shows the characteristics of the ECA cohort from 1981 through 2005 and the distribution of sociodemographic characteristics during each wave. We removed 297 prevalent cases of diabetes in 1981 and 1982 from analysis. We removed 4 cases from analysis because the age of diabetes onset was reported as prior to 1980. Overall, we found 169 incident cases of type 2 diabetes among 1803 persons at risk in the cohort that provided age of onset information such that survival models could be developed, representing 36–523 person-years. The average age of depression onset was 30.9 years, and the average age of type 2 diabetes onset was 55.0 years. The average difference between onset of depression and onset of diabetes was 16 years (data not shown).
Participant Characteristics by Wave: Baltimore Epidemiological Catchment Area Study, Baltimore, MD, 1981–2005
Persons with incident diabetes tended to be older (P < .01), have less education (P < .05), and drink less often (P < .01) and were less likely to be current smokers (P < .05) than were persons who did not develop diabetes between 1981 and 1993. From 1993 to 2005, incident case respondents continued to have less education (P < .01) but were also more likely to have a family history of diabetes (P < .01) and a higher BMI (P < .01) than were persons who never developed diabetes. Depression was positively associated with smoking status and alcohol use, both of which were inversely associated with diabetes risk, likely because of confounding by indication (i.e., persons with diabetes would have been advised by their health care providers to change their lifestyle). Depression was positively associated with antidepressant use (P < .001) which was nonsignificantly inversely associated with diabetes risk. The overall incidence of type 2 diabetes in the cohort was 5.1 per 1000 person-years (95% CI = 4.4, 5.9), and those with MDD had higher incidence than those without ().
Incidence of Type 2 Diabetes per 1000 Person-Years, by Age Group at Wave 3 and Depression Status: Baltimore Epidemiological Catchment Area Study, Baltimore, MD, 1993–1996
In initial Cox proportional hazards models of each potential mediator or confounder adjusted for sociodemographic characteristics, significant (P < .10) predictors of type 2 diabetes included education, BMI, alcohol use, family history of diabetes, frequency of eating balanced meals, and stairs climbed per day. After additional adjustment for BMI, frequency of social contact with relatives emerged as a predictor of type 2 diabetes (hazard ratio [HR] = 0.82; P < .10).
Over the entire study period (1981–2005), depression was associated with a higher risk of type 2 diabetes (HR=1.62; ). During 1993 to 2005, depression was associated with a higher risk of type 2 diabetes after additional adjustment for BMI, family history of diabetes, frequency of eating balanced meals, stairs climbed per day, and frequency of contact with relatives (HR=2.04). The association between MDD and type 2 diabetes increased when sociodemographic characteristics and health behaviors were added to the model, because these variables were differentially associated with exposure and outcome (e.g., women were more likely to have MDD, but being female was associated with a lower risk of type 2 diabetes).
Association of Depression and Risk of Type 2 Diabetes: Baltimore Epidemiological Catchment Area Study, Baltimore, MD, 1981–2005
The interaction term between MDD and education was marginally statistically significant after we adjusted for age, gender, race, BMI, and family history of diabetes (HR=0.79; P<.055), indicating that depression was associated with a lower risk of type 2 diabetes for each year of education attained. Interaction terms with other sociodemographic characteristics were not significant (data not shown). Among respondents with a high school education or less, the incidence of type 2 diabetes during the 23-year follow-up period was 6.1 per 1000 person-years (95% confidence interval [CI]=4.1, 7.2); the incidence among those with at least some college education was approximately 50% lower at 3.2 per 1000 person-years (95% CI=2.3, 4.4).
In models stratified by highest educational level attained (high school or less vs at least some college), depression was associated with a 73% increase (95% CI = 1.04, 2.90) in risk of diabetes among participants with less education after we adjusted for age, race, gender, smoking status, and alcohol use, and the association between depression and diabetes was attenuated among those with more education (HR = 1.12; 95% CI = 0.46, 2.76). Among respondents with less education, the association between depression and risk of diabetes increased to 2.26 (95% CI = 1.11, 4.60) after additional adjustment for BMI and family history. There was no association between depression and risk of diabetes among respondents with more education (HR = 0.45; 95% CI = 0.11, 1.80).
The joint effects of depression and low educational attainment are illustrated in . Compared with participants who never had MDD and had high educational attainment (reference group), those who had ever had MDD and had low educational attainment had 4.10 times (95% CI = 1.84, 9.16) greater risk of diabetes, after we adjusted for age, gender, race, smoking status, alcohol use, BMI, family history of diabetes, and social network characteristics. We observed no difference in risk between the reference group and either participants who had less education and had never had MDD (HR = 1.44; 95% CI = 0.82, 2.55) or participants who had more education and had ever had MDD (HR = 0.92; 95% CI = 0.27, 3.15).
Joint effects of lifetime major depressive disorder and low educational attainment: Baltimore Epidemiological Catchment Area (ECA) Study, Baltimore, MD, 1981–2005.