CHICAGO, July 21 (Reuters) - Eating plenty of fish, nuts and oil-based salad dressings that contain polyunsaturated fatty acids cuts the risk of developing Alzheimer's disease, researchers said on Monday.

A seven-year study of 815 nursing home residents -- 131 of whom developed the brain-robbing disease -- found those who reported eating fish at least once a week had a 60 percent lower risk of Alzheimer's compared to those who rarely or never ate fish.

Researcher Martha Clare Morris of Rush-Presbyterian St. Luke's Medical Center in Chicago, writing in The Archives of Neurology journal, credited polyunsaturated fatty acids found in fish, nuts and oily dressings for the protective effect. She said the fatty acids are also found in the membranes of brain cells, and may protect them from the ravages of Alzheimer's.

Previous studies have cited a healthy diet rich in antioxidants -- such as those found in red wine and various fruits and vegetables -- may stave off Alzheimer's, though the root cause of the brain-clogging plaque that afflicts an estimated 12 million people globally remains unknown.

In an accompanying editorial, Robert Friedland of Case Western Reserve University School of Medicine in Cleveland, Ohio, said a healthy diet containing fish could help ward off a host of ailments, not just Alzheimer's, though he warned of toxins such as mercury tainting some fish.

"A high antioxidant/low saturated fat diet pattern with a greater amount of fish, chicken, fruits, and vegetables and less red meat and dairy products is likely to lower the risk of Alzheimer's disease, as well as that for heart disease and stroke," Friedland wrote.