London, May 21 (IANS): People with depression may be more likely to develop Parkinson's disease, a large study has revealed.

"We saw this link between depression and Parkinson's disease during over a timespan of more than two decades. Depression may be a very early symptom of Parkinson's disease or a risk factor for the disease," said study author Peter Nordstrom, at the Umea University in Umea, Sweden.

The researchers also examined siblings and found no link between one sibling having depression and the other having Parkinson's disease.

"This finding gives us more evidence that these two diseases are linked," Nordstrom added.

"If the diseases were independent of each other but caused by the same genetic or early environmental factors, then we would expect to see the two diseases group together in siblings, but that did not happen," he continued.

For the study, researchers started with all Swedish citizens age 50 and older at the end of 2005.

From that, they took the 140,688 people who were diagnosed with depression from 1987 to 2012.

These people were then matched with three control participants of the same sex and year of birth who had not been diagnosed with depression, for a total of 421,718 control participants.

People with depression were 3.2 times more likely to develop Parkinson's disease within a year after the study started than people who did not have depression.

By 15 to 25 years after the study started, people with depression were about 50 percent more likely to develop Parkinson's disease.

The paper appeared in the journal Neurology, the medical journal of the American Academy of Neurology.