Income, education, and blood pressure in adults in Jamaica, a middle-income developing country

Michelle A Mendez1, Richard Cooper2, Rainford Wilks3, Amy Luke2 and Terrence Forrester3

1 Carolina Population Center, University of North Carolina at Chapel Hill, NC, USA.
2 Department of Preventive Medicine and Epidemiology, Loyola University, Chicago, IL, USA.
3 Tropical Metabolism Research Institute, University of the West Indies, Jamaica.


    Abstract
 Top
 Abstract
 Materials and Methods
 Results
 Discussion
 References
 
Background At present, little is known about how socioeconomic status (SES) is related to blood pressure (BP) and hypertension in developing countries. This cross-sectional study examined associations between SES and BP in 2082 adults from a peri-urban area of Jamaica, a middle-income developing country.

Methods Hypertension (systolic BP >=140 mmHg, diastolic BP >=90 mmHg or current hypertensive medication use) was estimated based on self-reported medication use and the mean of the second and third of three manual BP measurements. Income and education were self-reported. Linear or logistic regressions were used to estimate multivariate associations between BP or hypertension and SES.

Results Hypertension prevalence was 20% in men and 28% in women. In both men and women, the income distributions of BP and hypertension were non-linear, indicating elevated levels in low as well as in high-income groups. In contrast to the negative relationships typical for industrialized countries, multivariate-adjusted BP and hypertension were highest in the wealthiest women. In men with some high school education, income was positively associated with BP, while there were negative associations in men with lesser education. Unlike women, mean BP were highest in poor men with limited education. Low SES men were also least likely to receive diagnosis and treatment.

Conclusions Socioeconomic status is related to BP and hypertension in Jamaica, although relationships are non-linear. Behavioural and environmental factors that explain elevated BP among both low and high SES adults in developing countries must be identified to develop effective prevention strategies.


Keywords Hypertension, blood pressure, developing countries, Jamaica, socioeconomic income, education

Accepted 6 December 2002

Social and cultural transitions during the process of economic development are thought to influence the pace at which hypertension and other risk factors for cardiovascular disease (CVD) emerge in developing countries.1 Little is currently known about how socioeconomic factors may influence the distribution of blood pressure (BP) and hypertension in these societies, asfew studies have been published, and results have been heterogeneous.2–5 In contrast, there is a fairly consistent negative association between socioeconomic status (SES) and BP in industrialized countries.2,6 This negative association is thought to have emerged between 1940 and 1960; earlier studies in the US and UK found that high SES groups were at greater risk.6

The basis for the inconsistent associations between SES and BP in developing country studies is unclear. One explanation may be heterogeneity in stages of modernization and economic development. It has been hypothesized that there may be a transition in the relationship between SES and CVD risk, with the direction and magnitude of the associations changing from positive to negative with Westernization and economic development.7,8 While there is limited—and conflicting—empirical evidence of such a transition, plausible mechanisms have been put forth.8,9 In the least developed countries, high SES sub-groups may be early adopters of atherogenic and hypertensive lifestyles—characterized by smoking, sedentarism, and diets high in energy and fats. In more industrialized countries, high SES groups may be the first to adopt lifestyles that help to lower CVD risk, or to receive better treatment, while changes in the low SES environment may increasingly promote lifestyles adverse for health. Under this hypothesis, we would expect associations between SES and BP to be strong and positive in low-income countries, but variable in more industrialized, middle-income countries.

The relationship between SES and BP in the developing world may also be influenced by the emergence of new risk factorsfor hypertension and CVD. Though little is known about the distribution of potential risk factors such as fetal undernutrition and psychosocial stress, they may disproportionately affect the poor.1,10 The coexistence of emerging and ‘traditional’ risk factors may lead to elevated risk in both the poorest and relatively well-off subgroups.

