Diabetes is a growing global health problem, and an estimated 4.4% of the global population will have diabetes by 2030.1 The increased prevalence observed in recent years is mainly attributed not only to an increase in the incidence in type 2 diabetes mellitus (T2DM), but it could also be due to declining mortality among T2DM patients2–4 in some countries. This increased incidence is likely due to the increased prevalence of its main risk factors, such as obesity and sedentarism,1–6 while declining mortality among T2DM patients may be explained by improvements in treatment.3,4 Moreover, disease monitoring and improved access to health services play an important role in preventing diabetes-related complications.7

Inequalities in the incidence, prevalence and mortality of T2DM as a function of socioeconomic position (SEP) have been described in some populations.7–12 People with disadvantaged SEP (low income, deprived occupational social class and/or low educational level) experience higher rates of incidence and prevalence and are more likely to die from T2DM than those with high SEP.7 Some studies have suggested that the presence of SEP-related inequalities in T2DM are mediated13 by the unequal distribution of risk factors for T2DM between levels of SEP,9,14 and indeed obesity, physical inactivity, and unhealthy diet are most prevalent among those with the lowest SEP.15 For example, obesity has been reported to account for 26% and 36% of the variance in the prevalence of T2DM in middle-aged European men and women, respectively.16 However, there has been some debate regarding the existence of residual SEP-related inequalities in the incidence and prevalence of T2DM after adjusting for these traditional risk factors.9–12,14 It is also necessary to consider that these factors may in part be the result of the conditions in which people are born, grow, live, work and age, conditions that are shaped by the distribution of money, power and resources at global, national and local levels.

To date, no Europe-wide follow-up studies have been reported that analyze social inequalities in both incidence and prevalence of T2DM and the contribution of their risk factors to these inequalities. Studying inequalities in the incidence and prevalence of diabetes across Europe may help to identify general patterns that are independent of the characteristics of specific countries. Furthermore, assessing the role of potential mediators of the relationship between SEP and T2DM incidence or prevalence may aid the design of specific strategies aimed at reducing these inequalities. The aim of this study was to analyze SEP-related inequalities in the prevalence and incidence of T2DM in people aged 50 years and over in Europe between 2004 and 2006 and to describe the contribution of body mass index (BMI) and other possible mediators of SEP-related inequalities in T2DM.

A cross-sectional and longitudinal analysis of a group of individuals was performed on the basis of the results of two phases of the Survey of Health, Ageing and Retirement in Europe (SHARE) in 2004 and 2006. SHARE is a multidisciplinary and trans-national European study that has collected information on health, SEP and family networks of individuals aged ≥50 years. In this study we included men and women aged ≥50 years who resided in 11 European countries (Austria, Belgium, Denmark, France, Germany, Greece, Italy, Spain, Sweden, Switzerland and The Netherlands). These are the countries that participated in 2004 and had a follow up in the second wave (In 2004, the individual response rate ranged from 73.7% in Spain to 93% in Denmark). All countries attempted proper probability sampling. For more information about SHARE-project it is possible to consult its web page, where there is a lot of additional methodological information (www.share-project.org). The final sample size consisted of individuals who were part of the main sample of SHARE-project, aged 50 years or more.

The prevalence of T2DM was calculated among individuals enrolled in the first phase of the survey in 2004 (n=21,323). T2DM status was self-reported, using the same survey questions in 2004 and 2006: (1) “Has a doctor ever told you that you had any of the conditions on this card? Diabetes or high blood sugar”, or (2) “Do you currently take drugs at least once a week for problems mentioned on this card? Drugs for diabetes”. People who reported diagnosis of diabetes before 20 years of age (1.3%) were considered as they did not have T2DM since they are more likely to be affected by type 1 diabetes. Using data collected during the second phase of the survey in 2006, the cumulative Incidence of T2DM was calculated on the basis of individuals who did not have diabetes in 2004 (n=12,811; 39.9% were lost to follow-up).

Information on educational level (as a measure of SEP) was collected in the first phase of the survey and classified according to the International Standard Classification of Education (ISCED-11) (www.unesco.org). Some categories of this classification were collapsed because of the low numbers of individuals in some groups. The final variable was made up of the following categories: ISCED <3 (lower secondary education or less) or ISCED ≥3 (upper secondary education or more).

The following variables were collected as potential mediators of the association between education and T2DM: BMI was calculated using self-reported weight and height, and was analyzed as both a continuous variable and a 4-level categorical variable: (1) underweight (<18.5kg/m2); (2) normal (18.5–24.9kg/m2); (3) overweight (25–29.9kg/m2); and (4) obese (≥30kg/m2). Smoking status was classified as: (1) currently a smoker, (2) never a smoker, and (3) a former smoker. An individual's alcohol consumption was considered excessive if he reported drinking more than two glasses of beer, cider, wine, spirits or cocktails on five or more days a week. Physical inactivity was defined as never or almost never engaging in moderate or vigorous physical activity such as sports, heavy housework, or a job that involves physical labor. We note that although we have considered the risk factors mentioned above as potential mediators of the link between SEP and T2DM risk, some may not be part of the chain of causality between SEP and T2DM and therefore might be better considered as potential confounders.13 Age and country were treated as potential confounding variables.

