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ORIGINAL ARTICLES |
From the University of Helsinki, Helsinki, Finland (M.K., M.E.); the University of Jyväskylä, Jyväskylä, Finland (M.-L.K., T.P., L.P.); and the Finnish Institute of Occupational Health, Helsinki, Finland (M.K., J.V.).
Address reprint requests to: Mika Kivimäki, Department of Psychology, Finnish Institute of Occupational Health, Topeliuksenkatu 41 aA, FIN-00250 Helsinki, Finland. Email: mika.kivimaki{at}ttl.fi
| ABSTRACT |
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METHODS: One hundred and five boys and 101 girls who were 8 years of age at entry into the study were observed for 34 years in the Jyväskylä Longitudinal Study of Personality and Social Development, Finland. Data were gathered on educational attainment and occupational status, as indicators of SES, and potential explanatory factors related to 0, (14), 27, 36, and 42 years of age. SBP and DBP were assessed at 15 and 42 years of age.
RESULTS: In a structural equation model adjusted for sex and childhood SBP, educational attainment was inversely associated with adult SBP (structural coefficient -0.17, p< .05). Incorporating the effects of parental SES and adult body mass index into the model attenuated this association so that it was no longer significant. Variation in birth weight, unemployment, smoking, alcohol consumption, and use of antihypertensive medication had marginal or no impact on the educationSBP association. No socioeconomic variation was found for DBP or occupational status.
CONCLUSIONS: Prospective evidence suggests a weak association between low educational attainment and development of high SBP. Parental SES and adult BMI were the key explanatory factors for this association.
Key Words: socioeconomic factors, hypertension, blood pressure, life course, body mass index, health risk behaviors.
Abbreviations: BMI = body mass index;; BP = blood pressure;; DBP = diastolic blood pressure;; GFI = goodness-of-fit index;; JYLS = Jyväskylä Longitudinal Study of Personality and Social Development;; SBP = systolic blood pressure;; SES = socioeconomic status;; SRMR = root mean squared residual.
| INTRODUCTION |
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Early factors, such as birth weight and parental SES, may also be important in explaining socioeconomic differences in health (3, 16). Low birth weight and parents low socioeconomic position reflect increased likelihood of poor nutrition, parental smoking, and childhood illnesses, and they have been found to be associated with high BP (1719).
Research on the relationship between SES and BP has progressed, but the evidence relies mainly on information about adult populations (2023). The few prospective studies that have started from childhood have examined predictors of adult BP rather than explanatory factors for the SES-BP relationship in adulthood (24, 25). The ongoing Jyväskylä Longitudinal Study of Personality and Social Development (JYLS) provided an opportunity to overcome some of these limitations. The JYLS study, a 34-year follow-up of school children, focuses on potential explanatory factors for the SES-BP relationship. Thus, we examined the extent to which birth weight, parental SES, adulthood lifestyle, and BMI explained the association between SES and adult BP.
| METHODS |
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Socioeconomic Status and Blood Pressure in Adulthood
Information on educational attainment (1 = low, 6 = high) and occupational status (1 = blue collar, 2 = lower-grade white collar, 3 = higher-grade white collar), representing indicators of SES in adulthood, were requested from the participants in the survey at 42 years of age. BP was measured in a physical examination of the participants at 42 years of age with the use of a standard mercury sphygmomanometer (Mercurius SK Stator). A registered nurse measured BP from one hand after the participant had been seated quietly for at least 5 minutes. The first and fifth Korotkoff sounds were recorded as SBP and DBP, respectively. For each participant, 3 BP measurements were recorded during the visit. We used the average of the second and third measurements in the analysis.
Early and Adulthood Factors
Sex, birth weight, childhood SBP and DBP, and parental SES comprise the variables referred to as early factors, whereas unemployment, smoking, alcohol consumption, BMI, and antihypertensive medication are termed adulthood factors. We treated sex and childhood BP as covariates, and the other variables as potential explanatory factors for the SES-BP relationship in adulthood.
Birth weight was obtained from the records of child welfare clinics, and the participants SBP and DBP at 15 (± 3) years of age were furnished by school health services. These records were linked to the data using the personal identification number assigned to each Finnish citizen. Parental SES (1 = blue collar, 2 = lower-grade white collar, 3 = higher-grade white collar) was derived from responses to questions about fathers and mothers job titles in participants interview at 27 years of age. Information about the parent with the higher socioeconomic status was used in the analyses. This variable strongly correlated with parental SES measured at 14 years of age (Pearson r= 0.90, p< .001, N= 96).