Using income and education as socioeconomic indicators, this paper examines the association between SES and BP in Jamaica, a middle-income Caribbean country. Jamaica provides an opportunity to explore how SES is related to BP in a country where epidemiological and nutrition transitions are well underway, but coexist with problems related to undernutrition. Cardiovascular diseases are the leading cause of death, along with diabetes and cancer.11 Obesity and hypertension are highly prevalent.12 Nonetheless, nutritional deficiencies remain widespread, and nutrition-related disorders are also in the top five causes of death.11


    Materials and Methods
 Top
 Abstract
 Materials and Methods
 Results
 Discussion
 References
 
Study sample
Data were collected for an ongoing study initiated in 1993 as part of the International Collaborative Study of Hypertension in Blacks.12,13 The study was approved by ethics committees at Loyola University Medical Center, Chicago and at the University of the West Indies, Mona. Participants were recruited from in and around the city of Spanish Town, a large peri-urban area neighbouring the capital city of Kingston. Census data identified Spanish Town as a stable, residential community with similar variability in income and education as Jamaica overall.13 Using probability proportion to size to randomly select enumeration areas, a stratified sample of men and non-pregnant women in four age groups (25–34, 35–44, 45–54, and 55–74) was recruited by door-to-door solicitation of eligible residents. From 1993 to 1998, 2096 subjects (847 men and 1249 women) participated; 2082 (99%) had data on BP, age, and sex. The response rate was 61%.

Data collection methods
Trained staff measured BP using a standardized protocol.13 Arm circumference was measured and appropriate cuff sizes selected. After participants emptied their bladders and sat quietly for 5–10 minutes, three manual BP measurements were taken, with intervals of 1 minute between measures. The mean of the second and third measurements was used in all analyses. Hypertension was defined as self-reported current use of hypertensive medication, mean systolic blood pressure (SBP) >=140 mmHg or diastolic blood pressure (DBP) >=90 mmHg.14 Moderate/severe hypertension was defined as medication use or BP >=160/95 mmHg.

Trained field staff also collected anthropometric data. Digital scales were calibrated daily. Height and weight were measured without shoes and with light clothing. Body mass index (BMI, calculated as weight in kg/height in metres2) was used to classify participants as normal (<25), overweight (25–<30), and obese (>=30).15 Underweight (BMI<18) was omitted as there were few such subjects (5% of men, 6% of women) and this adjustment did not influence results.

Questionnaires were used to collect information on self-reported income, education, and other relevant social and biological factors, including family history of hypertension, occupation, marital status, oral contraceptive use (women), and lifestyle changes being used in attempts to lower BP (less salt, more exercise, dietary change, reduced alcohol use). Individual monthly income was reported in six categories: <=600 Jamaican dollars (J$), J$601–1000, J$1001–3000, J$3001–6000, J$6001–12 000, and >=J$12 000. Using category midpoints, income was deflated to J$1993 using consumer price index data from the Statistical Institute of Jamaica.16 As categories were fairly wide, only 2% of subjects were reclassified after deflation. The highest and lowest income groups were collapsed with neighbouring categories because of small numbers. Income was also dichotomized at J$3000/month (US$91.5 in 1993), approximately the national poverty line.17,18 Family income, reported by 997 subjects as part of an earlier sub-study, was used in supplementary models to validate results based on individual income.19 The Spearman correlation between individual and family income was 0.78 (0.65 for estimated per capita income); rank ordering was preserved for 73% of the sample using individual versus family income.

Statistical analysis
All analyses used Stata version 6.0 (College Station, TX). Age was strongly associated with income and education as well as with BP. Therefore, in descriptive analysis, mean BP and hypertension prevalence were age-adjusted by the direct method, using the total sample age distribution as the standard. Age-adjusted means and prevalences were calculated by sex, as well as by income and education level. The SES distribution of mean BP and hypertension was examined both for the entire sample and after excluding those on medication, as any influence of SES on access to care may have affected these distributions.

Multivariate-adjusted associations between mean BP and SES were estimated using linear regression; logistic regression was used to estimate associations between hypertension and SES. Models were fitted including and excluding treated subjects with similar results. Dummy variables were used for income and education to minimize assumptions about linearity. Based on the distributions observed, polynomials were used to test for quadratic trends; variables were centred to avoid collinearity. We evaluated confounding (15% change-in-estimate for parameters of interest) by the following variables: pulse rate, marital status, overweight status, fasting glucose level, household size, family history of hypertension, ambient temperature, alcohol and smoking use (past or current), and oral contraceptive use among women. Associations were significant at P < 0.05. We also assessed whether SES variables were modified by each other, or by overweight status; effect modification was deemed significant if P < 0.05 in linear and P < 0.20 in logistic models. Final models included only age groups, income terms, education, overweight status, and year of exam. Predicted BP from model equations were estimated to describe distributions after multivariate adjustment.

