is also a risk factor for subsequent depression (Nouwen et al, 2010). This suggests the two conditions share common pathways as both are associated with physical inactivity, obesity and cardiovascular disease.
Course of depression
Few studies have examined depression in people with diabetes but the available evidence suggests depression is more chronic in people with diabetes than in general population samples (Winkley, 2008). For example, 64% of people with both depression and diabetes have depression with a relapse-remitting course (relapse is defined as a worsening of mood within a nine-month period), 15% never recover from their depression, and only around 20% recover fully (Lustman et al, 1997).
Adverse effects of depression
The effects of depression can be detrimental to health, quality of life and personal relationships. However, when people also have diabetes, it is common for them to increase their use of health service resources and reduce their adherence to their diabetes self-management programme, such as eating less healthily, reducing physical activity and not adhering to oral medications. Cognitive dysfunction can also be problematic and is common with diabetes and depression.
It is not fully understood why risk of mortality is higher for people with diabetes and depression, but there is consistent, international evidence suggesting a link. In both the UK and US, studies confirm that affected patients are two to five times more likely to die earlier when they have both comorbid conditions (Ismail et al, 2007; Black et al, 2003).
In the UK, a cohort of patients with their first diabetic foot ulcer and with minor and major depression were three times more likely to die at 18-month follow-up and these effects persisted at five years (Winkley et al, 2012; Ismail et al, 2007). In the US Pathways study, a cohort of 4,385 people with type 2 diabetes followed up for three years found a 1.67-fold increase in mortality in those with minor depression and a 2.3-fold increase in those with major depression (Katon et al, 2005) compared with those with no depression.
The US National Health and Nutrition Examination Survey found that diabetes patients with depression were 2.5 times more likely to die from all-cause mortality and 2.43 times more likely to die from coronary heart disease than the general population (Egede et al, 2005). Finally, a longitudinal study of Mexican Americans with type 2 diabetes aged 65 years and over were found to be almost five times more likely to die if depressed at a seven-year follow-up and were significantly more likely to develop an earlier onset of diabetes complications than those without depression (Black et al, 2003).
Having depression and diabetes has significant costs to individuals and society but the mechanisms that start to explain why this is the case are complex and thought to involve interactions between biological, psychological and social processes.
Simple psychological explanations suggest people with depression and diabetes are less likely to look after themselves and perform adequate self-management, evidence of which is seen in the literature.
Social explanations might also add to this, for example if people are depressed they may be more likely to be socially isolated, without social support and perhaps at a disadvantage in maintaining good personal relationships. This can have far-reaching consequences, limiting their capacity for work and study, and increasing their chances of experiencing poverty.
Biological explanations suggest depression influences cardiovascular health and macrovascular complications through activation of the hypothalamic pituitary adrenal axis or through immune system dysfunction (McCaffery et al, 2006). In depression, the hypothalamic pituitary axis is overstimulated and the associated sympathetic response may increase cardiovascular risk in several ways. For example, more counter-regulatory stress hormones that raise blood-glucose levels are released leading to insulin resistance (Brotman et al, 2007); there is also increased platelet activity 