Elderly people whose diets are high in copper and heavy in saturated fats and trans fats risk faster mental decline that could be related to the onset of Alzheimer’s disease, researchers said on Monday.

In a six-year study of more than 3,700 people aged 65 or older, about 600 of the subjects consumed at least 1.6 milligrams of copper a day, along with foods heavy in saturated and trans fat. Many of those people added the equivalent of 19 years to their ages in terms of mental decline, according to the study.

While copper, zinc and iron are essential for brain development, too much copper in the bloodstream may block the body’s ability to rid itself of proteins that form plaques found to clog the brains of Alzheimer’s patients, study author Martha Clare Morris of Rush University Medical Center in Chicago found.

Copper, which has been found at higher levels in the blood of Alzheimer’s patients, is normally consumed in animal organs like liver, and in shellfish, nuts, legumes, some fruits, potatoes and chocolate. Drinking water that travels through copper pipes can also contain copper. Many of those in the study with high copper levels got it primarily through multivitamins.

The U.S. daily recommended intake of copper is 0.9 milligrams, while study subjects with the most copper consumed at least 1.6 milligrams per day.

Trans fatty acids, or trans fats, are created when oil reacts with hydrogen. 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.

The study was published in the journal Archives of Neurology.

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.