a Department of Medicine, Sahlgrenska University Hospital/Östra, Göteborg, Sweden
b Section of Preventive Cardiology, Karolinska Institute, Stockholm, Sweden
* Correspondence to: Annika Rosengren, Department of Medicine, Sahlgrenska University Hospital/Östra, SE-416 85 Göteborg, Sweden. Tel: +46-31-3434000; Fax: +46-31-259254
E-mail address: annika.rosengren{at}hjl.gu.se
Received 24 January 2003; revised 29 August 2003; accepted 4 October 2003
Abstract
Aims Low socio-economic status is a well-known risk factor for coronary heart disease (CHD), but the evidence concerning social network has been less consistent. In this prospective cohort study of men we sought to estimate the impact of social network factors on the risk of incident coronary heart disease and mortality from all causes.
Methods and results In a population of 741 men aged 50 at baseline 92 new cases of coronary disease were identified over a follow-up period of 15 years. Social factors included occupational class, two measures of social supportemotional attachment and social integration and a measure of global mental stress. Among the men in the lowest quartile of social integration, there were 13.6 cases (per 1000 observation years), compared to 8.9 in the intermediate two quartiles and 6.0 in the highest quartile (P for trend 0.003). After adjustment for all relevant risk factors the hazard ratio (HR) for the highest, compared with the lowest, quartile was 0.45 (0.240.84);P for trend 0.013. Emotional attachment was also associated with significantly reduced risk. The adjusted HR for the lowest quartile was 0.58 (0.370.91); P=0.019. No relation between mental stress and risk of CHD, or between low occupational class and risk of CHD was found.
Conclusion In this prospective study of men, we found two dimensions of low social supportlow social integration and low emotional attachmentto be predictive of coronary morbidity, independently of other risk factors.
Key Words: Coronary disease Social integration Emotional support Social class Psychological stress
1. Introduction
Several studies have demonstrated that social networks predict all-cause mortality.18Nevertheless, the most compelling evidence for the role of social ties in health is still derived from the early summary of five population based cohorts.9The authors concluded that deficient social networks convey the same mortality risk as smoking. Some studies have found increased risk of cardiovascular mortality in men with low social activity10or men who were socially isolated,11but in general, the evidence concerning the relation between cardiovascular disease and social network has been much less consistent than for mortality. Low socio-economic status has been linked to increased risk of coronary heart disease,12as well as to poor social network,13but despite the fact that low social support is often linked to low socio-economic status, the role of social ties with respect to the development of coronary disease has not been firmly documented. Most studies have not, however, assessed the functional aspects of social ties and there is a relative lack of prospective data with respect to cardiovascular morbidity.
In the baseline investigation in 1983 of men born in 1933 in Göteborg, Sweden, social network factors were introduced, along with data on occupation and mental stress. In a pilot study from the first 6 years of follow-up the risk of myocardial infarction was significantly lower in men with a well functioning social network.14The measures included data on social supports, both social integration and emotional support, as well as on mental stress and socio-economic status. The aim of the present study was, with an extended follow-up of 15 years, to examine the long term effects of social integration and emotional attachment on all cause mortality and on the specific incidence of coronary heart disease (CHD).
The locally appointed Ethics committee approved the study protocol, informed consent was obtained from all participants and the study was done in compliance with the Declaration of Helsinki.
2. Population and methods
The study group consisted of a random sample of half of all men born in 1933 and resident in the city of Göteborg at age 50.14Through the census register of the city, 1016 men were contacted and invited to participate in the investigation, which included both questionnaires and physical examination procedures. Seven hundred seventy six men (76%) agreed to take part in the study during 1983 to 1984. After exclusion of ten men with a documented prior history of coronary disease and 25 men with incomplete data the study group consisted of 741 men (73%).
Fasting venous blood samples were drawn in the morning. Plasma levels of total serum cholesterol and triglycerides were analysed by means of standard laboratory procedures. Blood pressure was measured with a mercury sphygmomanometer in a sitting position after 5min of rest. Body mass index (BMI) was calculated as weight (kg)/height2(m).
