Copper, zinc and iron are essential to good health. But a new study finds that when it comes to these metals, there also can be too much of a good thing.

Older adults with diets that include a high intake of copper and elevated amounts of saturated and trans fats may put themselves at additional risk for a decline in thinking, learning and memory abilities, according to a report in the Archives of Neurology.

Shellfish, organ meats, including liver, are foods with the highest levels of copper. Nuts, seeds, legumes, whole grains, potatoes, chocolate and some fruits also contain copper, which appears to play a role in the development of brain plaques associated with Alzheimer’s disease, the report notes. Researchers at Rush University in Chicago conducted the study.