© 2007 American Public Health Association DOI: 10.2105/AJPH.2005.078691
Mika Kivimäki is with the Department of Epidemiology and Public Health, University College London, London, England. Debbie A. Lawlor and George Davey Smith are with the Department of Social Medicine, University of Bristol, Bristol, England. Anne Kouvonen and Marko Elovainio are with the Department of Psychology, University of Helsinki, Helsinki, Finland. Marianna Virtanen and Jussi Vahtera are with Finnish Institute of Occupational Health, Helsinki. Correspondence: Requests for reprints should be sent to Dr. Mika Kivimäki, Department of Epidemiology and Public Health, University College London, 1-19 Torrington Place, London WC1E 6BT, UK (e-mail: m.kivimaki{at}ucl.ac.uk).
Objectives. We examined the associations between socioeconomic position, co-occurrence of behavior-related risk factors, and the effect of these factors on the relative and absolute socioeconomic gradients in coronary heart disease. Methods. We obtained the socioeconomic position of 9337 men and 39 255 women who were local government employees aged 1765 years from employers records (the Public Sector Study, Finland). A questionnaire survey in 20002002 was used to collect data about smoking, heavy alcohol consumption, physical inactivity, obesity, and prevalence of coronary heart disease (myocardial infarction or angina diagnosed by a doctor). Results. The age-adjusted odds of coronary heart disease were 2.12.2 times higher for low-income groups than high-income groups for both men and women, and adjustment for risk factors attenuated these associations by 13%29%. There was no further attenuation with additional adjustment for the number of co-occurring risk factors, although socioeconomic disadvantage was associated with the co-occurrence of multiple risk factors. The absolute difference in coronary heart disease risk between socioeconomic groups could not be attributed to the measured risk factors. Conclusions. Interventions to reduce adult behavior-related risk factors may not completely remove socioeconomic differences in relative or absolute coronary heart disease risk, although they would lessen these effects.
Coronary heart disease (CHD), a leading cause of morbidity and mortality in all Western countries, is more prevalent among lower socioeconomic position (SEP) groups than among groups that have higher SEP.17 Although the evidence of such a socioeconomic gradient in CHD is robust, the extent to which this gradient is the result of different distributions of coronary risk factors between SEP groups remains controversial. Several epidemiological studies suggested that most (60%95%) of the CHD burden can be attributed to established risk factors: smoking, hypertension, diabetes, unfavorable cholesterol profile, and physical inactivity; appropriately, public health interventions target these risk factors to reduce the CHD epidemic.813 However, several studies that compare the magnitude of the socioeconomic gradient before and after adjustment for these risk factors suggest that they explain only 15%40% of the association between SEP and CHD.714 Thus, the contradiction: most of the population burden of CHD can be attributed to established risk factors, but these same risk factors only explain a small part of the association between SEP and CHD. We raise the possibility that multivariable adjustment for risk factors may not have correctly estimated the contribution of these risk factors to SEP differences in the occurrence of CHD.12,15,16 In the most commonly used approaches, such as logistic regression and proportional hazards regression, risk factors are entered into the model to examine how they change the effects of SEP indicators on CHD.14 Any change in the effect estimate of SEPs effects on CHD after adjustment for these risk factors is then used as an indication of the extent to which the risk factors "explain" the relative socioeconomic gradient in CHD. Underestimation might result from the failure of such models to fully account for clustering of individual risk factors. The overall effect of the risk factors would be underestimated if they were clustered (i.e., there was a greater than expected number of persons with either no risk factors or many risk factors), and this clustering was substantially more common in low- than high-SEP groups.17 Underestimation can also occur if the risk factors have synergistic effects (i.e., the effect of combined risk factors exceeds the predicted effects from separate risk factors, which assumes independence within the particular multivariable model).18 In most studies, the effect of synergism is examined by including interaction terms, but then removing them from the final model if the associated P value is large (conventionally >.05). However, most studies have a limited ability to detect multiple statistical interactions, and very large data sets are required to examine this possibility. Multivariable adjustment may underestimate the effect of established risk factors when explaining most of the "excess" cases among the lower SEP groups (i.e., the effect of established risk factors on the absolute risk difference between SEP groups). A recent study of 2682 Finnish men in the Kuopio Ischemic Heart Disease Risk Factor Study found that although adjustment for smoking, hypertension, dyslipidemia, and diabetes resulted in a modest (24%) attenuation of the relative socioeconomic gradient of CHD risk, these same risk factors accounted for most (72%) of the absolute socioeconomic gradient, that is, the excess risk among those from the lowest SEP compared with those in the highest.19 The role of behavior-related risk factors in CHD, particularly those that might be modified through health promotion, is likely to lead to important policy implications. We used a large employee sample of people who were participating in the Finnish Public Sector Study20,21 to examine the associations between SEP, co-occurrence of behavior-related risk factors (smoking, physical inactivity, obesity, and heavy alcohol consumption),8,11,2225 and the effect of these factors on the relative and absolute socioeconomic gradients in CHD.