Using information from the participants interviews and surveys at 27, 36, and 42 years of age, we determined the number of years of unemployment between 27 and 42 years of age (27, 28) , smoking status at 42 years of age (0 = nonsmoker, 1 = ex-smoker, 2 = current smoker), and alcohol consumption at 42 years of age (0 = no or moderate consumption, 0 to 10,000 g of absolute alcohol per year in men and 0 to 7000 g for the women; 1 = risky consumption, 10,001 to 15,000 g for men and 7000 to 10,000 g for the women; 2 = heavy consumption, > 15,000 g for the men and > 10,000 g for the women) (29). During the medical examination at 42 years of age, physical measurements of height and weight were obtained to calculate the BMI, and the participants were asked whether they were being treated with antihypertensive medication (0 = no, 1 = yes).
Statistical Analysis
All of the study measures were treated as continuous variables. The data analysis included the following 3 parts: 1) an assessment of bivariate relationships between early factors, SES, and adult BP; 2) an assessment of bivariate relationships between adulthood factors, SES, and adult BP; and 3) tests of explanatory factors for the SES-BP relationship in adulthood.
In parts 1 and 2, bivariate relationships were studied with Pearson correlation coefficients in a sample combining the men and women. Since some variables may be gender sensitive, we also tested sex interactions in these relationships using regression models with cross-product terms, as suggested by Cohen and Cohen (30). Statistically significant sex interactions were illustrated by reporting correlations separately for the men and women.
In part 3, multiple linear regression models were used to identify early and adulthood explanatory factors for the association between indicators of SES and the development of BP. This association was first adjusted for sex and childhood BP to control for effects related to sex and base-line variation in BP. Then additional adjustments were made separately for early and adulthood factors. To evaluate the contribution of each explanatory factor, we compared the estimates of the association between SES and BP before and after adjustment for the factor. Greater attenuation in the estimate indicates greater explanatory power of the adjusted factor (31).
Using the findings of the multiple regression analyses, we formed a hypothetical model regarding the specific effects of explanatory factors on the SES-BP relationship and tested the model using structural equation modeling. Again we compared estimates of the SES-BP relationship between models without and with explanatory factors. The fit of a structural model to the data were judged acceptable if the p-value of the
-square test was > 0.05, the GFI > 0.90, and the SRMR < 0.05.
All the analyses were conducted with the SAS 8.12 statistical program package (SAS Institute, Cary, NC, USA), except structural equation modeling, for which LISREL 8.54 software was used (32). Data analyses were performed using maximum sample size except in multiple regression analyses, which were based on a cohort with complete data (N= 166 after list-wise deletion) to ensure comparability between models. We replicated all the other analyses with this cohort, but the results remained very similar and thus are not reported.
| RESULTS |
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Adulthood Factors, Socioeconomic Status, and Blood Pressure
Table 3 presents the correlations between adulthood factors, SES, and adult BP. Nonsmoking, lower alcohol consumption, and lower BMI were correlated with higher educational attainment and higher occupational status. The correlation between unemployment and low educational attainment was significant for the women (r= -0.34, p< .001) but not for the men (r= 0.02, ns)(p for sex interaction < 0.05). Unemployment correlated with low occupational status among both sexes.
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Tests of Explanatory Factors
The correlation between educational attainment and adult SBP was -0.20 (p< .01), and that between occupational status and adult SBP -0.08 (ns). The corresponding correlations for adult DBP were -0.16 (p< .05) and -0.07 (ns), respectively. No sex interactions were found for these relationships. After adjustment for sex and childhood BP, only the relationship between educational attainment and SBP remained significant. Tests of explanatory factors therefore involved this relationship.
Table 4 illustrates the relationship between education and SBP after adjustment for sex and childhood SBP, and the effect of additional adjustments for potential explanatory factors. Adjustment for parental SES and adult BMI attenuated the relationship by 36% to a nonsignificant level. Thus these factors may partially explain the SESSBP relationship. Adjustment for other factors had little effect on this relationship.