Before the examination, all participants had completed a postal questionnaire dealing with smoking habits, physical activity, family history of myocardial infarction and occupation. The answers were checked by the examining physician. Smoking habits were coded into five categories: former smoker of more than 1 month's duration, smoking 114g of tobacco per day, smoking 1524g, and smoking 25g or more per day, with never smokers as a reference. One cigarette was considered to contain 1g of tobacco, a cigarillo 2g, and a cigar 5g of tobacco. Leisure time physical activity was coded according to a four-graded scale, with 1 representing sedentary activity, 2 moderate exercise like walking or light gardening work during at least 4h per week, and 3 to 4 representing regular, strenuous or very strenuous activity. Family history of myocardial infarction was considered to be present if father, or mother, or both, had a history of myocardial infarction. Occupation was coded according to the socioeconomic classification system (SocioEconomic Index=SEI) elaborated by the Central Bureau of Statistics in Sweden.15This classification, in the aggregated form used in this study, consists of five occupational classes: (1) unskilled and semiskilled workers (2) skilled workers (3) foremen in industrial production and assistant non-manual employees (4) intermediate non-manual employees (5) employed and self-employed professionals, higher civil servants, executives. Men who were not classifiable (n=58), mainly men with disability pensions who had not stated their profession and non-professional self-employed subjects, were assigned the number zero in the statistical analyses.
Psychological stress was assessed using one simple question defining stress as feeling tense, irritable, anxious or having sleeping difficulties as a result of conditions at work or at home. Response alternative were (1) never felt stress (2) one or two periods of stress ever 34) several periods of stress either during the last year or the last 5 years (5) permanent feeling of stress the last year (6) permanent feeling of stress during the last 5 years. The two last were grouped together and defined as severe stress. In a previous publication this item was found to predict coronary disease and stroke in a large cohort of Göteborg men.16
2.1. Assessment of social support
We attempted to include both quantitative, structural and qualitative, functional aspects in our assessment of social support. Structure refers to the number of persons in the social network, and the frequency of contacts with them. Function refers to what these persons provide in terms of support. Recognizing the paucity of adequate instruments, which could be used to measure these aspects in cohort studies, we developed such a measure.17We departed from the Interview Schedule for Social Interaction (ISSI),18a comprehensive, psychiatric interview questionnaire, yielding two subscales attachment, describing the availability of close emotional support and social integration, describing the availability of appraisal and tangible support as well as social network characteristics. This instrument was, however, too resourceful and time consuming to be used in cohort studies. Preserving the essential meaning of the interview questionnaire and its subscales, we reduced the number of items and adapted all questions and response alternatives so that they could be self administered as a paper and pencil test. The testretest reliability, the internal consistency and the validity of the two subscales (attachment and social integration) were examined using state of the art psychometric methodology and found to be satisfactory,17as well as predictive of cardiovascular risk in previous publications.14,19,20
In the short version, availability of social integration, in the following termed social integration, comprises six main questions addressing quantity and quality of daily social contacts, their role and function for the mastery of daily stresses and strains. The questions include items concerning number of people the respondent meets during a week, or shares interest, or can come and visit at any time, or with whom respondent can talk freely. Response alternatives are scored from 1 to 6. Two of the six items are yes/no items and deal with whether there is someone available whom respondent can ask small favours, or to whom respondent can turn in times of difficulties. A higher score always means better support, but the assumptions of a truly continuous scale are not fully satisfied, as the meaning of a change in one score is not the same at all points of the scale. Accordingly, the men were classified into high, medium and low support groups according to their scale scores for social integration. Availability of attachment, termed emotional attachment consists of six questions with yes/no answers scored 1/0, thus yielding a scale score from 0 to 6. The questions address the very close emotional support, whether there is someone to lean on in real hardships, who feels close to respondent, someone to share feelings with, to confide in, to hold and comfort, or who really appreciates what respondent does for him/her. The responses to this scale was strongly skewed to the right, with most men estimating their emotional attachment as good; the lowest quartile was defined as low emotional attachment, and compared with all other men.
2.2. Follow-up procedures
All men in the study were followed until 31 December 1998. The Swedish national register on deaths due to specific causes and the Swedish hospital discharge register were matched against a computer file of the men in the study, after approval of the review board of the local University Ethics Committee and in agreement with the declaration of Helsinki. The two end-points analysed in this study were coronary disease and mortality from all causes. For the first 10 years vital status was also checked manually. By the end of that period, five men had emigrated and could not be traced, otherwise vital status at least until 1993 could be determined for all men. However, as the register covers all deaths in Sweden, the number lost to follow-up after 1993 will have been negligible.
