The results of this meta-analysis, based on 47 independent samples totaling over 17,000 patients, suggest that depression is significantly associated with nonadherence to diabetes self-care with an effect of moderate strength relative to the range of effect sizes typical in the social sciences. The size of this overall effect (r
= 0.21) is identical to the effect (r
weighted by n
− 3 = 0.21) obtained by DiMatteo et al. (7
) based on a meta-analysis of 12 studies of patients adhering to treatment regimens of other chronic illnesses. The effect is also similar in magnitude to the effects obtained from meta-analyses of the relationship between depression and hyperglycemia (r
= 0.17) (2
) and between depression and diabetes complications (r
= 0.25) (4
). To the extent that self-care is causally related to hyperglycemia and diabetes complications, our results suggest that impairment in self-care may be one pathway through which depression is associated with these negative health outcomes in patients with diabetes.
We found that the relationship between depression and worse self-care differed depending on the type of self-care measured. The effect was strongest and homogeneous for studies of missed medical appointments. Adherence to this type of self-care is unique in that it requires interpersonal behavior. Clinically, depression is associated with impairments in interpersonal behavior such as social withdrawal, disengagement from important activities, avoidance, and often with disruption of interpersonal relationships. Patients with increased levels of depression have been shown to report more dissatisfaction with their providers (27
). Considering that missed appointments are also often associated with increased provider frustration (28
), decreased empathy and patient-provider communication (29
), and less continuity of care (29
), this relationship may have important ramifications for depressed patients with diabetes.
Studies that used composite measures of diabetes self-care that tapped into more than one aspect of self-care also found stronger effects. This may be because comprehensive measures are more robust methodologically, or because the effects of depression are more global and are better captured by a more global measure of self-care. Although we found somewhat weaker relationships between depression and diet and medication adherence, studies that used stronger methodology found larger effects. If these effects (r
values = 0.20–0.28) are considered to be the best estimates of the true effect, then it appears that the effects on diet and medication adherence are also in the moderate range. Surprisingly, the relationship between depression and exercise was relatively weaker and homogenous. The effect for self-monitoring of blood glucose was small, but studies that measured monitoring as a continuous variable found significantly stronger effects. Also, the two studies (25
) that used objective measures of glucose monitoring found significantly stronger effects than those that did not, and the weighted average of these effects (r
= 0.22) was near the medium effect size benchmark. Finally, foot care was not significantly related to depression overall. However, only 2 of 47 effects measured foot care, which suggests this aspect of self-care may not be given sufficient attention in research or in providers’ instructions to patients.
With regard to population moderators, we examined differences between type 1 and type 2 diabetes and between children/adolescents and adults. We focused on these contrasts because it was plausible to expect that self-care routines are sufficiently different between these groups so as to affect the relationship between depression and self-care. We found no evidence to suggest that the relationship between depression and self-care varied as a function of type of diabetes. We did find that studies of children and adolescents tended to report larger effects than studies of adults, although effects in both populations were significant. This finding could also support more research on the impact of depression on diabetes self-care in children and adolescents, particularly for self-monitoring of blood glucose. We are unaware of depression treatment interventions that have targeted children or adolescents with diabetes to examine impacts on self-care or control. Our results suggest that, if the relationship between depression and worse self-care is causal, interventions in youth may have more of an impact on diabetes self-care and control than interventions with adults.
The results of this meta-analysis also provide important information on the methodological moderators of the effect between depression and diabetes self-care. We did not find any evidence to suggest that studies using stronger methodology found weaker effects; to the contrary, the effects were significantly larger when stronger methods were used. This was especially true for studies that analyzed self-care as a continuous variable: larger effects were obtained in the overall analysis, for medication adherence, and for glucose monitoring. Significantly larger effects for diet were also found in studies that used longitudinal designs. Also, studies that used objective measures of glucose monitoring found significantly stronger effects than those that relied on self-report or other methods more vulnerable to bias. Further, in the analyses aggregated by type of self-care, the strongest effects were found for appointment keeping, and each of the studies that measured this type of self-care used objective methods to measure it (i.e., medical records data). Thus, assuming that studies that use more rigorous methods will provide more accurate estimates of the true effect, it appears that our overall effect of r = 0.21 may actually underestimate the true association between depression and poorer self-care.
Future studies could be improved by using more rigorous measures and avoiding dichotomization of variables. The question of dichotomization is not only a statistical consideration that reduces power and accuracy (8
); it also reflects a limitation in the conceptualization of depression and adherence as purely categorical constructs. For example, in a previous article based on a large sample of primary care patients with type 2 diabetes, we showed that the relationship between depressive symptoms and worse diabetes self-care was not limited to patients likely to meet diagnostic criteria for major depressive disorder. We found the same magnitude of effects between symptoms of depression and worse self-care in a subsample of patients who did not meet screening criteria for major depressive disorder as we did in the overall sample (13
). These findings, along with the results of this meta-analysis, suggest that it would be inaccurate to think that the relationship between symptoms of depression and poorer diabetes self-care is limited to those who have clinically significant levels of depression. Rather, it appears that increases in depressive symptoms (measured by a variety of methods) are incrementally associated with decreases in diabetes self-care. We would also argue, both on conceptual grounds and based on the results of our moderation analyses, that it is also inaccurate to conceptualize self-care adherence as a categorical construct and to think of “adherent” versus “nonadherent” patients.
The results of this meta-analysis may have implications for patient interventions. It should be stated at the outset that none of the studies reviewed here provide conclusive evidence that the relationship between depression and poorer diabetes self-care is causal. However, one study used a longitudinal design and found significant relationships between depressive symptoms at baseline and increases in symptoms of depression over time and a variety of self-care behaviors assessed 9 months later, even when baseline levels of self-care were controlled (14
). Another study of an open-label treatment trial of bupropion hydrochloride in patients with type 2 diabetes and major depressive disorder found that depression severity, BMI, total fat mass, and A1C decreased during the acute phase of treatment, whereas adherence to diet and exercise improved significantly (30
). The pattern of findings from these studies is consistent with a causal relationship but does not provide conclusive proof. The current meta-analysis excluded randomized controlled trials, which would provide the strongest causal evidence, because we sought to evaluate the naturally occurring relationship between depression and poorer self-care, without the influence of intervention. Yet, evidence from randomized controlled trials suggesting that treating depression in patients with diabetes has positive effects on diabetes self-care has been lacking. Trials of antidepressants (31
), cognitive behavioral therapy (33
), and stepped-care case management (34
) have had positive effects on depression but have generally failed to have a positive impact on self-care behaviors. However, these trials have avoided integration of diabetes self-management training with the treatment of depression to isolate the effect of treating depression alone on diabetes outcomes.
The limitations of the available intervention literature suggest there may be an opportunity to maximize effects on diabetes control by developing comprehensive interventions to improve both depression and self-care (37
). It may be necessary but not sufficient to treat depression to improve self-care in patients who are struggling with depression and problems with diabetes self-management. Comprehensive interventions that address both self-care and depression management are likely to be more successful, and guidelines for how such interventions may be implemented in practice have been proposed (38
). Because even nonclinical levels of depressive symptoms can be associated with nonadherence (13
) and are associated with significant increases in risk for complications, functional impairment, and death (5
), perhaps all interventions oriented toward self-care or adherence should include a component to address psychological/emotional distress. Previous research has shown that such integrated programs can have positive effects on both self-care and emotional distress (39
). Taken together with the findings of this meta-analysis, this work could guide the design of well-powered randomized controlled trials of such interventions.