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-square = 1.41, df= 1, p= .23, GFI = 1.00, SRMR = 0.03; Model 2:
-square = 5.12, df= 3, p= .16, GFI = 0.99, SRMR = 0.03).
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| DISCUSSION |
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People do not randomly achieve different levels of SES. Although there was an overall increase in the occupational status of the participants when compared with that of their parents, the likelihood of high educational attainment and high occupational status was greater for those whose parents had a high SES, especially for the men. Our findings accord with previous cross-sectional and prospective evidence on intergenerational trends in social position (3, 39). Low birth weight (a correlate of low parental SES) predicted a lower level of SES in adulthood, but only among women and in relation to 1 of the 2 indicators of SES. Thus, birth weight may not be as important a determinant of SES as parental SES is.
Adult SES and lifestyle were inter-related. In line with other studies (3, 9, 10, 12), people with high occupational status and, in particular, high education attainment were less likely to smoke, drink excess alcohol, and be obese than their counterparts with low SES. Causal relationships among these factors are likely to be complex and the observed associations may partially stem from early factors, such as shared lifestyle between parents and children (40, 41).
There was a weak inverse relationship between SES and BP. Higher educational attainment was associated with lower SBP, but the associations involving occupational status or DBP did not reach statistical significance. Stronger links with lifestyle risk factors may partially explain the greater BP differences between educational levels than between occupational statuses. Education is also less open to change than other SES indicators are, and it has been shown to be a stronger correlate of atherosclerosis than occupation and income in Finnish samples (42, 43).
The weak association between SES and BP in our study is in line with the findings of prior research (38). For example, the Whitehall study found nearly threefold gradient in prevalence of coronary heart disease and stroke, but the difference in systolic BP was no more than 3 to 5 mm Hg between the highest and lowest employment grade (44, 45). In the INTERSALT study, an inverse association between years of education and BP was found for men in 28 of 47 populations and for women in 38 of the same 47 populations (21). The US HANES III study showed no association between SES and BP (46, 47).
The results of our structural equation modeling suggest that the links between parental SES, education, and adult body mass are important in understanding socioeconomic differences in BP. Lower parental SES predicted lower educational attainment, which, in turn, was associated with increased BMI, a predictor of higher BP. Previous research consistently shows a positive relationship between body weight and BP (4851). In the cross-sectional INTERSALT study, the explanatory factors for the SES-BP relationship included both BMI and the intake of alcohol, potassium, and salt (21). Our finding that parental SES may also be an explanatory factor extends prior knowledge on the socioeconomic differences in adult BP.
In our study, there were only marginal attenuations in the education-BP relationship after adjustment for birth weight and unemployment. This finding suggests that the SES gradient in BP may not be accounted for by these factors. Use of antihypertensive medication also did not explain the relationship between SES and BP, a result corresponding with those of some earlier studies with shorter follow-up periods (13).
Limitations
Our results should be interpreted in light of some limitations of the study. The drawbacks include the relatively small and homogenous sample (all white participants) and the restricted follow-up period (ie, the participants had only reached their early 40s and thus the effect of normal aging on BP trajectories was possibly constrained). Moreover, 44% of the original cohort was lost during the follow-up; the primary factor responsible for the sample attrition was missing BP recordings. Future research with more diverse and larger samples is needed to evaluate the generalizability of our findings and to more closely explore sex differences in explanatory factors for the SES-BP relationship.
We measured base-line BP at 15 years of age, a measurement that may reflect both genetic and environmental influences (eg, through childhood BMI). Lack of information about the reliability of the instruments used to assess base-line BP is a potential weakness of this study. However, the time 1-time 2 correlation for SBP from childhood to adulthood during the 34-year follow-up was r= 0.33; this result corresponds to earlier findings of standardized BP assessments. For example, the tracking correlation of high BP for 337 school children from East Boston, Massachusetts, was 0.55 with an 8- to 12-year measurement interval (25). Several studies suggest that ambulatory BP measures are superior to casual measures as predictors of cardiovascular morbidity and mortality (52, 53). It would be beneficial to repeat the current assessments with portable BP monitors and multiple readings to better control for "white coat hypertension" and the nuisance effect of within-person variability in BP (25).
| CONCLUSIONS |
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| ACKNOWLEDGMENTS |
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Received for publication June 24, 2003.
| REFERENCES |
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