IJE Advance Access originally published online on August 19, 2004
International Journal of Epidemiology 2005 34(2):268-275; doi:10.1093/ije/dyh267
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Article |
Education, socioeconomic and lifestyle factors, and risk of coronary heart disease: the PRIME Study
1 Belfast-MONICA, Department of Epidemiology and Public Health, Queen's University Belfast, Belfast, UK
2 MONICA-Strasbourg, Laboratoire d'Epidémiologie et de Santé Publique, Strasbourg, France
3 MONICA-Lille, INSERM U508, Institut Pasteur de Lille, Lille, France
4 MONICA-Toulouse, INSERM U558, Faculté de Médicine Purpan, Toulouse, France
5 INSERM U258 Hôpital Paul Brousse, Villejuif, France
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Abstract |
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Background Socioeconomic differentials have been described in the risk of coronary heart disease (CHD) but the extent to which these differentials are explained by lifestyle factors has been examined to a lesser degree. We have examined the contribution of socio-economic factors to risk of CHD in a large cohort study in France and Northern Ireland.
Methods In all, 10 593 men aged 50–59 years were examined between 1991 and 1994 in centres in Northern Ireland, Lille, Strasbourg, and Toulouse. Details were obtained for a number of socio-economic indicators from the men at the baseline examination. Men were also screened for evidence of CHD and followed annually by questionnaire for incident cases of coronary disease. Coronary events (coronary deaths, myocardial infarction, and angina) were documented by clinical records and were reviewed by an independent medical committee.
Results In all, 842 men (8%) showed some evidence of CHD at screening examination and these men were more likely to be living in poorer material circumstances, be unemployed, or have had less full-time education than men without CHD at screening in both France and Northern Ireland. These relationships persisted following adjustment for all known risk factors for CHD. Among men who were initially free of CHD there were clear socio-economic differentials (years of full-time education, unemployment, and educational level) in the distribution of several risk factors for CHD, notably smoking habit (which differs in France and Northern Ireland), systolic blood pressure, body mass index, and fibrinogen. Total cholesterol in contrast showed no socio-economic differential whilst those with a shorter period of full-time education and the unemployed tended to be high consumers of alcohol. In this cohort of men free of CHD at baseline few socio-economic indicators showed relationships with risk of CHD by 5 years of follow-up. Only years in full education, educational level, and unemployment status when adjusted only for age and country showed significant relationships with CHD risk, but these became non-significant following adjustment for major CHD risk factors.
Conclusions Socio-economic differentials in long-term risk of CHD are apparent in both cohorts of men from France and Northern Ireland, particularly in men with evidence of CHD at baseline. Among men free of CHD at baseline, although there is strong evidence of socio-economic differentials in cardiovascular risk factors these do not contribute independently to risk of CHD at 5 years of follow-up in this large cohort of men from France and Northern Ireland.
Keywords Coronary heart disease, epidemiology, socioeconomic factors, lifestyle, risk
Accepted 1 June 2004
Socio-economic inequalities in health have been a major concern of public health since its inception as a scientific discipline in the 19th century. In Britain clear evidence exists that health inequalities persist despite general improvements in mortality, and marked differences in premature mortality in men under 65 years of age in unskilled and in skilled and professional occupations have been shown.1 A similar pattern has been shown in France.2,3
In Britain and other European countries various measures of socio-economic position in society have been used to demonstrate health inequalities. These include occupation, length of education, home ownership, car ownership etc. and these and other demographic characteristics such as marital status have been linked with health-related behaviours.4,5 Others have raised the possibility that psychosocial factors may explain in part health inequalities between groups within and between countries6 but in the PRIME study we could not detect a major effect of psychosocial factors in the risk of CHD at 5 years of follow-up.7 A study in Finland in occupational groups found that only certain workplace conditions contributed to excess risk of cardiovascular disease and that this excess risk was largely mediated by known risk factors.8 A review of studies in western Europe demonstrated a north–south gradient in the risk of CHD by occupational class: in England and Wales, Ireland, Finland, Sweden, Norway, and Denmark there was a strong association but in France, Switzerland, and Mediterranean countries this was lacking,9 although other data from France showed conflicting results.2,3
In the present report we examine the association between socio-economic indicators and CHD in men aged 50–59 years from the general population of three French cities and their environs and Northern Ireland (Greater Belfast). Since chronic CHD may affect employment and socio-economic status cross-sectional associations are examined initially in the complete screened population, and subsequently the prevalence of cardiovascular risk factors in men stratified by socio-economic status during five years of follow-up is examined in those free of coronary disease at baseline examination. In this paper we examine to what extent socio-economic status contributes to risk of CHD independently of known risk factors and whether socio-economic differentials in risk differ between middle aged men in France and Northern Ireland.