The Finnish Public Sector Study focused on all local government employees of 10 towns and all employees in 21 public hospitals that provided specialized health care in the districts where the towns are located.20,21 These employees cover a wide range of SEPs, from city mayors to semiskilled cleaners. The largest groups were nurses and teachers. A total of 48 592 (9337 men, 39 255 women), aged 1765 years responded to a questionnaire survey in 20002002 (response 68%). Women were slightly overrepresented among the respondents (81% women) compared with eligible employees (n = 70 961, 76% women), but the differences in mean age (44.7 vs 44.0 years) and SEP (15% vs 17% performing manual labor) were small. The gender and age of respondents were also representative of Finnish public sector employees (77% women; mean age 44.6 years).26 However, the predominance of women did not correspond to the gender distribution of the Finnish general working population (48% female; mean age 45.5 years).26 Occupational status, income, and education were used as indicators of SEP. We obtained the participants occupational titles from the employers records (1931 different titles)27 and used the occupational classification by Statistics Finland27 to classify individuals into 3 categories on the basis of these titles: upper nonmanual workers, lower nonmanual workers, and manual laborers. Average monthly income figures for men and women were obtained by occupational title from Statistics Finland,27 and the distribution was divided into thirds separately for the men and women (referred to as high-, intermediate-, and low-income groups). Educational level was self-reported in the surveys and was categorized as primary or secondary versus tertiary. We assessed 4 behavior-related risk factors by using standard questionnaire measurements in the surveys. We requested the participants smoking status and the habitual frequency and amount of beer, wine, and spirits intake. Responses to the alcohol questions were transformed into units of alcohol per week.28 Binge drinking was determined by requesting whether the participant had passed out as a result of alcohol consumption more than once during the past 12 months. Physical activity was measured by the metabolic equivalent task index29 and was expressed as the sum score of metabolic equivalent task-hours per day (h/d). Self-reported weight and height were used to measure body mass index (kg/m2).
CHD was measured by using a self-administered checklist of common chronic diseases.30 For each disease, the respondent was asked to indicate whether or not a physician had diagnosed him or her as having the disease. Prevalent CHD was determined by affirmative responses for myocardial infarction or angina. The agreement between these self-assessments and data from medical records has previously shown to be substantial for myocardial infarction and angina (
We coded all of the risk factors as binary variables (0 or 1). Risks were defined as ever a smoker (current or past smoking), heavy alcohol consumption (>21 units of alcohol per week or binge drinking),22,23 physical inactivity (<2 metabolic equivalent task h/d),29 and obesity (body mass index>30 kg/m2).25 The age-adjusted association between SEP and binary risk factors was estimated in a logistic regression analysis that used upper nonmanual labor employees, high-income, and tertiary education as the reference groups. We counted the number of risk factors on the basis of these binary variables. Thus, the participants with all 4 risk factors had a score of 4; those with any 3 risk factors scored 3, and those with no risk factors scored 0. We performed a multinomial logistic regression analysis to examine the association between SEP and co-occurrence of risk factors (this analysis can assess associations that have a categorical outcome variable, such as ours).31 The multinomial models were used to assess the likelihood of having 1 risk factor, 2 risk factors, and 3 or 4 risk factors versus having no risk factors (the reference). The corresponding age-adjusted odds ratios were calculated for the levels of SEP by using the same reference groups that were used in the logistic regression analyses for each of the risk factors examined individually as described above. Finally, we tested the extent of clustered risk factors in the whole population and within each SEP group. The expected frequencies were those predicted given the prevalence of the risk factors within each SEP group (i.e., assuming independence or no clustering). Clustering is indicated when individuals are more likely to have no or many risk factors and are less likely to have a single risk factor than would be expected if the risk factors were independent (i.e., the observed-to-expected ratio is >1 for no risk factors, <1 for a single