This section presents our review on the association between socioeconomic status and the incidence of diabetes. Among the 68 articles selected, there were 11 studies that examined this association. All the studies were carried out in countries with very high human development: nine in United States and two in United Kingdom. Education and occupation were the most common measures of SES used in these studies. The studies covered 39 associations: 13 in men; 14 in women; and 12 in both sexes combined.
Overall and in each sex group, there was approximately the same number of negative associations and “no associations”.
A longitudinal study found that lower education, income and occupation were
associated with an increased incidence of diabetes (Maty et al., 2005). However, these associations disappeared after adjusting for demographic confounders (age, gender, race and marital status), and other components of the causal pathway (physical inactivity, smoking, alcohol consumption, body composition, hypertension, depression and health care access). Moreover, time dependent SES effects were not significant after full adjustment, and the SES variables were not measured simultaneously. The sample covered adults from the Alameda County, California, who were free of diabetes at baseline. The participants were followed during five waves for 34 years. It concluded that education was a good predictor of incidence at baseline; occupation was a better predictor in middle or later adulthood; and time dependent income was a weak predictor of diabetes. Limitations of the study included the use of self-reports of diabetes;
difficulties in distinguishing individuals with type 1 and type 2 diabetes; and survival bias.
Another study showed a negative association between the incidence of diabetes and income, education and occupation among women in US; and only with income and education among men (Robbins et al., 2005). Initial analyses were adjusted for age and
ethnicity. After adjustment for body size, diet, physical activity, alcohol and tobacco use, most of the associations were attenuated, and the association between household income and diabetes incidence disappeared. Therefore, potential mediators did not account completely for the association between diabetes incidence and SES. The simultaneous effect of the three SES measures was not analyzed. The study covered men and women from the NHANES I Epidemiologic Follow-up Study 1971-1992 (NHEFS), who were free of diabetes in 1980. The mean follow up was 10 years. In addition to self-reports, record of hospitals admissions or discharges were used to identify diabetes.
Two previous studies confirmed a negative association between education and the incidence of diabetes in the NHEFS. However, they were limited to African Americans and non-Hispanic whites. A study found a negative association between diabetes incidence and education, but only in the entire cohort, all women, and white women (Lipton et al., 1993). This association was independent of age, sex, race, BMI,
subscapular triceps, systolic blood pressure, and activity level. The study was based on the NHANES I Epidemiologic Follow-up Study 1971-1987. Although some of the information was collected from proxies, almost half of the diagnoses were verified by multiple sources. In addition, no distinction was made between types of diabetes.
Another study confirmed a negative association between education and the incidence of diabetes in men and women separately (Resnick et al., 1998). The associations were independent of BMI and subscapular-to-triceps skinfold ratio. The study included adults from the NHFES 1971-1992, five more years of follow-up than in the previous study. In contrast with the previous study, more adults were excluded due to more restrictions in the definition of diabetes at baseline.
An analysis showed that lower adult SES (spouse‟s education) was associated with a higher incidence of diabetes, independently of childhood socioeconomic status
(measured by father‟s occupation), (Lidfeldt et al., 2007). Obesity partly accounted for these associations. The analyses included married or widowed women from the Nurses Health Study. The participants were followed up by questionnaire every two years during ten years. Although diabetes was self-reported, it was confirmed by questions on tests and medications. Moreover, the participants were homogenous in terms of
education and occupation. In addition, the analyses were adjusted for BMI, physical
activity, diet, alcohol consumption, smoking, hypertension, hypercholesterolemia, family history of diabetes, menopausal status, use of hormone replacement therapy, ethnicity, birth weight, and breastfeeding.
There were three studies based on data from the San Antonio Heart Study follow up.
The participants enrolled any year from 1979 to 1988 and then had a 7-to-8 year follow up examination. Diabetes was defined by self-reports, and then confirmed by medical examinations or use of medications. However, the response rate was low (61-68%). One study found a negative association between incidence of diabetes and education
(Haffner et al., 1991). This association was independent of age, sex, ethnicity and BMI.
The study included Mexican Americans and non-Hispanic whites. A study among Mexican-Americans found no association between SES and diabetes incidence neither in men nor in women (Monterrosa et al., 1995). SES was measured by the Duncan Socioeconomic Index, a measure of occupation prestige. BMI was a strong predictor of diabetes especially in women. A posterior study found a negative association between the incidence of diabetes and the neighbourhood SES that was independent of BMI (Burke et al., 1999). However, BMI reduced the odds ratios of neighbourhood SES considerably. No association was found between diabetes incidence and the Duncan Socioeconomic Index. The study included Mexican Americans and non-Hispanic whites who did not have diabetes at baseline and whose diabetes status was known. It was concluded that the rise in the prevalence of diabetes was due to an increased number of cases more than to an increased survival of people with diabetes.
A study in the U.S. revealed a negative association between military rank and diabetes incidence (Paris et al., 2001). The study was restricted to the military population on active-duty status. Individuals were selected if they had an initial diagnosis of type 2 diabetes. Then, they were age-matched to control subjects on a 4-to-1 basis. After recruitment, the mean time of service at diagnosis was 13.5 years. Diabetes was assessed by military records and confirmed in a small sample by registers at a medical treatment facility. However, misclassification of diabetes could have occurred because the criteria for diagnosis could have varied by physician.
A study in nine British towns revealed a higher incidence of type 2 diabetes among towns with worse SES (Barker et al., 1982). The towns were selected to represent each
of the three different latitudes (north, centre and south) and each of the three different SES (better, intermediate and worse): York, Wakefield, Preston, Chester, Derby, Stoke, Ipswich, Plymouth, and Newport. SES was calculated using a combination of the towns levels of income, overcrowding, unemployment, and car ownership. Moreover, a lower incidence of diabetes was observed in the two lowest social classes. New cases of diabetes were identified through records from hospitals. Incidence rates were compared using different standardisations to control for several biases: differences in rigor of screening between general practitioners; social class measured by occupation; and duration of residence. However, no measure of the strength of the association was presented and there was a lack of adjustment for individual risk factors.
In the Whitehall II study, lower employment grade was associated with an increased risk of the incidence of diabetes among men, but not among women (Kumari et al., 2004). The association was independent of age, length of follow-up, ethnicity, family history of diabetes, height, systolic blood pressure, electrocardiographic abnormalities, BMI, exercise, and smoking. The participants were followed-up during 5 phases, from 1985 to 1999. According to civil service grade and salary, employment grade was classified as: administrative, executive and clerical. Housing tenure, car ownership and material problems were also examined. Material problems were associated with an increased incidence of diabetes among both men and women. Not having a car was associated with a higher incidence of diabetes only among men.
In conclusion, this section presented our review on the association between socioeconomic status and the incidence of diabetes. There was a small number of studies that examined this association and all of them were set in highly developed countries. Risk factors tended to attenuate or to vanish the associations between the incidence of diabetes and SES. There was approximately the same number of negative associations and “no associations”. Negative associations were more common with the variables education and area SES.