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Methods |
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Study design
Between 1991 and 1994, a sample of 10 593 men aged 50–59 years was examined in three centres in France: Lille in the north, Strasbourg in the east, and Toulouse in the southwest, and one centre in Northern Ireland (Belfast). The sample was recruited in factories and in various working organisations, in occupational medicine, health screening centres, and general practice. Subjects were informed of the aim of the study and agreed to an annual follow-up. Approval from the appropriate local Ethical Committee was obtained in each study centre. A more detailed description of the study populations and general methodology has been reported elsewhere. In each centre the sample broadly matched the demographic and socioeconomic structure of the background population.10
Questionnaires
Each subject completed self-administered questionnaires. These questionnaires on demographic, socio-economic factors, and dietary habits were checked by specially trained medical staff. Additional questionnaires on tobacco consumption, medical history, chest pain, medication etc were administered at the clinic.
A score for material conditions was devised which was based on the number of cars, baths and/or showers, toilets, and on home ownership.11
Clinical examination
Baseline investigations included anthropometric measurements, waist:hip ratio, a 12-lead electrocardiogram and standardized blood pressure measurements using an automatic sphygmo-manometer (Spengler SP9) as reported in detail previously.10
Pre-existing coronary disease
Subjects were considered as having history of coronary disease at entry if they reported one of the following events: (1) myocardial infarction and/or angina pectoris diagnosed by a physician; (2) electrocardiographic evidence of myocardial infarction, defined as major or moderate Q waves coded using the Minnesota system,12 (3) a positive chest pain questionnaire.
Follow-up
Subjects were contacted annually following their baseline examination by letter, and a clinical event questionnaire was filled up and sent to the centre. If the subject did not respond, a telephone contact was established with the subject or with his general practitioner. For all subjects reporting a possible clinical event, clinical information was sought directly from hospital or general practitioner notes. All details of electrocardiographs, hospital admissions, enzymes, surgical operations, angioplasty, treatment, etc. were collected. Death certificates were checked for supporting clinical and post-mortem information on cause of death. Whenever necessary, death circumstances were obtained from the subject's practitioner or family.
A medical committee was established, comprising one member of each PRIME centre, including the co-ordinating centre, and three independent cardiologists (two from France and one from the UK). All medical information related to any available events was examined and the committee decided a code for each, according to the following protocols: myocardial infarction (MI) was defined by one of the following sets of conditions: (1) new diagnostic Q wave or new other typical aspect of necrosis on electrocardiographs; (2) typical or atypical pain symptoms and new (or increased) ischaemia on electrocardiographs and a myocardial enzyme level higher than twice the upper limit; or (3) post-mortem evidence of fresh MI or thrombosis. Angina pectoris was defined by the presence of chest pain at rest and/or on exertion and at least one of the following criteria: (1) angiographic stenosis over 50%; or (2) a positive scintigraphy (if no angiographic data); or (3) positive exercise stress test (if no angiographic or scintigraphic data); or (4) electrocardiographic changes at rest, (if no angiographic, scintigraphic or exercise stress data), but without any set of conditions for MI and no evidence of a non-coronary cause in the clinical history. Coronary deaths were defined from death certification with supporting clinical or pathological information whenever possible. Total coronary cases were defined as all subjects with at least one diagnosis of coronary death, non-fatal MI or angina pectoris. Further details of the classification of outcome events are described elsewhere.13
Statistical analysis
Statistical analysis was conducted using SPSS software. Hard coronary events were counted only once, whether or not preceded by a diagnosis of angina. Conversely, angina pectoris events were counted once (stable or unstable according which occurred first), but only when they were not preceded by a MI. Results were expressed as mean ±SD or as percentage and (number of subjects). Statistical significance was assessed for P < 0.05. Relative odds were computed using standard logistic regression analyses with coronary prevalence or incidence as the dependent variable. Interaction terms were fitted when appropriate.
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Results |
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The distributions of demographic and socio-economic variables in the whole cohort examined at baseline were broadly comparable between Northern Ireland and France, although some differences were found. For example the Northern Ireland centre had a higher proportion of households with 41 individuals (37% versus 23%), significantly fewer households scoring high on the rating scale of material conditions (39% versus 67%), and fewer subjects on sick leave, disabled, or retired (6% versus 15%). Differences between the three French centres tended to be small and non-significant.