risk factor, and >1 for 3 and 4 factors). We calculated The associations of SEP, risk factors, and the number of co-occurring risk factors with CHD were studied first with age-adjusted logistic models. To estimate the contribution of the risk factors to the association between SEP and CHD, each risk factor, all their interactions on a multiplicative scale, and the number of co-occurring risk factors were added to the model as covariates. We wanted to examine whether greater attenuation was achieved if the risk factors were more finely categorized and linear associations were not assumed. To do this, we split body mass index, physical activity (metabolic equivalent task h/d), and alcohol consumption (units per week) into fifths of their distributions and entered these into the regression model as 3 variables together with 4 indicator variables that represented past smoking, current smoking, high alcohol consumption (> 21 units per week), and binge drinking. Finally, we estimated the absolute risk of CHD associated with SEP in the whole population and in a low-risk group (anyone who was free of all of the measured risk factors) to determine how many excess cases among the lowest (compared with the highest) SEP group would be removed if these risk factors were eliminated. The analyses were performed separately for men and women and for each SEP indicator using SAS 8.2 (SAS Institute Inc, Cary, NC) software. The findings were consistent across all of the 3 SEP indicators, so we reported full results for income only (indicator with evenly distributed categories) and have summarized the main findings for occupational status and education in the text.
Information was missing on income for 2227 (5%) of the participants, on any risk factor for 4044 (8%) of the participants, and on CHD for 4700 (10%) of the participants. A total of 39 631 employees (82% of all of the respondents) had full data for all of these variables. They differed slightly from participants who had some missing data in terms of gender (81% vs 79% women), mean age (44.2 vs 46.8 years), and SEP (15% vs 18% manual laborers). In spite of this, participants who had complete data were representative of all of the respondents (81% women, mean age 44.7 years, 15% manual).
In men and women, 31%45% had no risk factors, 53%63% had 1 or 2 risk factors, and
Table 1
In Table 2
Table 3
To illustrate what might happen to CHD risk and absolute SEP gradient if risk factors were removed from the population, we formed a low-risk subgroup that consisted of all the employees who had none of the measured risk factors (Table 4
Evidence from a large contemporary population suggests that SEP is associated with the co-occurrence of behavior-related risk factors such as smoking, heavy alcohol consumption, physical inactivity, and obesity. Men and women who have low SEP tend to have multiple risk factors more often than those who have higher SEP. Although these risk factors were clustered in the total sample, no evidence was found that clustering was more common in low-SEP groups or that the risk clusters had a synergistic effect on reported CHD. The effect of adjusting for risk factors on the relative socioeconomic gradient of CHD was to produce a modest (13%29%) reduction, which is a similar magnitude to reduction found in several other studies.7,14,19 Furthermore, when we examined absolute risk, we found that by removing the behavior-related risk factors, important reductions in the prevalence of CHD in all SEP groups would result without removing the socioeconomic gradient. These findings were replicable across the 3 SEP indicators of income, occupational status, and education. Thus, according to this study, behavior-related risk factors do not explain a large part of either the relative or absolute socioeconomic gradient in prevalent CHD. Few data have been published on the association between SEP and the clustering of risk factors, and those studies were on the basis of much smaller sample sizes than was used in our study. Consistent with our finding, an investigation of 2900 older women in the United Kingdom found evidence of risk factor clustering in all of the SEP groups and no evidence that the extent of the clustering was greater among those from the manual-labor SEP than among those from the non-manual ones.17 By contrast, a study of 3600 Australian adolescents found that both the co-occurrence and clustering of smoking, high levels of television watching, overweight, and high blood pressure were more common in families that had a low SEP than in other families.32 Similarly, a study of 480 young adults, aged 1824 years, reported that clustering of behavior-related risk factors was more common among participants who had a history of unemployment and less common among students.33 Thus, clustering of risk