Coronary disease was defined as (1) acute myocardial infarction (AMI) i.e. a discharge or death with an ICD code of 410 (ICD 8 and 9, until 1996) or I21 (ICD 10) as the principal or underlying diagnosis, or (2) coronary revascularization defined as discharge with any operation code of 3066, 3067, 3127, 3080, FNA, FNC or FNG, along with a diagnosis of CHD, or (3) acute hospitalization with a discharge diagnosis of angina (ICD 8 and 9 code 413 or ICD code I20) with either angiography performed or evidence of other vascular disease. A manual search of registers of the city hospitals until 1993 of AMI cases according to predefined criteria was also done. One of the AMIs identified by the manual search was not identified by the national register, and one AMI occurring in another city was identified by the national register but not by the manual search. In another study of Göteborg men it was estimated that fewer than 3% of cases of specified cardiac diseases registered by the city hospitals were missed by the national register.21The national registers employed in the present study have also been used to create the national AMI register in Sweden that has been validated by comparing data from the register with a random sample of records from patients discharged with a diagnosis of either acute myocardial infarction or other coronary disease and found to be in very good agreement with predefined AMI criteria.22
During follow-up 96 men either were admitted to hospital with a discharge diagnosis of coronary disease or died from coronary disease (underlying cause ICD 8 or 9 410414 or ICD 10 I21I25). Four of these were excluded from further analyses because they had only been admitted with a diagnosis of angina at one or two single occasions, with no further objective verification. Thus, 92 hospitalizations or deaths due to CHD were identified in 741 men. Nine of these were revascularizations without MI, 14 were acute hospitalizations with a discharge diagnosis of angina, and 69 were myocardial infarctions or coronary deaths. There were 84 deaths from all causes.
2.3. Statistical methods
We used the SAS statistical package (version 8e). Simple correlation tests were used to assess cross-sectional associations for continuous variables whereas MantelHaenszel tests or Fischer's exact tests were used for categorical variables. Proportional hazards analyses were used to calculate P values and relative risks in the prospective part of the study. We checked the assumption of proportional hazards through entering in the Cox regression model time-dependent variables related to the factors we studied. The impact of these variables was not significant on the model fit, which indicate that the assumption holds. For the calculations with respect to social integration the lowest quartile was used as reference and two dummy variables were created for the intermediate two quartiles and the highest quartile, respectively. Social integration was also analysed as a continuous scale from 1 to 3. Because the distribution of scale scores on emotional attachment were skewed towards the high, positive values, the three highest quartiles were compared with the lowest. In the multivariable adjusted analyses occupational class was assessed as a continuous variable from zero to five. Smoking was entered as never, former or current smoking of 114, 1524 and >24g tobacco/day, diabetes was coded as 1 for yes and 0 for no, systolic blood pressure, lipids, and BMI were entered as continuous variables as were serum cholesterol and serum triglycerides. Leisure time physical activity was entered as 1 to 3. In the final analyses we considered all variables that were associated with the end-point under study, with a P level of <0.2 in univariate analysis. All tests were two-tailed and a P value of <0.05 was considered significant.
3. Results
Table 1shows baseline characteristics of the study participants and the relations between social integration and emotional support, and cardiovascular risk factors. Of the biological and lifestyle cardiovascular risk factors low social integration was associated with smoking (P=0.04), sedentary lifestyle (P<0.0001), and with low serum cholesterol (P=0.04). There were no significant associations between low emotional attachment and smoking or leisure time physical activity. Men with low emotional attachment had significantly lower systolic blood pressure (P=0.04) and serum cholesterol (P=0.0008).
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4. Discussion
We present evidence that social supports may be protective of new coronary events in middle-aged men over an extended period of 15 years. The lowest risk of new events (HR=0.40) was found in the highest quartile of the social integration score, intermediate risk (HR=0.63) in the two middle quartiles, as compared to the lowest quartile of integration. The risk ratios remained virtually the same after multivariate control for standard risk factors (HR=0.45 and HR=0.64 respectively). Standard risk factors included smoking, sedentary habits, systolic blood pressure, serum cholesterol and serum triglycerides, BMI, history of diabetes, and family history of coronary disease. The findings confirm previous tentative results from the first 6 years of follow-up.14Although the number of end points at that time was small, only 25, a barely significant protective effect was found for the same social integration scale.
Being socially integrated by this type of measure, means having perceived access to both practical help (tangible support), help to master difficulties in life (appraisal support) and having a sense of embeddedness, by sharing values and interests with a group of people (belonging support). This scale thus provides measures of three of the four basic support dimensions as defined in social psychological theory. The fourth dimension, esteem support, is assessed by the attachment scale, which defines the close emotional ties, that provide basic trust, identity and self esteem to the individual.23Although not as strongly related as social integration, this scale was also protective of new CHD events (HR=0.6) for high compared to low support.
