Depression and diabetes are highly prevalent in the U.S. Over 6.5% of the U.S. adult population has been diagnosed with diabetes (1), and the incidence of type 2 diabetes is increasing, in part due to the national increase in obesity. Approximately 16% of U.S. adults will suffer major depressive disorder at some point in their lives, and this proportion is greater when other forms of depressive disorder, such as dysthymia and
minor depression, are included (2). Thus, the hypothesis that depression and diabetes are causally related deserves attention from researchers and policy-makers alike. Depression is associated with poor health behaviors (i.e., smoking, physical inactivity, caloric intake) that increase
risk of type 2 diabetes (3). Depression is also related to central obesity and potentially
to impaired glucose tolerance (4). Depression is associated with physiological abnormalities, including activation of the hypothalamic-pituitary-adrenal axis, sympathoadrenal system, and proinflammatory
cytokines, which can induce insulin resistance and contribute to diabetes risk (5). Diabetes may increase
risk of depression because of the sense of threat and loss associated with receiving this diagnosis and the substantial lifestyle changes necessary to avoid developing debilitating complications. Also, studies
suggest that late-life depression is associated with a history of vascular disease, including
diabetes (6). In sum, evidence suggests that the exposure/outcome relationship between these conditions is bidirectional and may change over the life course.