factors may be more common among individuals who have lower SEP in adolescence and young adulthood, perhaps because, at these ages, peer pressure related to the initiation or noninitiation of some risk behaviors is very socially patterned. However, the socioeconomic gradient in clustering does not appear to be retained in later adulthood. In this large study, we had the adequate ability to test for statistical interactions between behavior-related risk factors and their effect on CHD, yet we found none for either gender. Participants who had more behavior-related risk factors had greater risk of CHD, but this effect was consistent with the additive rather than synergistic effects of each risk factor. There was no evidence of differences in risk factor clustering by SEP and no evidence that the risk factors combined synergistically. It is, therefore, not surprising that we found that a more-complex adjustment (either including interaction terms or including both individual risk factors and a score for the number of risk factors) did not result in greater attenuation of the relative socioeconomic gradient in CHD than did simple adjustment for each individual covariate. We examined absolute differences in the prevalence of CHD within a group that was free of all measured behavior-related risk factors in order to illustrate the expected effects of removing these risk factors from the population. Our findings confirmed the modest contribution of these risk factors to the absolute socioeconomic gradient. The low-risk population had a lower prevalence of CHD throughout the entire social hierarchy, but the absolute (and relative) socioeconomic gradient in risk remained. This finding is in contrast to that of the Kuopio Ischemic Heart Disease Risk Factor study, in which the absolute socioeconomic gradient largely disappeared in a subgroup that was free of measured risk factors.19 A potential reason for the discrepancy between these studies involves the selection of risk factors. Three risk factors: hypertension, dyslipidemia, and diabetes, which were included in the Kuopio study but not in our study, are physiological markers of the underlying pathophysiological processes that end in manifest CHD. Only 5% of the CHD events occurred among the low-risk population in the Kuopio study. Because the elimination of these major disease mediators appears to eliminate much of the absolute socioeconomic gradient in CHD, it is likely that any underlying factors, whether socioeconomic, psychosocial, psychological, early life or genetic, may have their major influence through these disease mediators. In our study, with the exception of obesity, all of the measured risk factors were purely exogenous, which reflects the lifestyle of the participants. Of the CHD cases, almost 30% had none of these behavior-related factors. Therefore, it seems that several etiological pathways to CHD that can be related to SEP remain uncovered by these more distal risk factors.
Limitations The cross-sectional design of this study is open to reverse causality, healthy-worker bias, and survivor bias. If individuals who are diagnosed with disease change to adopt a healthier lifestyle, the associations of risk factors with CHD may be underestimated, and the extent to which these risk factors explain socioeconomic gradients may also be underestimated. However, the magnitude of the associations that we found between SEP, behavior-related risk factors, and CHD are similar to those reported in prospective studies,3743 which suggests that the cross-sectional nature of the study did not result in a major bias. The only exception was heavy alcohol consumption, which was not associated with CHD, even though it has predicted CHD events in several, though not all, prospective studies.37,40 Replication of our risk cluster analyses with a prospective investigation on incident CHD is important, but the challenge will be to achieve sufficient statistical power, which will require a very large cohort that is followed for many years. Our findings may not be generalized to other populations. However, a socioeconomic gradient in CHD of a similar magnitude has been reported throughout several different European and US populations. In addition, the associations between behavior-related risk factors and CHD risk are similar across these different populations. Thus, it is likely that our findings could be generalized to most developed countries.
Conclusions
The work presented in this article was supported by grants from the Academy of Finland (projects 117604 and 105195) and the participating towns and hospitals. The work of Debbie A. Lawlor was supported by a United Kingdom Department of Health, Career Scientist Award.
Human Participation Protection
Peer Reviewed
Contributors Accepted for publication March 11, 2006.
