© 2004 by the American Diabetes Association, Inc.
Explanations of Socioeconomic Differences in Excess Risk of Type 2 Diabetes in Swedish Men and Women
1 Department of Molecular Medicine, Endocrine and Diabetes Unit, Karolinska Institutet, Stockholm, Sweden Address correspondence and reprint requests to Emilie E. Agardh, RN, MPH, Diabetes Prevention Unit, Karolinska Hospital, S-171 76 Stockholm, Sweden. E-mail: emilie.agardh{at}ks.se
OBJECTIVEWe investigated to what extent socioeconomic differences in type 2 diabetes risk could be explained by established risk factors (obesity, physical inactivity, smoking, and heredity) and psychosocial factors (low decision latitude at work and low sense of coherence). RESEARCH DESIGN AND METHODSThis cross-sectional study comprised 3,128 healthy Swedish men and 4,821 women, aged 3556 years, living in the Stockholm area. An oral glucose tolerance test identified 55 men and 52 women with type 2 diabetes. The relative contribution of established and psychosocial factors to socioeconomic differences in diabetes risk was assessed by comparing analyses with adjustment for different sets of these factors. RESULTSThe relative risks (RRs) for type 2 diabetes in middle and low socioeconomic groups in men were 2.4 (95% CI 1.05.3) and 2.9 (1.55.7), respectively, and in women 3.2 (1.56.6) and 2.7 (1.35.9), respectively. In men, the RRs decreased to 1.9 (0.84.4) and 2.1 (1.04.2) after adjustment for established risk factors; no further change was found when psychosocial factors were included. In women, the RRs changed to 2.4 (1.15.2) and 1.6 (0.73.8) by including established risk factors and to 2.3 (1.05.1) and 1.9 (0.84.3) by inclusion of psychosocial factors. After adjustment for both established and psychosocial factors, the RRs were 1.4 (0.63.6) and 1.0 (0.42.5), respectively. CONCLUSIONSIn men, the excess risk of type 2 diabetes was partly explained by established risk factors (3642%), whereas psychosocial factors had no effect. In women, most of the socioeconomic differences in type 2 diabetes were explained by simultaneous adjustment for established risk factors and psychosocial factors (81100%).
Abbreviations: FHD, family history of diabetes IGT, impaired glucose tolerance OGTT, oral glucose tolerance test SOC, sense of coherence
Type 2 diabetes is more prevalent in lower socioeconomic groups in Western societies (16). Obesity, physical inactivity, and smoking are implicated in the development of type 2 diabetes (7,8) and are also associated with low socioeconomic position (911). In addition, a range of psychosocial factors is involved in socioeconomic inequalities in health (12,13). However, the research explaining socioeconomic differences in type 2 diabetes risk has mainly focused on established risk factors such as obesity, physical inactivity, and smoking (1,2,5). In a recent study (14), we found that psychosocial factors, such as low decision latitude at work and low sense of coherence (SOC) (a factor in successful coping with stressors), seemed to be associated with type 2 diabetes in women. Decision latitude at work derives from the demand-decision latitude model introduced by Karasek and colleagues (15,16). The model proposes that the combination of high demands and low decision latitude increases the risk of coronary heart disease. However, publications show that the decision latitude dimension is consistently related to coronary heart disease, whereas the demand dimension is not (17). In addition to established risk factors, we suggest that psychosocial factors may be of importance for socioeconomic differences in type 2 diabetes risk. We investigated to what extent the excess risk of type 2 diabetes in lower socioeconomic groups can be explained by established risk factors (obesity, physical inactivity, smoking, and diabetes heredity) and psychosocial factors (low decision latitude at work and low SOC).