Among the participants of phase I, 65% of men and 53% of women were overweight or obese and 92% of men and 87% of women performed some type of physical activity (Table 1). Fifty-five percent of men and 43% of women had upper secondary education or higher. The individuals included in the second phase of the study had similar prevalences of these risk factors, but were generally younger and more physically active than those included in the first phase (both with p<0.01).

Prevalence of T2DM in men ranges from 8.4% [95% CI: 6.3–10.4] in Denmark to 15.3% [95% CI: 12.5–18.1] in Spain, while in women ranges from 4.5% [95% CI: 2.6–6.5] in Switzerland to 14.2% [95% CI: 11.8–16.6] in Spain. The two-year cumulative incidence of T2DM in men goes from 2.2% [95% CI: 0.8–3.7] in Denmark to 7.1% [95% CI: 4.9–9.4] in The Netherlands, and in women goes from 1.9% [95% CI: 0.7–3.1] in Denmark to 7.8% [95% CI: 5.1–10.4] in Spain (Fig. 1). The age- and country-adjusted prevalence [95% CI] of T2DM in the first phase was 10.2% [95% CI: 9.5–10.8] in men and 8.5% [95% CI: 7.9–9.1] in women. The age- and country-adjusted cumulative incidence of T2DM in the period 2004–2006 was 3.8% [95% CI: 3.3–4.3] in men and 3.4% [95% CI: 2.9–3.9] in women (not shown in the tables). BMI was the strongest predictor of the prevalence (PR among obese versus normal weight 2.92 [95% CI: 2.46–3.47] in men and 3.67 [95% CI: 3.12–4.31] in women) and incidence (RR among obese versus normal weight 3.34 [95% CI: 2.33–4.80] in men and 4.07 [95% CI: 2.85–5.80] in women) of T2DM (Table 2).

Figure 1.

Prevalence (year 2004) and cumulative incidence (2004–2006) of T2DM aged 50 years by gender and country.

(0.16MB).

In relation to SEP-related inequalities (Fig. 2 and Table 3), women with a lower secondary level education or less had a higher prevalence of T2DM (PR=1.61 [95% CI: 1.39–1.86]) in the first phase of the survey and a higher incidence (RR=1.88 [95% CI: 1.35–2.62]) during follow-up, compared to women with a upper secondary or more. A similar pattern of SEP-related inequality in the prevalence (PR=1.29 [95% CI: 1.12–1.50]), but not the incidence (RR=1.04 [95% CI: 0.78–1.40]) of T2DM was observed among men. The SEP inequalities in incidence and prevalence in each country are shown in Fig. 3. Overall SEP inequalities in the incidence and prevalence of T2DM were consistent across countries (Fig. 3). In both prevalence and incidence of T2DM, SEP inequalities seemed to be higher in women than in men. For example, point estimates of SEP inequalities in T2DM incidence among men were very close to 1 in each country, while among women they were higher than 1 in the majority of countries. For overall estimations, SEP-related inequalities in the prevalence and incidence of T2DM were significantly greater in women compared to men (p-value for the interaction between sex and SEP: 0.017 for prevalence; 0.005 for incidence).

Figure 2.

Age-and country-adjusted prevalence (year 2004) and cumulative incidence (2004–2006) of T2DM according to educational level in Europeans aged 50 years and over.

(0.16MB).

Figure 3.

Age-adjusted SEP-related inequalities in the prevalence and incidence of T2DM according to educational level in Europeans aged 50 years by gender and country, 2004–2006.

(0.32MB).

The most important mediator risk factor for SEP-related inequalities in both the prevalence and incidence of T2DM was BMI, accounting for 23.0% and 15.9% of the inequality in prevalence and incidence among women, respectively (Table 3). After adjusting for all the potential mediators simultaneously, SEP-related inequalities in the prevalence of T2DM were reduced by 44.8% and 26.2% in men and women, respectively, while inequalities in incidence (that was found statistically significant only among women) were reduced by 21.6% in women. In men, SEP inequalities in incidence of T2DM were not found.

In this study, which tries to ensure a large and representative sample of the European population over 50 years of age, we identified SEP-related inequalities in the prevalence of T2DM in men and women and in the two-year incidence of T2DM among women. BMI appeared as the main mediator variable explaining these inequalities.

In this study we observed significant SEP-related inequalities in the prevalence of T2DM in a representative sample of European men and women aged 50 years or over. Similar inequalities in the incidence of T2DM were observed in women but not men. These inequalities are partially mediated by BMI and other factors. Future studies should explore structural and psychosocial factors and dietary behavior in order to understand why SEP-related inequalities in diabetes persist in Europe.

A. Espelt, C. Borrell, L. Palència, L. Font-Ribera and A. Kunst contributed to the conception and design of the study. L. Palència contributed to data management. C. Borrell, A. Kunst, R. Gnavi, T. Spadea, L. Palència, L. Font-Ribera, A. Goday and A. Espelt discussed, interpreted and contributed to the results interpretation. A. Espelt wrote the first draft of the paper, which was revised with contributions by all authors.

None.

One of the authors (C.B.) belongs to the Gaceta Sanitaria editorial committee, but was not involved in the editorial process of the manuscript.