The other two social measures included in the baseline assessments, low occupational status, (blue collar vs white collar job) and self-reported experience of stress (severe stress vs all other response categories) were not predictive of new CHD events. When these factors were entered separately, or together, into the multivariate model, results remained virtually the same. Thus, of the social factors assessed, the availability of good social integration was the best protective factor with respect to coronary disease. Previous reports of a strong interrelationship between low socio-economic status and low social integration may provide an explanation. It has been suggested that the often reported finding of low socio-economic status as a risk factor may operate through this mechanism, the support dimensions being less available to men and women in lower social strata.13,24
In the present study there was no relation between occupational class and incident coronary heart disease. Given the strong relation between low socioeconomic status and CHD incidence in a previous cohort ofGöteborg men born between 1915 and 192515these findings are intriguing. One possible interpretation might be that the occupation-based scale used in the older cohort, no longer is a good measure of socio-economic status in men, and that social integration captures the protective element in high socio-economic status more precisely. In men and women of the Göteborg MONICA study low socio-economic status was found to be more strongly associated with an adverse cardiovascular risk factor profile in women than in men.25Even so, low occupational class was a strong predictor of mortality from any cause in the present study, underscoring the complexity of the associations between various social factors and health outcome.
In previous studies we also found psychological stress to predict coronary heart disease in men,16a finding that was not replicated in the present study. However, the increase in risk of AMI that was associated with increased levels of stress was fairly modest, with an odds ratio of about 1.5 after adjustment for smoking and other factors. The absence of a significant effect in the present study could be due to insufficient power to detect such modest effects.
The present findings, of a functional social support measure predicting specific disease outcome, add to and advance our knowledge and understanding of this area. The most common findings in large scale epidemiological studies of scarce social networks are an excess all cause mortality. House et al concluded from their early review of the field, that the mortality risk associated with poor social networks was considerable and comparable in magnitude to that of smoking.9Data are less consistent with respect to the effect of social ties and activities on cardiovascular disease, but some studies have reported an association with coronary disease.10,11,26However, scientists are not unanimous as to the possible mechanisms or biological pathways of the social supporteffects.27
Putative effects of social factors may be mediated through effects on the coagulation system.2830The effects on the atherosclerotic process, however, have been studied and associations between different aspects of social support and presence or extent of coronary artery disease have been found in angiographicstudies.31,32In a study of women admitted with acute coronary syndrome and who also underwent quantitative coronary angiography the same social integration measure was related to both the angiographic assessments of the severity and extension of coronary artery disease as well as with event free survival.19Social integration protected from both severe coronary atheromatosis development and from a poor prognosis in women with coronary disease.19,20Patients with CHD and low emotional social support who express anger outwardly were shown to be at a highly increased risk of disease progression, independent of medication or other risk factors.33
4.1. Limitations
In the present study there were a limited number of coronary end-points and consequently, the findings must be interpreted with caution. Furthermore, two thirds only were myocardial infarctions or coronary deaths; the remainder of the end-points were revascularizations or cases of unstable angina. However, in the light of decreasing incidence of myocardial infarction in men34and increasing rates of hospitalizations for unstable angina35the inclusion of these categories may better reflect the current composition of patients with coronary artery disease. The homogeneity of the study population, exclusively men and of the same age, could contribute to the strong associations found, but on the other hand, the results may not be generalisable to women, to persons of other ethnicity or older or younger persons. Among the limitations we also count that we may have been unable to estimate accurately the influence of other risk factors, given that social factors may influence the development of other risk factors over time, for instance quitting smoking.
The measurement of social supports and social influences is often believed to be imprecise and unreliable. In the present study a well validated and standardized measure was used, which had been shown to predict CHD in previous studies of both men and women. Psychometric properties, including internal consistency, content validity and repeatability over time, were found to be satisfactory. Psychosocial factors are interrelated and tend to cluster together in the same individuals and groups. A conceptual and contextual overlap between psychosocial factors is commonly found. Individuals who report low social support are more often living alone and more socially isolated. In contrast, men in higher occupational strata may have more efficient social resources, which provide them with tools to obtain more and better social support and integration. In this way socio-economic status and social networks, social integration and support may represent different points on a continuum, rather than separate and distinct entities. These social entities are important means to cope with stresses and difficulties in life. Thus social integration may be one of the pathways, by which high socio-economic status is protective against CHD.
5. Conclusions
The long term predictive capacity of social integration in this study, from age 50, past retirement at age 65, suggests a stable and consistent protective role in middle aged men. The finding that lack of social integration predicts CHD, over and above the effect of standard coronary risk factors, further confirms its role as a core psychosocial risk factor for coronary disease.
Acknowledgments
This study was supported by the Swedish Research Council, the Swedish Council for Planning and Coordination of Research (FRN), and the Swedish Heart and Lung Foundation.
References