We initially examined whether there was a relationship between socio-economic indicators and prevalent CHD. All men with historical, symptomatic, and asymptomatic evidence of CHD were categorized as having pre-existing or prevalent disease using questionnaire and electrocardiographic diagnostic criteria defined previously in the methods section. Some 842 men (8%) showed some evidence of prevalent disease and in both countries were more likely to live in rented accommodation, to be without a car, to have had a shorter period of full-time education and to be unemployed or on sick leave, disabled, or retired, particularly in Northern Ireland. All socio-economic data were fully adjusted for all other risk factors for CHD (Table 1). Other demographic indicators also showed an association in France where single men had a higher risk of prevalent disease and where greater household density showed a lower risk.
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Since there was a clear association between socio-economic variables and the risk of prevalent disease subsequent analyses were done on subjects without historical, symptomatic, and asymptomatic evidence of CHD at baseline. We firstly examined the distribution of cardiovascular risk factors in these subjects according to years of education, level of education, and employment status. These data are shown in Table 2.
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The pattern of smoking habit shows some difference between Northern Ireland and France. In Northern Ireland all three markers of socio-economic status showed significant gradients with smoking habit but in France only unemployed men showed a significantly higher smoking habit. The remaining variables show similar associations in France and Northern Ireland (a greater proportion of high alcohol consumers [top 20% of the distribution], higher systolic blood pressure, fibrinogen and, body mass index in lower socio-economic groups whilst cholesterol levels did not differ between the groups).
Although there were strong associations between most risk factors and socio-economic status in these subjects without evidence of CHD at baseline, at 5 years of follow-up only a few socio-economic indicators predicted risks of subsequent CHD when these variables were adjusted only for age. Results were pooled for Northern Ireland and France following an examination of interactions between socio-economic indicators and country. Since only one interaction (marital status) was statistically significant from a total of 8 this was considered to be a chance occurrence and the interaction effect was therefore ignored. Table 3 shows the association of socio-economic factors and 5-year risk of CHD (in subjects initially free of CHD) adjusted only for age and also fully adjusted for conventional cardiovascular risk factors (as in Table 1).
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Men who had spent
15 years in full time education and those who had received a higher education had a significantly reduced odds of a subsequent coronary event if no allowance was made for other risk factors (other than age). Additional adjustment for these reduced the strength of the association to non-significance but did not abolish the tendency towards a reduced risk. Unemployed men showed an increased risk of a coronary event which was also reduced to non-significance after adjustment for other risk factors. Conversely, men who lived in households with 4 others showed increased risk and, following adjustment for other risk factors, this achieved statistical significance. Further examination of these data indicated that adjustment for smoking habit ‘explained’ the significant association between household density and risk of CHD. Conversely, adjustment for smoking habit was responsible for the non-significance of unemployment in the multivariable moel whereas no single factors were responsible for non-significance for years of full-time education or educational level.
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Discussion |
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Our previous cross-sectional report10 demonstrated that, although our cohorts were based on volunteers from each study centre in France and Northern Ireland, the men were broadly representative of the general population from which they were drawn. The overall level of material conditions was lower in Northern Ireland and the majority of socio-economic indicators showed relative disadvantage compared with indicators in the French centres. However, these socioeconomic indicators did not appear to be closely linked with biological factors such as fibrinogen levels, which have been implicated in this study as explaining 30% of the difference in incidence in coronary events between the French centres and Northern Ireland.14
Socio-economic indicators were closely associated with historical or clinical evidence of CHD obtained at screening examination (Table 1). The pattern of associations was broadly similar in France and Northern Ireland (associated with rented accommodation, lack of a car and shorter period of full education) but the risk of pre-existing CHD was severalfold times greater in men on sick leave, disabled, retired, and in the unemployed in Northern Ireland compared with men in the same situation in France. These associations may have arisen in part, possibly in large part, as a consequence of underlying cardiovascular disease rather than because of cardiovascular risk factors, such as those due to lifestyle. As the burden of cardiovascular disease is much greater in Northern Ireland than in France in men of the same age group and more likely to appear earlier in life, the possibility that the presence of cardiovascular disease could influence socio-economic status is more likely in Northern Ireland than in France.