The design of the baseline study of the Stockholm Diabetes Prevention Program has been described previously (14,18). In brief, this cross-sectional study comprises 3,129 men and 4,821 women between the ages of 35 and 56 years at the time of examination (19921994 and 19961998, respectively). The male study sample was selected from four municipalities: Sigtuna, Värmdö, Upplands Bro, and Tiresö in Stockholm. The female study sample was selected from the same municipalities and one other: Upplands Väsby. All study individuals were identified by the Stockholm County Council register. The selection of the study sample was obtained in two steps. First, a questionnaire containing questions on country of birth, diabetes diagnosis, and family history of diabetes (FHD) was sent to all men and women in the appropriate age-groups living within the study areas (n = 32,368). Completed questionnaires were received from 26,717 people (83%). At this stage, 12,979 (49%) were excluded due to already known diabetes (2%), insufficient knowledge of FHD (12%), incomplete responses (28%), and foreign origin (6%). In addition, 1% of the females were excluded due to death, mental retardation, and moving from the municipality. In the second step, the study sample was divided into two groups: those with FHD, defined as having at least one first- (parent or sibling) or two second-degree relatives (grandparents, uncles, or aunts) (n = 5,689) with diabetes, and those without FHD (n = 7,625). All those with FHD, along with a random sample of those without FHD (n = 5,921), were invited to undergo a physical health examination. In addition, 424 women reporting previous gestational diabetes were invited. In total, 8,108 (67% of those invited) individuals agreed to participate in the health examination. During the visit at the primary health care center, an oral glucose tolerance test (OGTT) was administered and the participants answered a detailed questionnaire about lifestyle factors. In addition, weight, height, and blood pressure were measured. The information on FHD was also verified. At this stage, 33 (1%) men were excluded due to insufficient FHD and 129 (2%) women were excluded due to breast-feeding, pregnancy, certain medications, or insufficient FHD. The final study sample consisted of 3,129 men and 4,821 women.
Classification of disease
Measurement of socioeconomic position
Measurement of established risk factors
Measurement of psychosocial factors
SOC is a paradigm developed by Antonovsky (23) and is based on three dimensions: comprehensibility, meaningfulness, and manageability. The original questionnaire contains 29 items. In our study, we analyzed SOC from three questions, one on each dimension (comprehensibility, meaningfulness, and manageability). The three-dimensional SOC has been tested for reliability and been previously recommended. The weighted
Data analysis
The proportion of smokers and FHD were higher among men and women with low and middle socioeconomic positions compared with those with high position (Table 1). In addition, low physical activity was slightly more prevalent and mean BMI somewhat higher among individuals with low and middle socioeconomic positions. With regard to psychosocial factors, low decision latitude at work and low SOC were more common in the middle and low socioeconomic groups. As compared with high socioeconomic position, middle and low socioeconomic positions were associated with higher type 2 diabetes prevalence in both men and women (Table 2).
In men, BMI, physical inactivity, smoking, and FHD explained a similar percentage of the excess risk of type 2 diabetes in middle and low socioeconomic groups (Table 3). The established risk factors together explained 36% of the excess risk in the middle group and 42% in the low group. When we adjusted for psychosocial factors, low decision latitude at work explained a minor part of the excess risk. Low SOC did not change the RR, and we could not identify any change in either socioeconomic group when adjusting for both psychosocial factors. Hence, when combining established risk factors and psychosocial factors, the RR was virtually the same as when adjusting for the established risk factors alone.
In women, smoking and BMI seemed to be important in middle and low groups (Table 4). Physical inactivity and FHD did not explain the excess risks to any important extent. When clustering the established risk factors, 36% of the excess risk was explained in the middle group and 65% in the low group. In women, on the other hand, low decision latitude at work together with low SOC explained a considerable part of the excess risk in the middle (38%) and low socioeconomic groups (53%). When we adjusted for both established risk factors and psychosocial factors, 81% of the excess risk of type 2 diabetes was explained in the middle group and 100% in the low socioeconomic group.
We performed additional analyses with regard to IGT with the same procedure as that for type 2 diabetes. In women, the RRs for IGT in association with middle and low socioeconomic groups compared with that of high socioeconomic groups were 1.5 (95% CI 1.02.2) and 1.9 (1.32.8), respectively. These excess risks were partly explained by established risk factors (40% in the middle socioeconomic group and 44% in the low socioeconomic group) but not by psychosocial factors. In men, on the contrary, no obvious association was found between IGT and the lower socioeconomic groups: 1.1 (0.71.7) and 1.2 (0.81.7), respectively (data not shown).
