1 Department of Psychiatry and Behavioral Sciences, Duke University Medical Center, Durham, NC
2 Danish Epidemiology Science Centre at the Institute of Preventive Medicine, University of Copenhagen, Copenhagen, Denmark
3 Centre for Alcohol Research, National Institute of Public Health, Copenhagen, Denmark
4 Epidemiological Research Unit, Bisbebjerg University Hospital, Bisbebjerg, Denmark
5 Department of Cardiology, Hvidovre University Hospital, Hvidovre, Denmark
6 Department of Cardiology, Rigshospitalet, Copenhagen, Denmark
Reprint requests to Dr. John C. Barefoot, Box 2969, Duke University Medical Center, Durham, NC 27710 (e-mail: foot{at}acpub.duke.edu).
Received for publication April 20, 2004. Accepted for publication January 4, 2005.
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ABSTRACT |
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epidemiologic studies; heart diseases; mortality; myocardial ischemia; social support
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INTRODUCTION |
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One individual source of social contact that has received extensive attention is marital status. The predominant finding is that there is an interaction of marital status with gender. The presence of a spouse or partner seems to convey more health protection for men than it does for women (4, 5
). This suggests that there may be gender differences in the impact of other aspects of the social network as well. Here, the evidence is not consistent, with some studies reporting no gender differences (6
8
) and others reporting a greater protective effect for men (9
, 10
).
Another issue that has generated conflicting findings is whether the relation between social contacts and health risk takes the form of a threshold or a continuous relation. Some studies (9) have observed elevated risk for only the most isolated persons. Others have found graded associations between social network indicators and health outcomes (6
, 10
). With some exceptions (8
), the form of the relation has not been explicitly tested.
This investigation utilized a large representative community sample to evaluate the potential impact of social contacts on the incidence of ischemic heart disease (IHD) and total mortality. It examined the individual contributions of family members (spouse, parents, children, other relatives) and acquaintances (friends, work colleagues, and neighbors) to the effect of the social network while controlling for established risk factors. The potential moderating role of gender and the form of the associations with the outcomes received special attention.
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MATERIALS AND METHODS |
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Additional models tested for interactions of individual categories of social contact with gender and for the possibility that a more graded measure of contact frequency would provide additional predictive information to that provided by the binary indicator of presence or absence of contact. We performed the latter tests by comparing the model chi-squares of the basic binary models to 3-df models that used four levels of contact (described below) as categorical independent variables.
Finally, we constructed models with two summary diversity indices that combined information from the various categories of social contacts. One summed across all sources of contact and the other used only those types of contact that were indicative of probable intimate relationships.
Effect sizes were expressed in terms of hazard ratios and 95 percent confidence intervals. The relations observed between the covariates and the outcomes were essentially as expected.
Social contact measures
Individual sources of contact.
The social network measure contained a subset of questions drawn from an instrument used in a previous study by Orth-Gomer and Johnson (7). Participants were asked to indicate how frequently they had contact with persons in the following categories: parents, children, other family members, a spouse or partner, colleagues from work (after work), neighbors, friends from youth, other friends, and home help. The home help category was omitted because few respondents said that they had home help and because the presence of home help is a likely indicator of disability (12
, 13
). The categories of friends from youth and other friends were combined for these analyses to simplify their presentation. Specific modes of contact (e.g., telephone vs. face-to-face) were not measured separately.
Response options were "daily," "weekly," "monthly," "rarely," "never," and "no one available." For initial analyses of binary contact variables, the responses of "no one available," "rarely," and "never" were combined into a "no contact" category. Responses of "daily," "weekly," and "monthly" were grouped into one category indicating that contact was present. A spouse or partner was available for 57 percent of the respondents. Frequencies of other types of contact are presented in table 2.
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One index was based on all contact sources and another used only those types of contacts pertaining to relationships that were likely to be intimate. Probable intimate relationships were defined as those with parents, children, family, and friends, while work colleagues and neighbors were not included. These choices were supported by the participants' reports of the people they considered to be confidants. Spouses and partners were named as confidants by 83 percent of those who had spouses or partners available. This was followed by 56 percent for children, 53 percent for friends, and 32 percent for other family members. In contrast, only 18 percent of persons with work colleagues named them as confidants. For neighbors, the rate was only 8 percent.