In the remaining data analysis subjects with pre-existing CHD, using both clinical and sub-clinical definitions of CHD, are excluded, and men free of disease were followed for subsequent coronary events during a 5-year follow up. Few socio-economic or demographic variables showed consistent relationships with risk of subsequent coronary disease, in spite of large differences in cardiovascular risk factors. This is in contrast to the findings in the Copenhagen Heart Study which showed a stronger relationship between socio-economic status at 8 years of follow-up to that at 22 years.15 In the Copenhagen study only men with a clinical history of CHD were excluded from the analysis which may account at least in part for the difference in the results. The present study is limited to a narrow age range of men (50–59 years at entry) and a short period of follow-up but is able to directly compare risk of CHD by socio-economic characteristics in two distinct countries and cultures.
No clear explanation exists for the reduced risk among widowers, divorced, and separated men in the Northern Ireland population, nor in the increased risk in both countries in men from households with 4 residents. However, men who had higher level education or 15 years in full-time education showed reduced risk which was attenuated by additional adjustment for other risk factors such as smoking habit. We had previously shown the classical risk factors to be important risk factors in this cohort14 but cross-sectionally10 and prospectively14 they did not explain the large difference in incidence of coronary disease between France and Northern Ireland.10 Smoking and other cardiovascular risk factors have been shown in the UK,16,17 France,3 other European countries18,19 and in the USA20 to be associated with socio-economic factors such as occupation and length of full-time education. Davey Smith et al. have compared these indicators for cardiovascular risk in two large occupational cohorts of men in the UK16,17 and have linked education with socio-economic circumstances in early life. In an introductory review of European studies Vartiainen et al.18 noted that cardiovascular risk factors were estimated to contribute up to a half of the variation in cardiovascular mortality between the range of socio-occupational groups. In the present report only comparative data for length of education are available and smoking habit is clearly related to length of education in Northern Ireland. A British randomised controlled trial suggested that less-educated groups responded better to practical demonstrations of lung damage due to smoking than did better educated groups who responded more to statistical arguments.21 Socio-economic differentials in mortality appear to be widening in several European countries22 along with differentials in health-related behaviours such as smoking habit, particularly among younger men and women,23 and long-term efforts to reduce these may need to adopt different approaches in different socio-economic groups. Our cross-sectional data show a strong socio-economic gradient in men with clinical or historical evidence of CHD but, in men without evidence of CHD, socio-economic indicators do not appear to contribute independently from major biological and behavioural risk factors for subsequent risk of CHD during 5 years of follow up. An additional period of follow up and comparable analyses of other cohorts may be helpful in establishing whether socioeconomic indicators contribute independently of other established factors to risk of CHD.
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KEY MESSAGES
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Acknowledgments |
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The PRIME Study is organized under an agreement between INSERM and the Merck, Sharpe and Dohme-Chibret Laboratory, with the following participating Laboratories: The Strasbourg MONICA Project, Department of Epidemiology and Public Health, Faculty of Medicine, Strasbourg, France (D Arveiler, B Haas); The Toulouse MONICA Project, INSERM U558, Department of Epidemiology, Paul Sabatier-Toulouse Purpan University, Toulouse, France (J Ferrières, JB Ruidavets); The Lille MONICA Project, INSERM U508, Pasteur Institute, Lille, France (P Amouyel, M Montaye); The Department of Epidemiology and Public Health, Queen's University, Belfast, Northern Ireland (A Evans, J Yarnell); The Department of Atherosclerosis, INSERM UR545, Lille, France (G Luc, JM Bard); The Laboratory of Haematology, La Timone Hospital, Marseilles, France (I Juhan-Vague); The Laboratory of Endocrinology, INSERM U326, Toulouse, France (B Perret); The Vitamin Research Unit, The University of Bern, Bern, Switzerland (F Gey); The Trace Element Laboratory, Department of Medicine, Queen's University Belfast, Northern Ireland (D McMaster, Jayne Woodside, Ian Young); The DNA Bank, INSERM U525, Paris, France (F Cambien); The Co-ordinating Center, INSERM U258, Paris-Villejuif, France (P Ducimetière, A Bingham). We thank Dr Chris Patterson for statistical advice; also the following organizations which allowed the recruitment of the PRIME subjects: the Health screening centers organized by the Social Security of Lille (Institut Pasteur), Strasbourg, Toulouse, and Tourcoing; Occupational Medicine Services of Haute-Garonne, of the Urban Community of Strasbourg; the Association Inter-entreprises des Services Médicaux du Travail de Lille et environs; the Comité pour le Développement de la Médecine du Travail; the Mutuelle Générale des PTT du Bas-Rhin; the Laboratoire d'Analyses de l'Institut de Chimie Biologique de la Faculté de Médecine de Strasbourg; the Department of Health (NI); and the Northern Ireland Chest Heart and Stroke Association.
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