Depression: epidemiology

The close relationship between depression and diabetes has been recognised since the 17th century when Thomas Willis noted that diabetes often appeared among patients who had experienced ‘significant life stress, sadness or long sorrow’. However, it was not until the late 20th century that epidemiological studies started to discover the complexities of the interrelationship between these two conditions.

Thomas Willis
Thomas Willis

Prevalence of depression in diabetes

It is now commonly accepted that the prevalence of depression, the total number of cases at a given time, is increased in people with diabetes. A meta-analysis of 42 studies has indicated that overall the odds of depression in people with diabetes are double that of people without diabetes, Odds Ratio (OR) = 2.0, 95% CI 1.8 – 2.2)[1]. In studies using self-report measures of depression, the pooled point prevalence of depressive symptoms was about 26% in people with diabetes compared to just over 14% in people without diabetes. When diagnostic criteria were used (DSM-IV, ICD-10) the point prevalence of major depressive disorder was 9% in the diabetes population versus 5% in the general population. However, the odds ratios did not differ by type of diabetes, sex or depression assessment method.

In a more recent meta-analysis that only included studies on depression in type 2 diabetes, a slightly lower odds ratio was found (OR = 1.59, 95% CI 1.5 – 1.7) [2]. This study found that, in concordance with data from the general population[3], the pooled point prevalence was higher for women with type 2 diabetes than for men (23.8% vs. 12.8%, respectively). However, the odds of depression was higher for men with type 2 diabetes (OR=1.9, 95%CI 1.7; 2.1) than for women (1.3, 95%CI 1.2, 1.4).

To date, no meta-analysis has been carried out that specifically looked at depression in type 1 diabetes. However, a systematic review of 4 controlled studies reported that the prevalence of clinical depression was 12.0% for people with diabetes compared to 3.2% in people without diabetes[4].

The increased prevalence of depression in diabetes is a global phenomenon. In a survey, carried out in 60 countries across the globe, 1-year prevalence of ICD-10 depressive episode in people with diabetes was 9.3% (95%CI 7.3 to 11.3) compared to 3.2% in people without a co-morbid condition[3]. However, the prevalence of depression was greater in people with arthritis (10.7%, 95%CI 9.1 to 12.3), angina (15.0%, 95%CI 12.9 to 17.2) and asthma (18.1%, 95%CI 15.9 to 20.3)[5]. There appears to be large variability across countries with prevalence of possible depressive episodes in people with diabetes ranging from 0% in the Comoros and Vietnam to 53% in Brazil and a whopping 78% in the Congo[6]. How much this reflects true differences in prevalence rate rather than case ascertainment is not certain.

The prevalence of depression can also vary between the sub-populations within countries. For example, more African Americans with diabetes suffer from depression than their counterparts of White Northern European ancestry[7][8]. Other studies have found even higher levels of co-morbid diabetes and depression in the US Hispanic population[9][10][11][12] and in Native Americans[13] although a more recent study among Pima Indians in Arizona found no significant differences in depressive symptoms between those with and without diabetes[14]. It is likely that differences in cultural and social dynamics along with social stressors shape the way illness and health are experienced.

In summary, there is now firm evidence that the prevalence of depression is increased in people with type 1 and type 2 diabetes. This increase is seen in studies from around the world. However, levels of depression may vary between countries and between populations within countries and between the sexes.

Depression in undiagnosed diabetes

In contrast to the increased risk of depression in people with known diabetes, a recent meta-analysis showed that the risk of depression was not increased in people who have diabetes but are unaware of having the condition. Compared to people with normal glucose metabolism, the risk of elevated depressive symptoms did not differ in people with undiagnosed diabetes (OR = 0.94, 95% CI 0.71, 1.25)[15]. This result could not be attributed to differences in blood glucose levels between diagnosed and undiagnosed diabetes. The risk of depressive symptoms in people with impaired glucose metabolism was also similar to those with normal glucose metabolism (OR 0.96; 95% CI 0.85–1.08).