Overall, 18% of women and 17% of men declined to report income. There were no differences in age, BP, hypertension prevalence, education level, or obesity among participants with versus without income data. To further assess bias, supplementary linear models were fitted using estimated income values based on multivariate imputation, with age, sex, education level, marital status, interview date, employment status, and occupation category as predictors. Results (not shown) were very similar to main findings based only on subjects with reported income; coefficients for imputed and reported income were not significantly different. Other diagnostics included collinearity assessment, and confirming similar results after substituting log transformed BP or excluding multivariate outliers.20,21 To assess any influence of temporal changes, we also confirmed that findings were similar using samples limited to 1993–1996 and 1994–1998 data.

Finally, we estimated associations between SES and hypertension awareness, treatment, and control. Numbers available for this analysis were limited as there were few hypertensive men and women in each income and education category. Hypertensive participants who reported being told by a doctor or nurse that they had hypertension were defined as aware; one participant who reported pregnancy-related hypertension only and one who was uncertain about prior diagnosis were excluded. Control was defined as SBP<140 mmHg and DBP<90 mmHg among all hypertensives. In addition to age-adjusted prevalence estimates, logistic regression was used to estimate associations between SES and awareness, treatment and control, adjusted for age.


    Results
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 Abstract
 Materials and Methods
 Results
 Discussion
 References
 
Sample characteristics
Table 1Go provides basic descriptions of sample demographics, BP, and hypertension. The mean (SD) age was 46.1 (13.6) in women and 46.4 (14.5) in men. Mean BP was similar in both sexes, although hypertension prevalence was higher in women than men (28.7 versus 20.1%), largely as a result of more treatment in women (17% of women; 8% of men). Poverty was highly prevalent, and most participants had <=6th grade education. Spearman correlations between income and education were 0.25 for women, and 0.39 for men, reflecting heterogeneity in earnings at each level of education.


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Table 1 Socio-demographic characteristics, blood pressure and hypertension prevalence among Jamaican adults
 
Socioeconomic status and age-adjusted blood pressure/hypertension
Figure 1Go shows the distribution of age-adjusted BP and hypertension prevalence by level of income and education. Participants in both the lowest and highest income groups had elevated BP and hypertension prevalence relative to those in intermediate categories. Mean BP and hypertension were generally highest in the top income group, creating a J-shaped pattern. Differences in mean BP and hypertension prevalence across income groups were substantial. Compared with the group with lowest mean BP, participants in the top income category had 2–5 mmHg higher age-adjusted SBP, and 4–5 mmHg higher age-adjusted DBP. Similarly, there was a substantial difference in hypertension (9% among men, 11% among women) across income groups with the highest versus lowest prevalence.



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Figure 1 Age-adjusted distribution of blood pressure and hypertension by socioeconomic status

 
Distributions of BP and hypertension by level of educational attainment were also J-shaped among men, with the highest levels among college-educated men (Figure 1Go). Among women, however, the distribution was more irregular, with the lowest BP and hypertension prevalence in the college-educated group. However, there were few college-educated subjects in the sample (n = 56 men, 50 women).

Multivariate associations—main effects
Multivariate associations using dummy variables indicated that the wealthiest women had significantly higher mean DBP and hypertension than women in intermediate income groups (Table 2Go). Differences between the highest and lowest income levels were non-significant, reflecting the J-shaped distributions described above (Table 2Go). Because of the non-linear pattern, substitution of the cruder income variable dichotomized at the poverty line obscured these relationships (e.g. DBP coefficient for poverty 0.51 P = 0.56). Systolic blood pressure model results were similar, but did not reach significance, nor did associations with education. All main effect associations were insignificant in men.


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Table 2 Main effect associations between income, education, and blood pressure (BP)/hypertension in Jamaican men and women
 
When using continuous measures, although main effects of income were non-significant, significant income2 terms in women reflected quadratic trends. Income2 was significant for SBP (income –1.23 P = 0.114; income2 1.31 P = 0.049) and DBP (income –0.41 P = 0.47; income2 0.96 P = 0.044) among women not using medication, as well as in hypertension models (income –0.06 P = 0.57; income2 0.19 P = 0.03). Quadratic terms in men were significant only for DBP (income –1.05 P = 0.17; income2 1.06 P = 0.047). Education polynomials did not reach significance.

