Discussion
In this population-based cross-sectional study of individuals with type 2 diabetes born in Norway, the results show that higher education levels are associated with lower odds for CHD and CKD. These associations persisted after adjusting for the potential mediating cardiovascular risk factors HbA1c, LDL-cholesterol, systolic BP, smoking and diabetes duration. We found associations between education level and stroke, retinopathy and foot complications after adjusting for age and sex, but not statistically significant after adjusting for the above-mentioned potentially mediating factors. The significant association between education level and CHD was found in both imputed and complete case analyses.
Our results show an association between education level, used as a marker of individual-level SES, and CHD in individuals with type 2 diabetes in a European country with equal access to healthcare, including both men and women in all age groups. Previous studies have reported similar findings, but the number of studies is low, representing selected populations and study designs with limitations.7 16–18 In the Whitehall cohort study the prevalence of heart disease in British male civil servants aged 40–64 years was higher in the lowest social group (measured as employment grading).7 These results are in line with a previous small survey on individuals with diabetes, a large diabetes study with self-reported data, and similar to a multinational study of highly selected individuals ≥55 years old diagnosed with type 2 diabetes after the age of 30 years with one or more macrovascular or microvascular diabetes complications or additional cardiovascular risk factors.16–18 In our study the odds for CHD remained unchanged when adjusting for potentially mediating risk factors. This is in line with the findings from a computer simulation study of the general US population aged 35–64 years, reporting that traditional risk factors for CHD explained 40% of excess events among those with low SES, with the remaining 60% attributable to other risk factors.19 We found that statin prescription was more frequent in high education groups. Due to our cross-sectional design, levels of LDL-cholesterol at the start of statin prescription and whether statin prescription was initiated before or after a cardiovascular event are not known.
Consistent with four other studies, CKD was more common among individuals with low as compared with high individual SES.3 7 16 18 Similar to a Chinese study we found no significant association between education level and stroke when adjusting for all risk factors.17 Different from our findings most studies report an SES-level gradient associated with retinopathy.3 4 16 17 20–24 However, three of these studies included less than 1200 individuals. Low education level (≤9 years) increased the risk of retinopathy at time of diagnosis by 44% in Swedish individuals with type 2 diabetes and latent autoimmune diabetes in the adult.25
There are limited studies on the association between individual SES and foot complications. Two studies from France and Finland report an association between low SES and increased risk of the outcome.2 26 In a recent UK study on individuals newly diagnosed with type 2 diabetes, social deprivation, measured by a deprivation score, was an independent risk factor for the development of diabetes-related foot disease, peripheral vascular disease and lower limb amputation.27
Differences in vascular complications according to education level might be affected by social factors such as low income, employment insecurity, poor living conditions and chronic stress contributing to type 2 diabetes and acting as parts of a cyclical process both resulting from and contributing to adverse outcomes.28 Poor health literacy is more common among individuals with low educational attainment.29 Moreover, level of education is considered to affect the individual’s ability to turn information into practical measures and behavior, affects access to recourses, employment-related problems and social exclusion if unemployed. Among individuals with type 2 diabetes in primary care, inadequate health literacy has been independently associated with worse glycemic control and higher rates of retinopathy.30 In a Danish study, individuals with high education levels were favored or more proactive in receiving services and more willing to accept rehabilitation services and seek specialist care.31 In the diabetes population included in our study 34.0% had completed compulsory education, 49.0% upper secondary education and 16.9% higher education, compared with 26.9%, 40.9% and 32.2% in the general Norwegian population at the time of the study.
Comparing our results with other studies is complicated by differences in healthcare systems and insurance policies affecting healthcare delivery, possibly mediating the effect of education level on vascular complications. Furthermore, SES can be measured by income, level of education or occupational status. Each indicator measures different aspects of the socioeconomic gradient and may be more or less relevant to different health outcomes studied.15 Income may change in a short time and a high proportion of our study population were, according to the mean age, retired, possibly affecting income. We therefore considered education status as the most appropriate measure for SES as it is relevant regardless of age and working status.
The main strengths in this study include the large sample size, individual register-based information on education level and the high-quality data collection done by experienced staff in a country with equal non-insurance-dependent access to healthcare and theoretically full availability of healthcare and higher education. Furthermore, the study included both men and women ≥18 years living in three out of four health regions in Norway, covering both urban and rural areas, ensuring that our findings are representative for individuals with type 2 diabetes born in Norway. Missing data were imputed, including missing measurement of HbA1c, BP, LDL-cholesterol, BMI, smoking status and diabetes duration, which may reduce the possibly biased estimates from complete case analyses. The imputation was done under the assumption that data were missing at random. However, we cannot exclude the possibility of sampling, ascertainment and detection bias. Although the trend of lower OR for higher education groups is present for all complications, there are few observations for some complications. Due to this there might be uncertainty related to the estimates, as seen for retinopathy.
A limitation is that the cross-sectional design prevents us from drawing conclusions regarding causality. Further, we did not have information on lifestyle factors like nutrition, diet including alcohol consumption and physical activity. Furthermore, heredity for disease, adherence to therapy and factors important in healthcare delivery affecting the risk of developing vascular complications are unknown. We lack information on cumulative lifetime exposure for potential risk factors and the development of risk profile over time. We had no information on albuminuria as a marker for CKD. Time period bias caused by time frames up to 36 months for included variables cannot be excluded, though 88.2% of HbA1c values, 73.9% of LDL-cholesterol values, and 83.1% of S-creatinine values were recorded within the last year.
The proportion of the population with higher education has changed in recent decades and longer education is now more common. Cohort effects may be present, as older cohorts will be over-represented among those with low education. Moreover, the meaning of education levels differs across cohorts, both qualitatively and quantitatively, and access to and structure of educational systems have changed over time. When tested, there was no significant interaction between education level and age in our study (data not shown). The OR for CHD remained largely unchanged when repeating the analyses using age as a categorical variable, but this does not exclude the cohort effect.