Thus, despite having higher blood glucose concentration than people with normal glucose metabolism in people with impaired glucose metabolism and undiagnosed diabetes, the evidence, to date, seems to indicate that higher blood glucose levels per se are not associated with an increased level of depressive symptoms. Rather, these findings are consistent with the hypothesis that the psychological burden of having to live with and manage diabetes and cope with its complications contributes to higher levels of depression. However, this meta-analysis only included a small number of studies (N=11) and further research is needed before firm conclusions can be drawn, not least because one study from the UK showed a linear association between blood glucose and depressive symptoms[16].

It is currently unclear how and when depression develops after diagnosis of type 2 diabetes and results are equivocal. While some studies found no increased risk of depressive disorder in people with newly diagnosed diabetes in comparison with non-diabetic cohorts[17][18], a large prospective study found that the risk of depressive disorder increased in the two years after diagnosis of type 2 diabetes even in the absence of diabetes complications [19]. Other studies found no overall increases of depression after the diagnosis of diabetes but also found that a significant number of people had persistent depressive symptoms during the first year after diagnosis[20]or that use of antidepressant medication was increased[21]. While there are multiple risk factors for the development of depression in people with diabetes [22], diabetes complications, and in particular microvascular and macro-vascular disease [23], and more intensive treatment for diabetes[24] figure prominently amongst them.

A bi-directional relationship between diabetes and depression?

It is hypothesised that the temporal relationship between depression and type 2 diabetes is bi-directional in nature with depression increasing the risk of developing type 2 diabetes, and type 2 diabetes increasing the risk of depression.

To date, no study has examined whether depression is a risk factor for the development of type 1 diabetes. In contrast, the incidence of type 2 diabetes as a result of depression has been the topic of numerous studies. Four meta-analytic studies[25][26][27] concluded that depression was associated with a significantly increased risk of incident type 2 diabetes. In the most recent meta-analysis[27], which included 22 studies, a moderately increased the risk of incident type 2 diabetes was found in depressed versus non-depressed individuals (adjusted OR = 1.38; 95%CI 1.23-1.55).

The reverse temporal relationship that diabetes may be a risk factor for depression has been the focus of two meta-analyses [26][28]. The more recent one, including 11 studies, found that the risk of developing depression was modest in people with type 2 diabetes (OR =1.24; 95% CI 1.09–1.40). However, this risk was significantly higher for studies relying on diagnostic criteria than for studies using self-report depression questionnaires (OR=1.47; 95%CI 1.34-1.60)[23]. See Figure 1.

Figure 1. Forest plots showing the RR of depression in type 2 diabetes for individual studies using depression self-report questionnaires, diagnostic criteria, and all studies combined.  Bars and the diamond indicate 95% CIs.  The size of the squares corresponds to the weight of the study in the meta-analysis[23].
Figure 1. Forest plots showing the RR of depression in type 2 diabetes for individual studies using depression self-report questionnaires, diagnostic criteria, and all studies combined. Bars and the diamond indicate 95% CIs. The size of the squares corresponds to the weight of the study in the meta-analysis[23].

To date, only one longitudinal study from the US has examined the development of depression in type 1 diabetes[29]. Repeated assessments of psychiatric problems showed that 27.5% of the cohort developed major depression over the 10-year study period with the majority during the first year after diagnosis of type 1 diabetes. Unfortunately, there was control group of people without diabetes included in this study.

Recurrence and persistence of depression

Recurrence of depression is common in people with diabetes, and episodes are likely to last longer[30][31][32]. However, persistence of depression is more likely among for people with type 2 diabetes having high depressive symptoms, compared to those diagnosed with major depressive disorder[33].

Only a few studies have examined the recurrence and persistence of depression in type 1 diabetes. One US study found that although episodes of major depressive disorder lasted longer in adolescents with type 1 diabetes than in control participants, no differences in recovery rates were found[29]. Having type 1 diabetes was found to lower the risk of persistent depression in another US study [32].


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