Risk factor covariates
In addition to age and gender, more completely adjusted models included covariates for a number of known risk indicators. Education was defined as a continuous variable based on number of years of schooling. Body mass index was calculated as weight (kg) divided by height squared (m2). Family history of coronary disease was coded positive if the respondent reported the presence of heart disease in either parent. Systolic blood pressure was measured in a sedentary position after 5 minutes' rest. A London School of Hygiene and Tropical Medicine sphygmomanometer was used. Measures of glucose, high density lipoprotein cholesterol, and total cholesterol were obtained from nonfasting blood samples.
Potential mediators
In the third set of analyses, we also controlled for health behaviors and psychological factors that might be mediators of the effects of social contacts. Smoking was measured as a three-level variable (nonsmoker, ex-smoker, or current smoker). Alcohol consumption was also defined as a three-level variable separating nondrinkers, moderate drinkers (121 drinks per week), and heavy drinkers (>21 drinks per week). This was based on the sum of reported servings of beer, wine, and spirits. Physical activity in leisure time was measured in four categories of weekly activity: sedentary or light exercise for less than 2 hours, light activity for 24 hours, light activity for more than 4 hours or strenuous activity for 24 hours, and more than 4 hours of strenuous activity.
Self-rated health was also included as a covariate. This measure came from a rating scale with the categories "extremely good," "good," "not so well/feeling bad," and "terrible." This was treated as a four-level variable. It can be argued that self-rated health is an indicator of baseline physical health and should be considered a baseline confounder. However, measures of self-rated health are heavily influenced by psychological well-being (14, 15
), which might be a product of social contacts. Therefore, it could also be classed as a potential mediator.
Endpoints
Participants were followed via the National Board of Health Registry and the National Hospital Discharge Registry for mortality and incident IHD until December 31, 1997. IHD was defined as International Classification of Diseases, Eighth Revision, codes 410414 until January 1, 1994, and International Classification of Diseases, Tenth Revision, codes 121125 thereafter. There were 1,089 deaths and 427 new cases of IHD during the follow-up period. These outcomes were evaluated in separate models. The mean duration of follow-up for living participants was 5.7 years, and duration ranged up to 7.2 years.
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RESULTS |
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Another interaction observed in the age- and gender-adjusted models was an interaction for work colleagues. Although there were no main effects for this category and no significant interaction for mortality, there was a gender interaction for IHD (p < 0.003). This was based on a trend toward lower risk with more frequent contact in women (HR = 0.56, 95 percent CI: 0.34, 0.93) but an opposite trend in men (HR = 1.37, 95 percent CI: 0.99, 1.90). The interactions were somewhat more apparent if persons with no available colleagues were omitted from the analysis. In those models, the interaction for mortality (p < 0.02) was based on hazard ratios of 0.76 (95 percent CI: 0.55, 1.05) for women and 1.21 (95 percent CI: 0.95, 1.54) for men. For IHD, the interaction had a p value of 0.001, with hazard ratios of 0.51 (95 percent CI: 0.30, 0.87) for women and 1.41 (95 percent CI: 0.99, 2.02) for men.
Finally, there was a weak interaction between gender and contact with children (p = 0.04) for mortality when results were adjusted for age. This model showed a tendency for contact with children to be more beneficial for men (HR = 0.80, 95 percent CI: 0.67, 0.94) than for women (HR = 1.06, 95 percent CI: 0.87, 1.31). This interaction was not significant if persons with no children available were omitted, as they were in the analyses described above.
Continuity of effects across the range of contact frequencies
For each contact source and outcome, models using a dichotomous indicator of contact frequency (rarely/never vs. at least monthly) were compared with models containing four categorical levels of contact frequency. Tests were based on comparisons of the log likelihoods of the binary and multilevel age- and gender-adjusted models. The addition of graded indicators of contact frequency did not improve the predictions of most of the binary models. Only three of the 12 models showed a significantly better fit than the corresponding binary models. Trends for these effects are presented in table 4. No such analysis was possible for the spouse/partner variable, because it was only measured dichotomously.
