Large amounts of copper — from food like organ meats, shellfish, nuts and some vitamin supplements — may speed the mental decline of older people, but only when combined with a diet high in saturated and trans fats, a new study reports. 

Researchers studied 3,718 people 65 and older over six years. They interviewed subjects about their diet and administered tests of thinking, learning and memory at the beginning of the study and then twice more at three-year intervals. 

Those whose diets were rich in saturated and trans fats and who also consumed the most copper — an average of 2.75 milligrams a day — suffered a rate of mental decline almost 1.5 times as great as those on high-fat diets who consumed an average of only 0.88 milligrams a day. The current recommended daily allowance of copper for adults is 0.9 milligrams.

The findings appear this month in The Archives of Neurology.

Martha Clare Morris, the study’s lead author and an associate professor of medicine at Rush University Medical Center in Chicago, said that it was hard to measure copper intake in food precisely, but that she and her colleagues also looked at consumption in vitamin supplements, a more accurate gauge.

“Knowing the specific amounts of copper in multivitamins gives you better data,” Dr. Morris said. “In the group on a high saturated fat and trans fat diet, you see a very strong correlation between copper in multivitamins and mental decline.” Still, the authors cautioned that further study was needed.

Whole milk dairy products and red meat are high in saturated fats; foods made with partly hydrogenated oils — including some cookies, crackers, snack foods and margarines — are high in trans fats, or trans fatty acids.