The debilitating disorder Parkinson's disease may be more likely to occur in depressed individuals.
Researchers in Taiwan tracked more than 22,000 adults for 10 years, and found those who had depression were more than three times more likely to develop Parkinson's than those who were not depressed.
"Depression is linked in other studies to illnesses such as cancer and stroke," Dr. Albert C. Yang, a psychiatrist with the Taipei Veterans General Hospital in Taipei, Taiwan, said in a statement. "Our study suggests that depression may also be an independent risk factor for Parkinson's disease."
Parkinson's disease is caused by the loss of a brain chemical called dopamine, which is responsible for muscle and motor control and other essential functions. Symptoms of the disease include tremors, stiffness, slowness of movement, posture problems and troubles with balance and coordination.
The disease is degenerative, meaning it gets worse over time. Tremors may eventually interfere with daily activities like walking, talking or eating, leading to permanent disability. Risk factors for Parkinson's include older age, heredity, and being exposed to toxins like pesticides, according to the Mayo Clinic. It is more common in men than in women.
Depression and Parkinson's disease have well-established links. Up to 60 percent of people with Parkinson's experience mild or moderate symptoms of depression, according to the Parkinson's Disease Foundation, with some research suggesting Parkinson's may cause chemical brain changes leading to a depressive disorder.
People with both illnesses tend to have more movement problems and anxiety than those who have just one of the conditions, the National Institute of Mental Health adds.
But could depression itself -- which affects about 7 percent of U.S. adults -- be what's leading to Parkinson's?
Researchers looked at more than 4,600 people with depression and more than 18,500 people without depression for 10 years to determine the number of patients who developed Parkinson's. People who developed Parkinson's within two to five years of their depression diagnosis were excluded from the analysis, because depression previously has been shown to be an early sign of Parkinson's.
After 10 years, 66 people with depression developed Parkinson's, while 97 people without depression did. That's 1.42 percent of the depressed people and 0.52 percent of the non-depressed group.
While researchers found triple the risk for people with depression, the overall rates were still low.
Yang pointed out to CBSNews.com that the study showed two groups in particular appeared to be at higher risk for Parkinson's: elderly people over 65 with depression, and people with difficult-to-treat-depression, who were defined as having to alter their antidepressant treatment regimen two or more times.
"The important message is that not all depression leads to Parkinson's," said Yang.
The study was supported by Taipei Veterans General Hospital and Taiwan National Science Council.
The authors did not look at what in the brain explains this link, but Yang speculates that both diseases' effects on neurotransmitters like dopamine and serotonin may play a role.
If a person falls into one of these two high-risk groups, they may benefit from speaking to their doctor to develop a carefully-monitored treatment plan that may include antidepressants and other therapies. Antidepressant use itself was not associated with an increased risk for Parkinson's.
"For elderly depression, it's also good to screen for any neurological disease as depression may be a red flag," Yang added.
The study was published Oct. 2 in Neurology.