In an article published in the Journal of the American Medical Association Psychiatry, researchers have provided evidence that people with both diabetes and depression have an even greater risk of developing dementia than the combined risk of people who have either diabetes or depression.
To identify people who had diabetes, depression, and eventually, dementia, scientists used various Danish national registries. Scientists identified people who had Type II diabetes from a national diabetes register. People with depression were identified if they received a diagnosis from a psychiatrist and were listed in a Danish psychiatric research register, or if they were prescribed anti-depressants, listed in a national prescription registry. People with dementia were identified by a physician diagnosis, listed in national patient or psychiatric research registers or if they were prescribed specific drugs, listed in the national prescription registry. Researchers then used statistical modeling to estimate the risk of developing dementia associated with having diabetes, depression, or both, compared to people who had neither diabetes nor depression.
THE FINDINGS
Included in the study were over 2.4 million Danish citizens aged 50 years and older who did not initially have dementia. Among them, about one in five had depression, with an average age of first diagnosis of 59 years old. About one in ten people had diabetes, with an average age of diagnosis of 63 years old. Finally, about one in twenty people had both depression and diabetes.
During the duration of the study, about one in fifty people were diagnosed with dementia at an average age of 81 years old. Of these, about one in five had depression, about one in ten had diabetes, and about one in sixteen had both depression and diabetes.
Having both diabetes and depression was associated with a 117% greater risk of being diagnosed with dementia
The effects of being diagnosed with depression on the risk of developing dementia
Within the first year of being diagnosed with depression, the researchers reported a nearly 7-fold increase in the risk of being diagnosed with dementia. Between 1 and 2 years after a diagnosis of depression, the risk of developing dementia decreased to about 3.5-fold. From 6 years after a depression diagnosis to over 10 years after, the risk of developing dementia stabilizes at about a 2-fold greater risk than those people who did not have depression.
The effects of being diagnosed with diabetes on the risk of developing dementia
Within the first year of being diagnosed with diabetes, the researchers reported a 30% increased risk of being diagnosed with dementia. Between 1 and 6 years following a diagnosis of diabetes, the risk of developing dementia decreases to just slightly greater than those people who do not have diabetes. However, from 6 to 10 years from a diabetes diagnosis, the risk of developing dementia increases again. From 10 or more years since a diabetes diagnosis, there was a 42% increase in the risk of developing dementia, compared to those people who did not have diabetes.
The effect of age on the risk of developing dementia
In their study, researchers compared the effects of having diabetes and/or depression on the risk of developing dementia in people under 65 years old, and people aged 65 years and older. People who had depression and were under 65, had an almost 3-fold increase in the risk of developing dementia. For people over 65 who had depression, the risk of developing dementia was almost double that of people who did not have depression.
People who had diabetes and were under 65, had an almost 2-fold increase in the risk of developing dementia. People over 65 who had diabetes, had a slightly greater risk of developing dementia than those who did not have diabetes. Finally, people who had both depression and diabetes, and were under 65 years of age had an almost 5-fold increase in the risk of developing dementia. People over 65 who had both depression and diabetes, had about a 2-fold greater risk of developing dementia than people who had neither depression nor diabetes.
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Summary of results
Researchers reported that having diabetes, having depression, and having both diabetes and depression were all associated with an increased risk of developing dementia. Having diabetes was associated with a 20% greater risk; being diagnosed with depression was associated with an 83% greater risk; and having both diabetes and depression was associated with a 117% greater risk of being diagnosed with dementia.
RELEVANCE OF THE RESEARCH
This study addresses a gap in the literature because prior to it being published, it was not known whether the risk of developing dementia in people who had both diabetes and depression is the same as when you add the risks of those people who have either diabetes or depression, or whether there was an increased risk of dementia. Having both depression and diabetes was reported to be associated with an even greater risk of developing dementia than the combined risk of having either diabetes or depression. This effect was larger still for people under 65 years of age.
The results of this study further advance our knowledge by providing evidence that having depression is associated with the highest risk of developing dementia relative to those who do not have depression or diabetes. In a separate study that aimed to create prevention programs to decrease the rate of new cases of dementia, it was reported that both an improved diet and effective treatment of diabetes and depression may reduce dementia diagnoses by 20%.