Joint effects of income and education
As illustrated in Figure 2Go, SES effects on BP in men were significant only after including an interaction between income and education level. Including this interaction showed that income was positively associated with BP only among men with >6th grade schooling (DBP coefficients for increasing versus lowest income group: 1.84 P = 0.62; 7.05 P = 0.02; 6.55 P = 0.03). In men with limited (<=6th grade) schooling, higher income was associated with reduced BP (DBP coefficients: –2.59 P = 0.17; –4.78 P = 0.01; –1.34 P = 0.51). Similarly, >6th grade education was negatively associated with BP among low-income (DBP coefficient –6.36 P = 0.01), but not higher-income men (coefficient 2.35 P = 0.10). Associations were similar for SBP, and similar but non-significant for hypertension. Poverty–education interactions remained at least marginally significant (P < 0.10) among normal weight as well as overweight men. Findings did not change when retirees and/or college-educated men were excluded (not shown). Interactions were not significant among women. As in women, mean BP were elevated in the wealthiest men. However, stratified results and interactions showed that while mean BP were highest among wealthiest women regardless of education level, BP were highest among the poorest men if they also had limited education (Figure 2Go).



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Figure 2 Predicted mean blood pressure by income and education level

 
Awareness, treatment, and control
As shown in Table 3Go, income and education were associated with hypertension awareness, treatment, and control in men. Hypertensive men with incomes below the poverty line were less likely to be aware (29–53%) than those with higher incomes (71%). The odds ratio (OR) comparing likelihood of awareness among poor men versus non-poor men was 2.7 (95% CI: 1.1, 6.5, adjusted for age, BMI, and education). Fewer poor than non-poor men received treatment, both currently (adjusted OR = 2.2, 95% CI: 0.9, 5.3) and in the past (not shown). Poor men were also less likely to have controlled hypertension (OR n.s., not shown). Compared with those who never attended high school, men with more education had higher levels of awareness (59% versus 66%), treatment (36% versus 53%), and control (17% versus 31). However, associations were not significant (not shown). Cell sizes were too small to explore income–education interactions.


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Table 3 Age-adjusted awareness, treatment, and control of hypertension by income and sex (among hypertensives)a
 
Women were more likely than men to be aware of hypertension, as well as to be treated and controlled (OR significant for sex differences in awareness, treatment, and control, not shown). Unlike men, neither income nor education appeared to strongly explain awareness or control among women. However, more women above the poverty line received hypertension treatment than did women who were poor (OR adjusted for age, BMI, and education 2.8, 95% CI: 1.5, 5.2).


    Discussion
 Top
 Abstract
 Materials and Methods
 Results
 Discussion
 References
 
In this middle-income developing country, BP and hypertension levels were elevated in low- as well as high-income groups. Adjusted mean BP and hypertension levels in women were most elevated in the top income groups, resulting in J-shaped distributions. Diastolic blood pressure and hypertension prevalence were significantly higher in the top versus intermediate income groups. Income was also associated with significantly higher BP among men with some high school education. In men with more limited education, however, there was a negative relationship.

In contrast to generally positive associations with income, associations with education were generally negative or insignificant. Non-significant negative associations were observed in women. Similarly, >=6th grade education was associated with significant reductions in BP among poor men. In non-poor men, though, there was a non-significant positive association.

Furthermore, disparities in BP across SES groups were substantial. Age-adjusted differences in BP across SES groups were in the order of 2–5 mmHg for SBP and 4–5 mmHg for DBP; multivariate-adjusted differences were at times higher. Differences of 5 mmHg in DBP have been estimated to predict a 21% lower rate of CHD and 34% lower rate of stroke.22 We also found substantial differences—of the order of 10%—in the prevalence of hypertension across SES groups.

