Beyond clinical thresholds: the continuum of depressive symptoms and their influence on diabetes outcomes

Worldwide, over half a billion people are living with diabetes, and this number is expected to rise to 783 million by 2045. The prevalence of depression in diabetes is doubled compared to people without diabetes. This comorbidity has been associated with greater risk of mortality, cognitive decline, and diabetes complications. The aim of this thesis was to advance knowledge on the role of depression in diabetes and the currently available pathways to care. A systematic review examining the longitudinal association between depression and diabetes complications (Study 1) revealed a bidirectional relationship, with depression being a stronger predictor of complications than vice versa. A dose-response relationship was found, with even low levels of depressive symptoms predicting complications. Study 2 examined the role of maladaptive schemas as a cognitive vulnerability factor for depression in diabetes. Results revealed elevated maladaptive schemas in current and past depression compared to never depressed people as well as a linear association with depressive symptoms, even in the never-depressed group, highlighting the importance of subthreshold symptoms. Further, the relationship between schemas and depression was moderated by perceived stress in three of the five schemas. Study 3 examined the pathways to mental health care in individuals with diabetes and depression across 12 countries and found an undiagnosed depression rate of 58.3%. With many of the diagnoses within the existing settings discovered in primary care and diabetes centres, these may be a future target to improve depression detection. Together, these results highlight the importance of subclinical levels of depression in individuals with diabetes. Consideration must be given to the adoption of a dimensional concept of mental health problems in favour of the current categorical approach. Further research is needed to examine the reasons for high rates of undiagnosed depression and to find effective treatments for subthreshold depression.