1. Paffenbarger RS, Hyde RT, Wing AL, Lee I-M, Jung DL, Kampert JB. The association of changes in physical-activity level and other lifestyle characteristics with mortality among men. N Engl J Med. 1993;328: 538545. 2. Stamler J, Stamler R, Neaton JD, et al. Low risk-factor profile and long-term cardiovascular and noncardiovascular mortality and life expectancy: findings for 5 large cohorts of young adult and middle-aged men and women. JAMA. 1999;282:20122018. 3. Yusuf S, Reddy S, Ounpuu S, Anand S. Global burden of cardiovascular diseases. Part II: variations in cardiovascular disease by specific ethnic groups and geographic regions and prevention strategies. Circulation. 2001;104:28552864. 4. Yusuf S, Hawken S, Ounpuu S, et al. Effect of potentially modifiable risk factors associated with myocardial infarction in 52 countries (the INTERHEART study): case-control study. Lancet. 2004;364:937952.[CrossRef][Web of Science][Medline] 5. Magnus P, Beaglehole R. The real contribution of the major risk factors to the coronary epidemics: time to end the "only-50%" myth. Arch Intern Med. 2001; 161:26572660. 6. Greenland P, Knoll MD, Stamler J, et al. Major risk factors as antecedents of fatal and nonfatal coronary heart disease events. JAMA. 2003;290:891897. 7. Lantz PM, House JS, Lepkowski JM, Williams DR, Mero RP, Chen J. Socioeconomic factors, health behaviors, and mortality: results from a nationally representative prospective study of US adults. JAMA. 1998;279: 17031708. 8. Marmot MG, Davey Smith G, Stansfeld S, et al. Health inequalities among British civil servants: the Whitehall II study. Lancet. 1991;337:13871393.[CrossRef][Web of Science][Medline] 9. Mackenbach JP, Bos V, Andersen O, et al. Widening socioeconomic inequalities in mortality in six Western European countries. Int J Epidemiol. 2003;32:830837. 10. Kaplan GA, Keil JE. Socioeconomic factors and cardiovascular disease: a review of the literature. Circulation. 1993;88:19731998. 11. Adler NE, Boyce WT, Chesney MA, Folkman S, Syme SL. Socioeconomic inequalities in health. No easy solution. JAMA. 1993;269:31403145. 12. Evans R, Barer M, Marmor T. Why Are Some People Healthy and Others Not? New York, NY: Aldine de Gruyter; 1994. 13. Macintyre S. The Black Report and beyond: what are the issues? Soc Sci Med. 1997;44:723745.[CrossRef][Web of Science][Medline] 14. Marmot MG, Bosma H, Hemingway H, Brunner E, Stansfeld S. Contribution of job control and other risk factors to social variations in coronary heart disease incidence. Lancet. 1997;350:235239.[CrossRef][Web of Science][Medline] 15. Davey Smith G, Shipley MJ. Confounding of occupation and smoking: its magnitude and consequences. Soc Sci Med. 1991;32:12971300.[CrossRef][Web of Science][Medline] 16. Begg CB. The search for cancer risk factors: when can we stop looking? Am J Public Health. 2001;91: 360364. 17. Ebrahim S, Montaner D, Lawlor DA. Clustering of risk factors and social class in childhood and adulthood in British womens heart and health study: cross sectional analysis. BMJ. 2004;328:861864. 18. Thompson WD. Effect modification and the limits of biological inference from epidemiologic data. J Clin Epidemiol. 1991;44:221232.[CrossRef][Web of Science][Medline] 19. Lynch JW, Davey Smith G, Harper S, Bainbridge K. Explaining the social gradient in coronary heart disease: comparing relative and absolute approaches. J Epidemiol Community Health. 2006;60:435441. 20. Kivimäki M, Virtanen M, Vartia M, Elovainio M, Vahtera J, Keltikangas-Järvinen L. Workplace bullying and the risk of cardiovascular disease and depression. Occup Environ Med. 2003;60:779783. 21. Vahtera J, Kivimäki M, Pentti J, et al. Organisational downsizing, sickness absence, and mortality: 10-town prospective cohort study. BMJ. 2004;328:555. 