A new study says that depression nearly doubles a person's risk of developing dementia later in life.
To reach the conclusion, researchers examined research data on 949 people with an average age of 79 from the Framingham Heart Study.
At the start of the study, participants were free of dementia and were tested for depressive symptoms based on questions about general depression, sleep complaints, social relationships and other factors.
A total of 125 people, or 13 percent, were classified as having depression at the start of the study.
The participants were followed for up to 17 years.
At the end of the study, 164 people had developed dementia with 136 specifically diagnosed with Alzheimer's disease. Nearly 22 percent of people who were depressed at the start of the study developed dementia compared to about 17 percent of those who were not depressed, a 70 percent increased risk in those who were depressed.
The 10-year absolute risk for dementia was 0.21 in people without depressive symptoms and 0.34 in people with depressive symptoms.
The results were the same regardless of a person's age, sex, education and whether they had the APOE gene that increases a person's risk of Alzheimer's disease.
"While it's unclear if depression causes dementia, there are a number of ways depression might impact the risk of dementia," said study author Jane Saczynski, with the University of Massachusetts Medical School in Worcester, MA.
"Inflammation of brain tissue that occurs when a person is depressed might contribute to dementia.
ertain proteins found in the brain that increase with depression may also increase the risk of developing dementia. In addition, several lifestyle factors related to long-term depression, such as diet and the amount of exercise and social time a person engages in, could also affect whether they develop dementia," Saczynski added.
Saczynski hopes the study, which is one of the largest and longest population based studies to date, helps clear up confusion over earlier studies that reported inconsistent results about the link between depression and dementia.
The research will be published in the July 6, 2010, issue of Neurology, the medical journal of the American Academy of Neurology.