Data from the long-running Framingham Heart Study has revealed that depression significantly increased the risk of developing dementia. Of the 125 people (13%) who were classified as having depression at the start of the study, 21.6% had developed dementia by the end of the study (17 years later). This compares to around 16.6% of those who weren’t depressed. When age, gender, education, homocysteine, and APOE gene status were taken into account, depressed participants had a more than 50% increased risk of developing dementia. Moreover, for each 10-point increase on the self-report scale used to measure depression (CES-D), there was a significant increase in the dementia risk. These findings, from one of the largest and longest population-based studies, should clarify the inconsistent results from earlier research.
There are several possible ways depression might increase the risk of dementia — for example, through the brain inflammation or the increased level of certain proteins that occurs during depression; or through the effects on lifestyle (reduced exercise, social engagement, poor diet).
 . Depressive symptoms and risk of dementia: The Framingham Heart Study. Neurology [Internet]. 2010 ;75(1):35 - 41. Available from: http://www.neurology.org/cgi/content/abstract/75/1/35