22. Murray RP, Connett JE, Tyas SL, et al. Alcohol volume, drinking pattern, and cardiovascular disease morbidity and mortality: is there a U-shaped function? Am J Epidemiol. 2002;155:242248. 23. Pletcher MJ, Varosy P, Kiefe CI, Lewis CE, Sidney S, Hulley SB. Alcohol consumption, binge drinking, and early coronary calcification: findings from the Coronary Artery Risk Development in Young Adults (CARDIA) Study. Am J Epidemiol. 2005;161:423433. 24. Rimm EB, Klatsky A, Grobbee D, Stampfer MJ. Review of moderate alcohol consumption and reduced risk of coronary heart disease: is the effect due to beer, wine, or spirits? BMJ. 1996;312:731736. 25. Kopelman PG. Obesity as a medical problem. Nature. 2000;404:635643.[Medline] 26. Statistics Finland Web site. Available at: http://www.stat.fi/index_en.html. Accessed March 1, 2006. 27. Classification of Occupations: Handbook 14. Helsinki: Statistics Finland; 1987. 28. Kaprio J, Koskenvuo M, Langinvainio H, Romanov K, Sarna S, Rose RJ. Genetic influences on use and abuse of alcohol: a study of 5638 adult Finnish twin brothers. Alcohol Clin Exp Res. 1987;11:349356.[CrossRef][Web of Science][Medline] 29. Kujala UM, Kaprio J, Sarna S, Koskenvuo M. Relationship of leisure-time physical activity and mortality. JAMA. 1998;279:440444. 30. Haapanen N, Miilunpalo S, Pasanen M, Oja P, Vuori I. Agreement between questionnaire data and medical records of chronic diseases in middle-aged and elderly Finnish men and women. Am J Epidemiol. 1997;145:762769.[Web of Science][Medline] 31. Hosmer DW, Lemeshow S. Applied Logistic Regression. New York, NY: John Wiley & Sons; 1989. 32. Lawlor DA, OCallaghan MJ, Mamun AA, Williams GM, Bor W, Najman JM. Socio-economic position, cognitive function and clustering of cardiovascular risk factors in adolescence: findings from the Mater-University study of pregnancy and its outcomes. Psychosom Med. 2005;67:862868. 33. Raitakari OT, Leino M, Rakkonen K, et al. Clustering of risk habits in young adults. The Cardiovascular Risk in Young Finns Study. Am J Epidemiol. 1995;142:3644. 34. Rowland ML. Self-reported weight and height. Am J Clin Nutr. 1990;52:11251133. 35. Stevens J, Keil JE, Waid LR, Gazes PC. Accuracy of current, 4-year, and 28-year self-reported body weight in an elderly population. Am J Epidemiol. 1990; 132:11561163. 36. Lawlor DA, Taylor M, Bedford C, Ebrahim S. Agreement between measured and self-reported weight in older women. Results from the British Womens Heart and Health Study. Age & Ageing. 2001;31:169174.[CrossRef][Web of Science] 37. Powell KE, Thompson PD, Caspersen CJ, Kendrick JS. Physical activity and the incidence of coronary heart disease. Annu Rev Public Health. 1987;8:253287.[CrossRef][Web of Science][Medline] 38. Rimm EB, Stampfer MJ, Giovannucci E, et al. Body size and fat distribution as predictors of coronary heart disease among middle-aged and older US men. Am J Epidemiol. 1995;141:11171127. 39. Corrao G, Bagnardi V, Zambon A, La Vecchia C. A meta-analysis of alcohol consumption and the risk of 15 diseases. Prev Med. 2004;38:613619.[CrossRef][Web of Science][Medline] 40. Lloyd-Jones DM, Wilson PW, Larson MG, et al. Framingham risk score and prediction of lifetime risk for coronary heart disease. Am J Cardiol. 2004;94:2024.[CrossRef][Web of Science][Medline] 41. Oguma Y, Shinoda-Tagawa T. Physical activity decreases cardiovascular disease risk in women: review and meta-analysis. Am J Prev Med. 2004;26:407418.[CrossRef][Web of Science][Medline] 42. Emberson JR, Shaper AG, Wannamethee SG, Morris RW, Whincup PH. Alcohol intake in middle age and risk of cardiovascular disease and mortality: accounting for intake variation over time. Am J Epidemiol. 2005;161:856863. 43. Kujala UM, Kaprio J, Sarna S, Koskenvuo M. Future hospital care in a population-based series of twin pairs discordant for physical activity behavior. Am J Public Health. 1999;89:18691872. 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