Does Diabetes Double the Risk of Depression?

It is generally agreed that the prevalence of depression is increased in people with [type 2] diabetes compared to the general population. However, recent findings have shown that the relationship between depression and diabetes is likely to be complex. For example, a large population-based study Pouwer et al. (2003) found that the rate of depressive disorder was higher in patients with diabetes complications than in those without complications, suggesting that the risk of depression is associated with increased burden of diabetes-related conditions. Recent studies show that the risk of depression in diabetes is not increased if people are unaware of having the condition (Holt et al, 2009; Knol et al., 2007) supporting the burden hypothesis

In a rather complex article ‘Does diabetes double the risk of depression?' O’Connor et al.(2009) compared the rate of new depression diagnoses in people with prevalent and incident diabetes from a very large dataset of a multispecialty medical group in Minnesota, providing care to approximately 225,000 patients. When controlling for age and sex, patients with diabetes had a higher risk of a depression diagnosis than outpatients without diabetes who consulted their GP. However, when they controlled for the number of primary care visits, this difference disappeared. Another intriguing finding was that the likelihood of a new depression diagnosis was decreased as the number of primary care visits increased. The authors concluded that “Patients with diabetes have little or no increase in the risk of a new diagnosis of depression relative to nondiabetic patients when analyses carefully control for the number of outpatient visits. Studies showing such an association may have inadequately adjusted for comorbidity or for exposure to the medical care system.”

How does this study contribute to solving the puzzle of depression in diabetes? First, if we assume with O’Connor et al. that the number of outpatient visits is related to co-morbidity or disease burden, their results provide support for the above mentioned notion that the incidence of depression in diabetes is mediated by the burden of diabetes and not by diabetes per se. However, this assumption is debatable because people consulting their GP are likely to have a variety of acute and chronic conditions. Although the control group did not have diabetes, this group was not a healthy control group (which is required when one aims to determine the impact of having diabetes versus not having diabetes), nor a group with other chronic conditions (e.g., Kessing et al., 2003). As such, the conclusions that can be drawn are limited. There are also limitations regarding the diagnosis of depression as no systematic diagnostic interview was done for all patients. As approximately 50% of depression in diabetes remains undetected in primary care (Katon et al., 2006), O’Connor et al are likely to have underestimated the number of depressed people. The same holds true for type 2 diabetes where there is still a considerable under-detection of diabetes.

To answer O’Connor et al.’s question: ‘Does diabetes double the risk of depression?’, the prevalence and incidence of depression in people with diabetes without complications, should be compared to those with diabetes and complications and a control group without a chronic condition.

References

1. Pouwer F, Beekman AT, Nijpels G, Dekker JM, Snoek FJ, Kostense PJ, Heine RJ, Deeg DJ. Rates and risks for co-morbid depression in patients with Type 2 diabetes mellitus: results from a community-based study. Diabetologia 2003 46:892-898
2. Holt RI, Phillips DI, Jameson KA, Cooper C, Dennison EM, Peveler RC; Hertfordshire Cohort Study Group. The relationship between depression and diabetes mellitus: findings from the Hertfordshire Cohort Study. Diabet Med 2009 26: 641-648.
3. Knol MJ, Heerdink ER, Egberts ACG, Geerlings MI, Gorter KJ, Numans ME, Grobbee DE, Klungel OH, Burger H. Depressive symptoms in subjects with diagnosed and undiagnosed type 2 diabetes. Psychosom Med 2007 69:300 –305.
4. Kessing LV, Nilsson FM, Siersma V, Andersen PK. No increased risk of developing depression in diabetes compared to other chronic illness. Diabetes Res Clin Pract 2003 62: 113-121
5. Katon WJ, Richardson L, Russo J, Lozano P, McCauley E.. Quality of depression care in a population-based sample of patients with diabetes and major depression. Med Care 2004 42: 1222-1229.

Competing interests:   None declared