Why depression and diabetes often go together ...but which comes first?

While researchers have long been aware that type-2 diabetes and depression often go together, which develops first has always been unclear.

A new study by researchers at the Johns Hopkins University, will hopefully shed some light on this chicken or egg first dilemma.

This new study suggests there is a dual answer - patients with depression have an increased risk of developing type-2 diabetes, and patients with type-2 diabetes have an increased risk of developing depression.

For the study diabetes expert Dr. Sherita Hill Golden and her colleagues used data generated by the Multi-Ethnic Study of Atherosclerosis (MESA).

MESA examined the risk factors for atherosclerosis, or hardening of the arteries, in an ethnically diverse group of 6,814 men and women between ages 45 to 84 who identified their cultural background when they enrolled.

The participants made three visits to clinics over the course of three years to be examined for various atherosclerosis risk factors, including type-2 diabetes and symptoms of depression, which could serve as a precursor for full-blown clinical depression.

The study also collected information on other atherosclerosis risk factors, such as body-mass indices (BMI), blood pressure, diet and exercise patterns and smoking habits, as well as other information associated with health in general, such as income and socio-economic factors.

Excluded from their analysis were all participants who had high fasting glucose, an indication of diabetes, at the initial clinic visit and the team then looked to see whether participants who initially had elevated symptoms of depression, as indicated through a questionnaire, were more likely than those who didn't, to develop high fasting glucose at the end of the three-year study period.

The results showed that those with elevated depressive symptoms were 42 percent more likely overall to develop diabetes by the end of the study than those without these symptoms and the stronger the symptoms, the higher the risk of diabetes.

Even when factors such as overweight, lack of exercise, and smoking, were accounted for the risk of developing diabetes was still 34 percent higher for patients with depressive symptoms.

The same pool of information was then used to investigate whether diabetes could lead to depression and those who had elevated depressive symptoms at the initial clinic visit, were excluded.

The researchers then checked whether those who had high fasting glucose - with or without a formal diagnosis of diabetes - were more likely to develop depressive symptoms by the end of the study.

The researchers found that patients treated for diabetes were about 54 percent more likely to develop elevated depressive symptoms than those without diabetes.

A surprising result the researchers are unable to explain was that those with prediabetes or untreated diabetes were about 25 percent less likely to develop elevated depressive symptoms than people with normal fasting glucose.

Dr. Golden, an associate professor of medicine and epidemiology at Johns Hopkins, speculates that depression may lead patients to develop behaviours that trigger diabetes or make it worse, such as overeating, not exercising or smoking.

Similarly, keeping up with the often extensive treatment regimens to care for their diabetes may make patients’ depression worse.

Dr. Golden says having both diabetes and depression can make it difficult for patients to get good clinical outcomes and understanding how one condition might lead to another could improve treatments for both problems.

The study is published in the June 18th issue of the Journal of the American Medical Association, and was supported by grants from the National Heart, Lung, and Blood Institute.