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(HealthDayNews) -- In a finding that deepens the mystery of what causes Parkinson's disease (news - web sites), researchers say people with high levels of iron and manganese in their diet are more likely to develop the illness.

Researchers suspect the minerals are somehow connected to the development of Parkinson's disease, but it's far from clear whether iron and manganese, found in a variety of healthy foods, actually make people ill.

"We don't know about cause and effect," says study author Karen Powers, a research scientist at the University of Washington. "We are not saying that we know what causes Parkinson's, and it's way too soon for us to make any recommendations about diet."

An estimated 1 million to 1.5 million Americans have Parkinson's disease, a brain disorder that can cause tremors, difficulty walking and rigid muscles. In severe cases, patients can develop dementia and die from the disease. Parkinson's is largely a disease of the elderly, and is estimated to affect one in every 100 Americans over the age of 60.

Researchers have suspected a link between iron and Parkinson's disease, but the new study is the first to look at the combined effects of both iron and manganese, Powers says.

Powers and colleagues looked at two groups of people -- 250 who were newly diagnosed with Parkinson's disease and 388 healthy people. The researchers interviewed members of each group about their diets. The findings appear in the June 10 issue of Neurology.

The 25 percent of the subjects with the highest levels of iron intake were 1.7 times more likely to be Parkinson's patients than those in the lowest 25 percent.

The Parkinson's rate was a bit higher -- 1.9 times more likely -- for those who had higher-than-normal intakes of both iron and manganese.

Sources of iron include cereal, bread, spinach, nuts, beans, meat, poultry and fish. In the United States, many grain products -- such as cereal -- are fortified with iron. Manganese is found in grain products and spinach, among other foods.

"These are essential elements in the diet; we absolutely need them," Powers says. But the idea that "if a little is good, a whole lot is better" is simply not true.

The link between manganese and illness is already well known. Miners and welders who are exposed to manganese on the job sometimes develop a Parkinson's-like illness, Powers notes. Iron can also cause problems in large doses.

If iron does contribute to Parkinson's disease, one theory is that excess amounts of the minerals may interact with dopamine -- a vital brain chemical -- and create new chemicals that hurt brain tissue, Powers says. Parkinson's develops when the brain doesn't make enough dopamine.

More research will be necessary to confirm these findings, Powers says, and two other experts agreed.

While diet may directly affect development of Parkinson's disease, it's possible other factors are in play that affect consumption of certain foods, says Dr. Paul Tuite, an assistant professor of neurology at the University of Minnesota. For example, it's possible people who develop the disease may eat more of certain foods because their bodies don't turn them into proteins properly.

Parkinson's disease itself could also cause changes in the diet of patients by affecting how they enjoy food and their willingness to eat certain things, says Dr. Irene Litvan, director of the Movement Disorder Program at the University of Louisville.