There are few studies addressing the relationship between socio-economic status and risk factors of cardiovascular disease in LMICs, so in the current study, we investigated this in a representative population sample from northeast Iran. A total number of 8733 participants with the average age of 49.67 ± 7.61 years were included and were classified based on education level, income, and occupation status into three classes of low, middle and high socio-economic status with frequencies of 44.4, 50.0 and 6.6%, respectively.
Metabolic syndrome was more prevalent in the intermediate SES population in our study, however, in comparison between high vs low SES, high SES group had more metabolic syndrome individuals compared with low SES, which is in the same direction with other studies (35–37). Cardel et al. have reported that Metabolic Syndrome severity was associated with both subjective social status (SSS) and objective social status among African American population and one-point increment in SSS score was related to a 0.04 decrease in Metabolic Syndrome severity score (38). Both education and income were shown to be associated with the presence of Metabolic Syndrome, but education was more prominent and had a greater effect on the risk of metabolic syndrome (39). This finding highlights the important effect of public awareness in the prevention of chronic diseases. Education increases the knowledge to acquire a healthy lifestyle and prefer healthy foods over unhealthy foods(40), and occupation could help in an individual’s psychosocial stress improvement and leads to metabolic syndrome prevention (41). Many studies have addressed gender-dependent associations, that low SES had more influence in developing metabolic syndrome in females rather than males (40, 42–45). Although the underlying mechanism for the gender-dependent association between SES and metabolic syndrome is unclear, some factors have been suggested, including parity, obesity-related effects on social mobility, and greater psychosocial risks in low SES women than low SES men (46).
Dyslipidemia was very common among all of our SES classifications. Lower SES group had slightly more participants with dyslipidemia, however, the differences between SES groups in terms of dyslipidemia prevalence was not remarkable. Li et al. showed that low SES (based on income, education, and occupation) is associated with higher rates of dyslipidemia and stated that people with low SES adopt unhealthy lifestyles such as inactivity, smoking, and alcohol consumption and therefore they are at higher risk for dyslipidemia (47). On the other hand, Nam et al. also reported that men who have higher levels of educational attainment and higher income are at more risk for dyslipidemia, however, educated women and women with higher income were at lower risk for dyslipidemia (48). In our population, high SES individuals had lower physical activity, which is known to be associated with dyslipidemia (49), and unhealthy behaviors may be a link between SES and dyslipidemia or other chronic diseases and effect as an intermediate mediator (50). This could be an explanation for why lower SES group in our study had a slightly higher prevalence of dyslipidemia. Findings regarding the correlation between SES and dyslipidemia are inconsistent especially in low and middle-income countries (48). One possible mechanism that may explain this inconsistency could be that people in different regions and social environments may behave differently in specific SES. For instance, high SES people in Iran tend to have more sedentary jobs and eat out more frequently, especially fast foods and junky foods. On the other hand, low SES people prefer to have homemade dishes (because of lower cost compared to fast foods), have more active jobs and are more predisposed to psycho-social stress, and smoke more cigarettes, and these associations might be completely different in high-income countries.
In our study hypertension was associated with higher SES and this was consistent with studies from Bangladesh and Uganda, indicating that people with higher education levels and higher incomes were at more risk for developing hypertension (51, 52).However, a meta-analysis by Leng et al. showed that higher blood pressure was associated with low SES, especially low education level. This association was significant in high-income countries, but not in some developing countries like India, Nigeria, Thailand, and Jordan (53). The difference between HICs and LMICs in case of SES effects on hypertension could be explained as follows: in developed countries, fast foods are usually the cheapest meals and are more affordable for low SES people, while in developing countries, high SES groups adopting western lifestyles, consume more fast foods and are at higher risk of hypertension.
Corsi et al. surveyed 758978 Indian participants and indicated a strong correlation between high income and higher prevalence of hypertension, diabetes, and obesity (14), which is similar to our findings in term of obesity, hypertension and diabetes. Other studies from India, Bangladesh, and China also reported that diabetes was more prevalent in high SES people (51, 54, 55), although studies by Kim SR et al.(among Korean population) and Jaffiol C. et al.(among French population) showed that people with low SES were at greater risk for diabetes (56, 57)and diabetes complications like retinopathy, were more common in low SES people (58). As it is concluded, like hypertension, findings about the relation between SES and diabetes vary from developing to developed countries and this inconsistency also has been reported in a systematic review by Zhiye Xu et al. that diabetes prevalence is increasing in developing countries and decreasing in developed countries(59).
We found an inverse gradient between SES and smoking. Concordant with Yiqian Zhan’s study, it is implied that people with higher education level and income tend to smoke less frequently (60), some other studies also determined the same findings (61, 62). Low SES smokers tend to have a higher risk of hypertension (63) and chronic diseases (64), hence more social support and attention are needed for them. In our study intermediate and high SES classifications had higher rates of smokers compared with low SES, however, former smokers did not differ among different SES categories. In a study carried out in Kingdom of Saudi Arabia, also individuals with higher education and higher income were more likely to be smoker (65). Although, other studies from China and India showed that smoking rate is higher among people with lower education and income (60, 66). In another study carried out by Ashleigh Guillaumier, demonstrated that majority of smokers were individuals with moderate education level rather than non-educated or tertiary education (67). The study also showed that majority of smokers income is within 200–400 $ rather than lower or above this amount (67). Inconsistent results regarding smoking like other cardiovascular risk factors are also seen lectures. Economical, regional and cultural factors and national’s policies towards tobacco use contribute to conflict results towards smoking status. It is also important to consider that smokers who are within lower SES group are at higher risk for chronic disease (64), therefore additional care should be provided for these disadvantaged people.
In this study higher SES increased the odds of being obese. However, studies have shown inconsistent findings (68–71). In our study, we did not assess education level and income as an indicator of SES separately, this makes it difficult to compare our findings with other studies because in some studies, education and income displayed opposed interaction with obesity (72, 73). Hua Zhang indicated that higher monthly income reduces the odds of being obese and higher education leads to being overweight; However, Mosli reported that in Saudi Arabia adults with lower education and higher income are more prone to obesity (72, 73). Physical activity decreases obesity and inflammation, while improves the endocrine system and energy expenditure (7). In our study, high SES individuals had lower physical activity, which could be an explanation for why higher SES leads to being overweight in our study and physical activity could be a link between SES and obesity as discussed thoroughly in the Pan et al. study (7).
Limitation and strengths
We did not explore the association between cardiovascular risk factors and SES indicators such as education, income, and occupation separately. Based on previous studies, these SES indicators displayed independent and diverse actions in the manner of socioeconomic status. Although we could consider this as a strength of our study because the socioeconomic status of a person is a combination and interaction of all indicators and they all have cumulative effects on overall SES. In this study we assessed the most important cardiovascular risk factors in a large population, this would give us a better outlook on the effect of SES on cardiovascular diseases and health care policies. By using more variables such geographic data maybe more accurate SES stratification could be developed which remains for future studies.