A better understanding of pathways linking the socioeconomic environment to BP is essential for designing and targeting interventions to reduce hypertension in rapidly developing countries where levels of CVD and hypertension are increasing.1 Hypertension prevalence in this study (20.1% in men, 28.7% in women) was similar to recent estimates for the US (22.8% for men and 18.0% for women).23 Environmental factors that contribute to the high prevalence of hypertension, the substantial disparities in BP across some SES groups, and differences in the magnitude and direction of relationships with income versus education, are not well understood. To our knowledge, the interaction between income and education observed in men has not been described in previous studies on BP. The interaction may well be a statistical artefact. Alternatively, it may be that in men with very limited education—and thus limited economic prospects—increased income may act as a buffer against psychosocial stress. Income may be associated with greater stress, or other adverse lifestyle practices, in more educated men. It is not certain why this interaction was not found in women. Studies in other contexts are needed to examine these relationships in men and women, and to explore underlying factors that might explain this phenomenon.

Income and education may be markers of a diverse range of community and individual factors associated with CVD risk. Studies in developed countries have found relationships between SES indicators and factors such as access to care, availability of healthy foods, individual dietary patterns, and physical activity.24–26 Similar factors may explain the SES gradient in BP in highly westernized, middle-income developing countries.

Most studies in developing countries have reported direct (positive) or no associations between SES and blood pressure, although a number of studies have reported negative relationships.2,5,27 Few studies, if any, have explicitly examined whether relationships are non-linear, or examined joint effects of income and education. The non-linear associations observed in this study may reflect widespread adoption of lifestyles characterized by factors such as sedentarism and high-fat diets across diverse SES groups. Relatively high BP among low SES groups may also reflect alternative risk factors such as early undernutrition or psychosocial stress. Indeed, previous studies in Jamaica found that growth retardation in infancy was associated with higher BP in late childhood.28 Further study of SES and BP in countries at different stages of development may provide insights on how socioeconomic transitions are linked to the emergence of hypertension.

As reported previously, treatment rates in this sample were high.29 In this study, we also showed that low income was associated with reduced awareness, treatment, and control of hypertension in men. Income was also weakly related to lower treatment rates in women. Given the high prevalence in low SES groups, ensuring adequate diagnosis and treatment of hypertension regardless of SES is essential. Disparities in diagnosis and effective treatment of early markers of CVD risk could create disparities in progression to more advanced disease. Indeed the observation of greater declines in CVD mortality among high versus low SES groups suggests such disparities have been problematic in industrialized countries.6

Although 18% of the sample did not report income, we found consistent results for men and women, and similar results based on imputed or family income, suggesting that the income distributions observed are credible. The observation of a similar pattern for education also reduces the likelihood that the income distribution in men might be attributable to reverse causality, such as adverse effects on current earnings as a consequence of health-related changes in employment. Despite a moderate response rate (61%), comparisons of mean income and unemployment against national data indicated that the sample was similar to Jamaica as a whole, suggesting our results are generalizable. The sample poverty level (45%) was similar to national estimates of 40–60% during 1990–1998.17

In spite of the higher prevalence in industrialized countries, developing countries contribute nearly twice as much as industrialized nations to the global CVD burden because of their larger population size.1,2 The prevalence of hypertension in these countries has risen dramatically in recent decades; further increases are anticipated with improvements in life expectancy and continued development.1,30 Effective primordial prevention is needed to mitigate this epidemic.


KEY MESSAGES

  • Adult hypertension levels in Jamaica, a middle-income developing country undergoing epidemiological and nutrition transitions, were comparable to levels in many industrialized countries.
  • In contrast to the negative associations observed in many industrialized countries, mean blood pressure (BP) and hypertension levels were elevated in both low and high socioeconomic status (SES) groups. Blood pressure levels were highest in the wealthiest women, and were elevated in the wealthiest men. However, mean BP was highest in poor men with only primary school education.
  • Rates of treatment and control were relatively low among the poor, particularly among men. This suggests that treatment efforts must give special emphasis to low-income groups to prevent the exacerbation of disparities in cardiovascular disease.
  • Behavioural and environmental factors underlying these disparities must be identified.

 


    Acknowledgments
 
The project was supported by NIH grant # HL 45508. M Mendez received funding from NIH grant # HL 53353 during the preparation of this manuscript.


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 Abstract
 Materials and Methods
 Results
 Discussion
 References
 
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