardised units per year (SU/y) among the whole sample population.
However, among the 604 people consuming a diet high in saturated fat and trans fat, a higher copper intake (2.75 milligrams per day) was linked to a higher rate of cognitive decline (6.14 SU/y).
In Europe, the recommended daily intake (RDI) of copper is 1.2 milligrams per day with an upper safe limit of 9 milligrams. In the US, the RDI is 2 milligrams with an upper limit of 10 milligrams.
The relationship between high saturated fat or trans fat individually and high copper intake was not as strong as when the fats were combined, and no association was observed in people with high copper intake and diets low in saturated fat and trans fat.
"These data suggest that high dietary intake of copper in conjunction with a diet high in saturated and trans fats may be associated with accelerated cognitive decline," wrote Morris.
The mechanism behind the high fat- high copper decline could be due to the adverse interaction between copper and so-called amyloid-beta peptides that have been linked to the formation of plaques in the brain. Such plaques are thought to be the cause of Alzheimer's disease.
"This finding of accelerated cognitive decline among persons whose diet were high in copper and saturated and trans fats must be viewed with caution," warned the researchers. "The strength of the association and the potential impact on public health warrant further investigation."
Public awareness and concern about the fat content of foods, particularly trans fats, is increasing, with pressure growing to cut, or at least label, the individual fat content in foods to better inform consumers.
Legislation introduced in Denmark in 2004 mandated that all oils and fats used in locally made or imported foods must contain less than 2 per cent industrially produced trans fatty acids.
This virtually eliminated trans fatty acids and had no effect on quality, cost, or availability of foods.
And in January 2006 the US Food and Drug Administration mandated that all food manufacturers provide the content of trans fatty acids and cholesterol in addition to saturated fat on nutrition labels for all manufactured foods.
The UK Food Standards Agency is now pressing for revision of the European directive that governs the content and format of nutrition labels on foods marketed in the United Kingdom and other European countries, so that these fats are labelled.
They believe that mandatory addition of the content of saturated fat and trans fatty acids to nutrition labels would enable consumers to make healthier food choices that could lower LDL concentrations and reduce the risk of coronary heart disease and other vascular events.