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The neighbor contact indicator was also significantly improved in its association with mortality when the four levels were considered (p = 0.01), but the form of the association appears to be opposite of that expected. There was a trend for more frequent contact to be associated with higher mortality, but the effect was not strong. The hazard ratio for the daily contact indicator approached statistical significance, showing elevated risk for persons with that response. A potential explanation for this trend is that persons who are disabled or in poor health may have neighbors who monitor their well-being frequently. This speculation receives some support from the observation that more participants who reported daily contact rated their health during the last year as "fair or poor" as compared with those who saw neighbors weekly or monthly (30.2 percent vs. 25.4 percent; p = 0.001).
The pattern of significant associations in the multicategory model of friend contact and mortality (p = 0.005) was difficult to interpret. Occasional (monthly) contact with friends was associated with lower mortality risk, but more frequent contacts were not. As with the neighbor contact variable, persons who had daily contact with friends were more likely to report fair or poor health than persons in the weekly or monthly contact category (30.7 percent vs. 24.6 percent; p = 0.001). In summary, findings regarding both neighbors and friends suggest a U-shaped function, with some contact being better than none but daily contact perhaps being indicative of poor health.
Summary indices of social network diversity
We calculated one diversity index by summing the presence of contacts across all sources. The mean score on the scale was 3.6 (of a possible score of 7), with 7.5 percent of participants reporting one contact or no contacts and 9.9 percent reporting six or more sources of contact. Results from models using this index are presented in table 5. This index was related to mortality but not to IHD in age- and gender-adjusted models. There was no significant linear trend for either outcome in fully adjusted models. The hazard ratios for persons with five or more sources of contact as compared with two or fewer sources (approximately the top quartile vs. the bottom quartile) were 0.72 (95 percent CI: 0.59, 0.88) for mortality and 0.90 (95 percent CI: 0.66, 1.22) for IHD. Results were weaker in fully adjusted models. The effects of the index were not significant in either the mortality model (HR = 0.89, 95 percent CI: 0.73, 1.10) or the IHD model (HR = 1.01, 95 percent CI: 0.74, 1.39).
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DISCUSSION |
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Although there have been theoretical debates in the past about whether multiple social roles should be beneficial or detrimental to health (16), this finding and other findings (2
, 3
) suggest that a greater variety of intimate social contacts is associated with better health. It has been proposed that this could be due to feelings of self-worth and purpose associated with multiple roles that are translated into more positive affective experiences (16
). These, in turn, could have beneficial physiologic consequences.
Of the seven types of social contact investigated, all but those with neighbors and work colleagues were associated as main effects with subsequent health. This argues for the special importance of intimate social ties such as those provided by a spouse (at least for men), friends, and family. However, this issue was not fully investigated in this study, because contacts with clubs and religious organizations were not measured. Those types of contacts have often been included in network measures used in previous studies.
With the exception of parents, contacts that occurred at least monthly were as strongly associated with favorable outcomes as those that were more numerous. This adds weight to the argument that the fulfillment of social roles is the critical psychosocial factor, even if those duties are performed only occasionally. This supports the notion that the diversity of social contacts rather than the frequency of contacts is important. However, this conclusion might not apply to persons who are severely isolated, who may have been underrepresented in this sample.
These data confirm previous findings that the presence of a spouse or partner has a greater health impact for men than for women (4, 5
). However, the importance of other aspects of the social network appeared to be similar for both genders. One exception was a tendency for contact with work colleagues to be more beneficial for women. While speculative, it is possible that this reflects more affiliative relationships among women in the workplace. The reason why contacts with children were more beneficial for men than for women is not immediately apparent.
A variety of plausible mechanisms have been proposed as potential explanations for the health consequences of social participation (1). Both animal and human studies have documented that socially isolated individuals have heightened cardiovascular reactivity, which has been linked to atherosclerosis (17
, 18
). Some studies have reported associations between social isolation and neuroendocrine output (19
, 20
). In addition, there are studies suggesting that relatively isolated individuals have impaired immune system functioning (21
, 22
). Physiologic correlates of social network diversity should also be investigated.
In summary, these findings illustrate the significance of social ties for health and argue for the notion that the variety of social roles fulfilled, especially roles involving family and friends, is a key factor in this phenomenon. Further efforts to more precisely identify the aspects of social networks that carry the most health impact should help in evaluating the mechanisms responsible for their effects.
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ACKNOWLEDGMENTS |
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References |
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