These data indicate that all-cause mortality rates in type 1 diabetes are significantly associated with education and income levels attained by early adulthood. Low income and low education groups had significantly higher rates of all-cause mortality compared to the local general population, whereas high income and education groups did not significantly differ from the general population in their mortality rates. Cox proportional hazards modelling revealed that education and income were similarly associated with mortality. This relationship became attenuated for education (HR=3.0 reduced to 2.1) after adjusting for other key potential mediators, while the relationship between mortality and income was largely unaffected by such adjustment (HR=3.2 to 3.0).
The strategy of identifying SES measures at a single point in time in young adulthood has been reported recently in the general population (Framingham Offspring Study).(17
) However, this strategy is even more important in a population with childhood-onset type 1 diabetes with an average diabetes duration of 20 years by the time they reach age 28. Given the very low levels of prevalent major complications (n
=3 for ESRD and n
=4 for hard CAD events), we believe that analysis using SES from this age 28 cohort most adequately captured the educational attainment and earning potential for this cohort prior to the adverse influence of complications on SES.
Clear evidence exists that low SES, both at and after onset of type 1 diabetes is associated with a number of risk factors: poor glycemic control,(18
) and multiple hospitalizations.(25
) All of these have been associated with increased mortality risk in type 1 diabetes. It is unclear, however, whether SES is an independent predictor of mortality or whether it merely reflects these other measures which drive mortality. Our data suggest that the effect of educational attainment on mortality in type 1 diabetes is reduced after accounting for other risk factors, as seen in other reports.(24
) Income, however, appears to be a stronger predictor of mortality, as its effect was not diminished with similar adjustment.
Gnavi et al. examined the role of SES (education level) in a 9-year follow-up of the Turin Longitudinal Study.(10
) In type 1 diabetes, individuals with only a primary school education or no formal education were 3–4 times as likely to die during follow-up as those who graduated high school (HR for T1D males=3.1, 95% CI 1.6–6.1, HR for T1D females= 4.4, 1.6–12.3), after adjusting for age and neighborhood. Similarly, in our 16-year follow-up, we found that T1D individuals with less education were 3 times as likely to die as their more highly educated counterparts, after adjusting for diabetes duration and sex. While previous SES studies in T1D have reported overall and sex-specific standardized mortality ratios (SMRs),(7
) our study is the first to report SMRs in T1D stratified by different SES measures, with dramatic results.
The strengths and weaknesses of this study should be noted. We did rely on SES measures from a single point in time for analysis, and repeated measures have been shown to provide a more accurate SES picture in the general population.(3
) However, a key objective of this study was to assess SES prior to advanced diabetes complications adversely affecting SES. So, while repeated measures at a later date may have improved our assessment of actual SES, they also would have made it more difficult to interpret the actual predictive value of SES before the development of advanced complications.
With 16 years of follow-up, this is the longest prospective study in type 1 diabetes to address the role of SES in early T1D mortality. Despite this, the sample size for our study is relatively small (n
=317) with only 10.7% dying during follow-up. This is due to the fact that our cohort was young at baseline (~age 28), and our oldest person at follow-up was only 53 years-old. However, we still found strong associations with SES and mortality in type 1 diabetes. These associations would likely persist with increased follow-up; however, it could be argued that the SES effect would be stronger at younger ages, based on the phenomenon recently termed “metabolic memory”, and seen in the Diabetes Control and Complications Trial (DCCT) and other observational studies.(27
) Metabolic memory represents the notion that early chronic exposure to hyperglycemia leads to irreversible microvascular changes occurring prior to modern advances in diabetes management. We hypothesize that the SES effect on access to diabetes care led to more irreversible microvascular damage in the low SES groups in this cohort.
The income variable was based on household income and could not readily be adjusted for household size. As such, the actual income available to the T1D individual might be over-estimated. We were also unable to adjust for inflation due to the nature of the original income variable (categorical ranges of income), and could only standardize the income levels nearly midway through the study cycles. Adjusting for inflation, however, would not substantively change the results, as the dichotomized income comparisons were between the highest income group and all others.
Finally, the Hollingshead Index is not thought by all to be the best SES measure for occupation.(12
) We dichotomized occupation (professional vs. non-professional) based on the Hollingshead Index and found little association () with mortality, an association which has been reported in both general and diabetes populations worldwide.(1
) However, with longer follow-up, the small differences apparent in and for occupation may reach statistical significance.
In conclusion, baseline education level at age 28 strongly predicted all-cause mortality in our type 1 diabetes population, suggesting the vital importance of education in diabetes self-care. This association was weakened by adjusting for HbA1c
, non-HDL cholesterol, hypertension, and presence of microalbuminuria, indicating that the relationship between education and mortality is partially mediated by these other risk factors. However, a larger type 1 diabetes prospective study in Germany showed that the effect of SES persists despite adjustment for known mortality risk factors.(29
). Consequently, the effect of SES in type 1 diabetes requires additional prospective research, as this population is already at very high risk of early mortality. Regardless, these data indicate the need for improvements in diabetes care and education, especially in those from lower socioeconomic backgrounds.