The bidirectional relationship of depression and diabetes: A systematic review
Highlights
► Depression and diabetes are serious chronic conditions common in Western cultures. ► Despite common comorbidity, the nature and direction of this relationship is unknown. ► Depression may be a consequence of diabetes due to the burden of chronic illness. ► Depression may be a risk factor for development of diabetes due to multiple factors. ► This paper reviews the current literature behind both lines of investigation.
Introduction
Diabetes and depression are serious chronic diseases that negatively affect quality of life, increase functional disability, and reduce life expectancy (Goetzel et al., 2003, O'Connor et al., 2009). According to the most recent comprehensive assessment of diabetes in the United States, approximately 25.8 million people in the United States (8.3% of the population) have diabetes (Centers for Disease Control and Prevention, 2011). Type 2 Diabetes Mellitus (T2DM) is the most common form of diabetes, and is characterized by high levels of blood glucose resulting from impaired insulin production by the pancreas and/or insulin resistance in the body's cells (American Diabetes Association, 2010). T2DM accounts for 90% to 95% of newly diagnosed cases of diabetes in adults (CDC, 2011) and the risk of developing this form of diabetes increases with age, obesity, and lack of physical activity (ADA, 2010). Diabetes, if unmanaged or poorly managed, can lead to serious medical complications such as limb amputation, advanced periodontal disease, blindness, neuropathy, cerebrovascular disease, kidney failure, and premature death (Kinder, Kamarck, Baum, & Orchard, 2002).
Depressive disorders are some of the most common of the psychiatric disorders, with a recent survey of 38 states reporting the overall prevalence of current depressive symptoms to be 8.7% (range by state and territory, 5.3%–13.7%) and 15.7% lifetime prevalence rate of a diagnosis of a depressive disorder by a doctor or health care provider (Strine et al., 2008). Major Depressive Disorder (MDD), the most serious form of the unipolar depressive disorders, is the most prevalent of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR; American Psychiatric Association, 2000) disorders among adults aged 18 years old and older, with a lifetime prevalence of 16.6% (Kessler et al., 2005). The nature of depression is such that sufferers experience dysphoric mood, loss of interest or pleasure, appetite and sleep disturbances, and changes in energy level (American Psychiatric Association, 2000). Thus, decreases in self-care behavior, such as decreased medication adherence, poor nutrition, and lack of exercise, are often associated with depression (Lin et al., 2004).
Furthermore, these common conditions are frequently comorbid (Knol et al., 2007). Research demonstrates that people with diabetes (either type 1 or type 2) are twice as likely to have depression compared to people without diabetes (de Groot, Anderson, Freedland, Clouse, & Lustman, 2001). Prevalence rates vary depending on the assessment method utilized; Anderson, Freedland, Clouse, and Lustman (2001) estimate that anywhere between 11% (based on standardized diagnostic interviews) and 30% (assessed by self-report questionnaires) of patients with diabetes have comorbid depression.
Of particular interest to researchers and clinicians is the nature of the relationship between depression and diabetes. One line of research seeks to demonstrate that depression is a consequence of diabetes. For instance, recent research indicates that people with diabetes are more likely to develop depression than those without diabetes, perhaps due in part to the burden of chronic disease or because of the biochemical changes that occur in diabetes (Kinder et al., 2002, Knol et al., 2007). Another line of investigation indicates that depressive symptoms are a risk factor for the development of diabetes. This relationship may be due in part to a decline in health-maintenance behaviors among depressed persons (Barbour and Blumenthal, 2005, Golden et al., 2008, Katon, Von Korff, Ciechanowski, et al., 2004, Kinder et al., 2002) or biochemical changes associated with depression (Bjorntorp, 2001, Knol et al., 2006).
Despite these promising, yet somewhat contradictory theories, the exact nature of the relationship, and the direction of the relationship, remain unknown. Lustman and Clouse (2007) liken this relationship to the chicken-or-the-egg conundrum—which came first? It is likely that the answer is not as simple as a unidirectional relationship, and rather that the comorbidity of depression and diabetes is part of a bidirectional interaction between the conditions. The implications for understanding this relationship are critical for public and personal health, as both conditions result in increased medical costs and complications. A synthesis of relevant literature is a necessary step toward increasing clinical awareness, which can lead to appropriate screening and treatment planning for these serious comorbid conditions.
This paper presents a review of the current literature on the complex relationship between diabetes (specifically, T2DM) and depressive disorders. To our knowledge, no review to date has integrated all relevant aspects of research findings that support depression both as a risk-factor for and as a consequence of diabetes. Clinical implications, as well as areas for future research, are discussed.
Section snippets
Method
To identify relevant literature, electronic databases PsycINFO (1872–July 2011) and PubMed (1966–July 2011) were searched for English language articles in peer-reviewed journals. Search terms included combinations of the following terms: “diabetes,” “direction,” “comorbid,” “relationship,” “risk factor,” and a truncated form of “depression.” The reference lists of earlier reviews and included studies were also examined. Studies were included if they were relevant to the scope of the review,
Depression as a consequence of diabetes
One line of research lends support to the hypothesis that an increased risk of depression is a consequence of diabetes. This association between T2DM and the onset of depression is often conceptualized as having two possible mechanisms. First, the psychosocial burden of a chronic disease such as T2DM may carry with it a risk for developing depressive symptoms (Knol et al., 2007). Cognitions related to diabetes, such as perceived disability and awareness of having a chronic illness, may impose
Depression as a risk factor for diabetes
The idea that depression is a risk factor for development of diabetes suggested by Talbot and Nouwen (2000) is the second mechanism for the diabetes/depression comorbidity presented in the literature. Knol et al. (2006) sought to establish a temporal relationship between depression, or elevated depressive symptomatology, and T2DM in their meta-analysis. The authors examined nine empirical studies that investigated the longitudinal relationship between depression and onset of T2DM. Their
Evidence for a bidirectional relationship
In light of the evidence reviewed, it is likely that the answer is not as simple as a unidirectional relationship. An oft-cited meta-analysis by Anderson et al. (2001) demonstrated that people with either type of diabetes were twice as likely as nondiabetics to be depressed. However, as the purpose of this study was to estimate the prevalence of comorbid depression and diabetes, the authors did not examine depression's role as a risk factor or consequence of diabetes. Instead, the authors
Limitations of current research
In most of the studies reviewed herein, changes in depressive symptoms were reported in terms of statistical significance and not clinical significance. In other words, when changes in depression scores or depressive symptomatology were reported, it is unknown if these changes were enough to effectively change a person's clinical picture or diagnosis. Also, the studies reviewed herein classified and assessed depression in a multitude of fashions, making it difficult to draw conclusions about
Clinical implications
The review of literature presented herein presents the high rate of comorbidity of depression and diabetes. The need for clinicians to recognize this is imperative, particularly in primary care settings. For example, the US Preventive Services Task Force (2009) estimates that up to half of all adults and nearly two-thirds of older adults who receive treatment for depression receive it in a primary care setting, and in these settings, the prevalence of MDD ranges from 5% to 13% in adults and
Summary
As we have reviewed, a significant literature provides well-documented support for a relationship between diabetes (specifically, T2DM) and depressive symptomatology. However, the present paper comprises the first attempt to carefully compare the relevant literature examining the depressive characteristics between people with and without comorbid diabetes mellitus. Still, the exact direction of this relationship remains unclear. The biochemical and physiological changes associated with
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