Diabetes and depression have each been identified as independent hazards to healthy brain aging – and depression is also known to raise the likelihood of diabetes. But what is the risk of poor cognitive fitness later in life for those people who have both diabetes and depression?

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Healthy cognitive aging is put in danger by depression and diabetes, especially both combined, the study suggests.

A study in the journal JAMA Psychiatry gives an answer to that question after finding that the risk of dementia was stronger among people with comorbidity than it was from depression or type 2 diabetes alone.

Data on medical databases derived from nearly 2.5 million people were analyzed by a team including Dr. Dimitry Davydow of the University of Washington School of Medicine in Seattle. The authors conclude:

“We found that depression and diabetes mellitus were both associated with a greater risk for all-cause dementia, Alzheimer disease and vascular dementia.

“These associations appeared to be stronger among those individuals with depression alone compared with those with diabetes alone.”

In the large Danish cohort study, 95,691 individuals had both depression and type 2 diabetes (3.9% of 2,454,532 adults). Compared with adults having neither condition:

  • Adults with both showed a 117% greater risk of dementia by the end of the study
  • Those with depression alone were associated with an 83% larger risk
  • And those with type 2 diabetes but not depression were linked to 20% more dementia risk.

The comorbid risk for dementia was stronger even than the risk suggested by adding together the separate depression and diabetes factors, concludes the study.

In other words, beyond the additive effect of each pure risk, there appeared to be an extra contribution towards dementia brought by an interaction between depression and diabetes.

The study started at the very beginning of 2007 to include all living people of 50 years of age and older who did not have dementia. The data were then followed up to the very end of 2013.

During that period, to put the above relative risk increases into perspective, 2.4% (59,663 people) developed dementia, with an average age at diagnosis of nearly 81 years. Of these people getting the cognitive decline brain diseases:

  • 26.4% (15,729 people) had depression alone and 10.8% (6,466) had type 2 diabetes alone
  • 6.7% (4,022) had both conditions.

The conclusion of the study considers the potential importance of preventive measures:

The interaction between diabetes mellitus and depression tended to be particularly strong for individuals younger than 65 years.

In light of the increasing societal burden of chronic diseases, further research is needed to elucidate the pathophysiologic mechanisms linking depression, diabetes mellitus and adverse outcomes such as dementia and to develop interventions aimed at preventing these dreaded complications.”

Why depression is linked to diabetes may be an easier question to answer and offers clues to the more difficult question of why depression is linked to dementia. The study authors cite, for example, “poorer adherence to diet, smoking cessation, exercise and medication regimens to control” diabetes.

“Depression is also associated with increased cortisol levels, autonomic nervous system dysregulation, and increased inflammation, all of which worsen glycemic control,” the authors say.

In a linked commentary, Dr. Charles Reynolds looks at healthy aging more widely and begins to expand on possible explanations for the links between depression and the biological diseases of brain aging. The cognitive decline as a result of dementia includes Alzheimer’s disease caused by neural changes; and vascular dementia caused by cerebrovascular disease, from the same disease process that leads to cardiovascular disease and heart attacks.

Dr. Reynolds writes, for example: “We do not yet know whether the treatment of prevalent cases of depression protects brain health and delays the onset of dementia, although this hypothesis is plausible.”

The specialist in geriatric psychiatry, of the University of Pittsburgh Medical Center, also develops the idea that poor mental health has direct effects on biological wellbeing, citing as the authors do, the effects on glucose control and diabetes management, but also citing less direct factors than diet and exercise, including:

“Inadequate sleep can induce carbohydrate craving, resulting in weight gain and worsening glycemic control, and can be a risk factor for depression.”

The commentary ends with:

In conclusion, the study by Katon and colleagues illustrates the need for convergent scientific approaches to meet the challenge of promoting healthy brain aging and cognitive fitness into the last years of life.

The convergence of expertise from epidemiology, behavioral and basic science in the biology of aging and brain health are all necessary ‘to move the needle’ in the demographic challenge that confronts the entire globe.”