People who have been diagnosed with depression may have an increased risk of developing Parkinson's disease later on, a new study suggests.

In the study, researchers looked at about 140,000 people who had been diagnosed with depression between 1987 and 2012. They matched each of them with three people of the same age and gender who had not been diagnosed with depression. The people in the study were all at least 50 years old by 2005.

The investigators followed the participants for 26 years, and found that 1 percent of the people with depression developed Parkinson's disease, whereas 0.4 percent of the people without depression developed the disease.

The researchers noted that Parkinson's disease is not common, even among people with depression. "Only a very little proportion of those with depression develop Parkinson's disease," said study author Peter Nordström, of Umeå University in Umeå, Sweden. [3 Myths About Parkinson's Disease]

Still, the link should be studied further because the new study adds to the growing body of research connecting Parkinson's disease with certain other health conditions and personality traits. For example, a 2012 study presented at the American Academy of Neurology meeting that year showed that people who are cautious and avoid taking risks are more likely to develop the disease.

Parkinson's disease results from a loss of the brain cells that produce the chemical dopamine. The condition affects body movements — it causes tremors, rigid muscles and impaired balance. About 1 million people in the U.S. have Parkinson's, according to the Parkinson's Disease Foundation.

The new study also suggested that people who have depression and develop Parkinson's do so earlier than people who have not been diagnosed with depression. The people who had depression were 3.2 times more likely to develop Parkinson's disease within a year after the study started, compared with people who did not have depression.

Moreover, the more severe a person's depression, the greater their likelihood of developing Parkinson's disease. For example, people who had been hospitalized for depression were more than three times more likely to be diagnosed with Parkinson's disease than those who had depression but had not been hospitalized because of it. People who had been hospitalized for depression five times or more were 40 percent more likely to be diagnosed with Parkinson's disease than those who had been hospitalized for depression once.

It is not clear whether depression might cause Parkinson's disease, the researchers said. It is possible that depression affects the brain in some way that contributes to the likelihood of developing Parkinson's disease, Nordström told Live Science.

It could also be that the drugs used to treat people with depression, such as antidepressants or antipsychotics, raise a person's risk of developing the disease. Alternatively, depression may be a symptom that precedes the manifestation of Parkinson's disease, the researchers said.

The study was published today (May 20) in the journal Neurology.

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