In our study, established risk factors could not fully explain the association between socioeconomic differences and the risk of type 2 diabetes in either men or women. This is in line with previous results. One study (2) examined the association of poverty-to-income ratio, education, and occupational status in African American and non-Hispanic white women and men and found an association between lower socioeconomic position (in all three measures) and type 2 diabetes in non-Hispanic women, but not consistently in men. In this study, the differences could not be explained by only including established risk factors. Another study (5) demonstrated a relationship between glucose intolerance (IGT and type 2 diabetes) and lower social position based on occupation. In women, this relationship was independent of BMI and waist-to-hip ratio, whereas in men it was nonsignificant after adjusting for obesity. Moreover, an association between deprivation (measured as residency in a deprived area) and type 2 diabetes has been reported (1), showing an increased BMI within the diabetic population. However, in the latter study, information on BMI in the nondiabetic population was lacking. In our study, low decision latitude at work and low SOC contributed in explaining socioeconomic differences in type 2 diabetes risk among women. On the other hand, in men these factors did not contribute at all. Both low decision latitude at work and low SOC were more prevalent in men with middle and low socioeconomic positions. However, they were not apparently associated with type 2 diabetes (RR 1.4, 95% CI 0.72.8 and 1.3, 0.63.2, respectively). Importantly, psychosocial factors did not explain the relationship between socioeconomic position and IGT in women. This suggests that the impact of different factors varies through the progression of type 2 diabetes.
Type 2 diabetes and cardiovascular disease share some causes (7). Hence, it is interesting to compare epidemiological findings in type 2 diabetes with data in cardiovascular disease. In cardiovascular research, the inclusion of psychosocial variables has made important contributions (29). For example, in one prospective study (27), psychosocial factors such as depression, hopelessness, marital status, and social support contributed to The mechanism through which low socioeconomic position and psychosocial factors could relate to type 2 diabetes is not known. One might speculate whether the psychosocial factors are directly linked to established risk factors. However, in our previous study in women (14), the associations between psychosocial factors and type 2 diabetes persisted after adjustment for BMI, physical inactivity, smoking, and FHD. In addition, low socioeconomic position has been found to be associated with higher cortisol values in relation to perceived stress (33), and it has been argued that type 2 diabetes may be mediated by the physiological response to chronic stress (34). It is not obvious why low decision latitude at work and low SOC were associated with type 2 diabetes (14) and also contributed to socioeconomic differences in diabetes risk in women but not in men. One explanation could be that work stress in women might reflect housewives who go out to work and are thus exposed to double stress. For this purpose, we analyzed the question, "How demanding is your work in the household?" The results did not show any association between type 2 diabetes and this variable (RR 1.0, 95% CI 0.61.8). Hence, we could not demonstrate that demanding work in the household influenced the effect of low decision latitude at work. Another explanation could be difficulties in the measurement of behavioral variables. However, the questions and circumstances under which the study population filled in the questionnaire were equal for men and women. Moreover, there is increasing evidence that low decision latitude at work is associated with coronary heart disease in men (17). There are some methodological issues in this study that should be noted when interpreting the results. First, to explain socioeconomic differences in diabetes risk, we investigated to what extent the association between socioeconomic position and type 2 diabetes was confounded by psychosocial and established risk factors. This is based on the belief that all investigated factors (socioeconomic position, psychosocial, and established risk factors) are correctly measured. However, imprecise measures may bias the results, and the magnitude of such error is unpredictable in a multivariate context (35). Thus, the attenuated risks observed between type 2 diabetes and lower socioeconomic position after adjustment for different factors may be biased due to imprecise measures. It is also worth noting that our results should be interpreted with some caution due to the limited number of cases in our study. Second, as this is a cross-sectional study, we cannot exclude the possibility that belonging to a low socioeconomic group is a result of poor health status. On the other hand, those with already known diabetes were excluded from the study, and study subjects who had type 2 diabetes received their diagnosis after answering the questionnaire. It could be hypothesized, however, that the patients who were excluded because their diabetes was already known belonged to a more privileged socioeconomic group with good access to medical care. If so, although not likely to be a pronounced phenomenon in the present region of Sweden, our selected group of patients with diabetes would possibly be enriched by relatively underprivileged individuals. Thus, the associations of type 2 diabetes and socioeconomic position could be to some extent overestimated. However, we are lacking information on those patients excluded due to already diagnosed diabetes. Third, the study population answered questions about former lifestyles. However, case subjects were not aware of having the disease before filling out the questionnaire, and hence, possible misclassification due to recall bias is negligible. In conclusion, our data show that in middle-aged men, 3642% of the excess risk of type 2 diabetes was explained by established risk factors, whereas the assessed psychosocial factors played no role. In women, however, both established risk factors and psychosocial factors contributed and together explained 81100% of the excess risk.
This study was supported by grants from the Stockholm County Council, the Swedish Council for Working Life and Social Research, Vårdalstiftelsen, and GlaxoSmithKline, Sweden. We thank the nurses and other staff members at the health care centers who carried out the OGTTs and other measures. Received for publication September 10, 2003. Accepted for publication December 16, 2003.
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