Major depression and type 2 diabetes mellitus are common chronic illnesses within the general US population, with prevalence rates of approximately 5% to 10% and 11%, respectively.^1,2 Moreover, depression and type 2 diabetes mellitus, individually, can be among the most disabling chronic disorders one can acquire, and when they occur comorbidly, they are even more detrimental. Together they exhibit a bidirectional relationship, with each disease an independent risk factor for development of the other.

In the presence of diabetes, the prevalence of depression increases to 15% to 30% depending on depression definition, population sample, and study type.^3,4 In depressed cohorts, the risk of developing diabetes has been shown to be twice that of nondepressed persons.^4,5 The bidirectional relationship between depression and diabetes is not limited to its effects on prevalence; instead, studies show that when these diseases are comorbid, they significantly amplify the cost, morbidity, and mortality expected from either one alone.

In the case of depression, changes in blood sugar levels have been linked directly to moods such as anger, anxiety, sadness, frustration, and even general well-being—common themes in depressed patients.⁶ Clinically, more than 70% of patients with diabetes have depressive episodes that last longer than 2 years.⁷ Dysthymia and double depression may also be more common in patients with diabetes. This was demonstrated in a large cohort of diabetic, primary care patients, well over half of whom had dysthymia.⁷ MDD is highly recurrent in diabetic patients—nearly 80% of depressed persons with diabetes experienced a relapse of symptoms, with an average relapse rate of nearly 1 episode every year.⁸

Patient disability experienced in the home, workplace, or otherwise is exacerbated by the relationship between comorbid diabetes and depression. Studies that examined disability have shown not only that depression itself is one of the most disabling chronic conditions but also that the functional impairment it causes is substantially worse when it occurs in the context of another chronic disease, such as diabetes.

The course of diabetes becomes profoundly worse in depressed patients. Depression in diabetes has been associated with decreased self-care, including decreased adherence to exercise, medications, smoking cessation, and eating a healthy diet.⁹ Depression contributes to the pathophysiology of diabetes by leading to greater body mass index, higher hemoglobin A_1c (HbA_1c) levels, and increased medical comorbidity.⁷ It has also been associated with a myriad of adverse outcomes, including microvascular complications such as retinopathy, nephropathy, neuropathy, and sexual dysfunction, as well as the macrovascular complications of coronary artery and cerebrovascular disease.¹⁰

Depression in diabetic patients is also a risk factor for dementia, hospitalization, and even death. In the case of dementia, diabetes and depression are independent risk factors for vascular and Alzheimer-type dementias, and comorbid they impart substantially more risk than either one alone. Effective treatment of depression and diabetes may be one of the most powerful interventions available for decreasing the prevalence of dementia. Treating depression in patients with diabetes and dementia may help slow the rate of functional and cognitive decline over time.

Not surprisingly, depression also contributes to decreased quality of life and increased costs of diabetes care. In one study of Medicare beneficiaries, the costs of the care for depressed versus nondepressed patients with diabetes was 4.5-fold higher.¹¹ A second study of mixed-age patients with diabetes found that comorbid depression severity influenced costs as well; costs for severely depressed patients were 86% higher than those for patients with less severe depressive symptoms.⁹

Diagnosis

The diagnosis of depression in the context of diabetes and other chronic medical conditions can be challenging. Many of the symptoms of depression can overlap with a chronic disorder such as diabetes, and depression itself is associated with an increased likelihood of the patient experiencing diabetic symptoms. Examples of overlapping symptoms include fatigue or decreased energy, change in appetite or weight, difficulty in concentration, and sleep disturbances.

Some general concepts can be helpful when thinking about depression in diabetes. Dividing the symptoms of depression into somatic (energy, sleep, appetite/weight, concentration) and psychological (mood, interest, suicidal thoughts, guilt, worthlessness) is important. The psychological symptoms of depression have little overlap with the signs or symptoms of diabetes, which make them particularly specific to depression in this context.

This is supported by studies that used the Beck Depression Inventory (BDI) to identify which symptoms successfully differentiated between the depressed and nondepressed, medically ill patients. Study findings included 6 symptoms that were discriminatory: a sense of failure, loss of social interest, feelings of being punished, suicidal ideation, dissatisfaction, and indecisiveness.^12-14

The somatic symptoms of depression are less specific to depression in patients with diabetes; however, that does not mean that they are without value. While the mere presence or absence of somatic symptoms is less helpful, there are many characteristics of these symptoms, such as severity, timing, etiology, and proportion, that can be useful. The more severe the somatic symptom, the more specific it is to depression.