A linear relationship between social support and health status has been well documented. 11, 30–32
Social isolation has been shown to prospectively predict mortality and serious morbidity both in general population samples33, 34
and in individuals with established morbidity35
, especially coronary heart disease36
. However, our understanding of how and why social isolation is risky for health or, conversely, how and why social ties and relationships are protective of health, still remains quite limited. Our data support a possible role for social isolation as a factor influencing left ventricular mass. It is possible that subclinical cardiovascular disease such as increased left ventricular mass may be a mediating mechanism or modifying factor in the pathway by which a lack of social networks contribute to cardiovascular morbidity and mortality but ours was not a prospective study. Our multivariate model controlled for standard cardiac risk factors, and such factors do not fully account for or explain the deleterious effects of social isolation on left ventricular mass among Hispanics.
Social isolation may be related to left ventricular mass through a number of underlying mechanisms including psycho-physiologic stress related to isolation, depression, and poorer regulation of risk factors due to decreased medication compliance and decreased participation in healthful activities. Poor social friendship networks may turn off the means of obtaining health-related information 37
and enhance health risk behaviors. 9, 38, 39
Poor social support may lead to increased left ventricular mass via mental stress that is not buffered by the supportive presence of others. Mental stress activates neuro-endocrine components including the hypothalamic-pituitary-adrenal-axis and autonomic nervous system 11
Mental stress may induce hypertension40
, and increase cardiovascular reactivity41
and endothelial dysfunction 42
, all of which lead to increased left ventricular mass. The deleterious effects of social isolation are likely multifactorial. Further study is warranted to test these hypotheses.
Our definition of social isolation represents the presence of a primary, informal network that incorporates friends or friend-neighbors. This type of primary support network would most likely be associated with tasks involving leisure activities, hobbies, companionship, community events, shopping, and perhaps religious services. Hence, lack of friendship networks may provide a mechanism for lack of access to a wide range of resources supportive of health, such as medical referral networks, access to others dealing with similar problems, or opportunities to acquire needed resources via jobs, shopping, or financial institutions.
The spouse/partner constitutes another type of primary support network that has been associated with decreased mortality, particularly among men.43
Indeed the proximity of friend networks may also take on a more prominent role in those who have lost a spouse. In our cohort, age was also associated with poorer social support as those socially isolated were older and had increased left ventricular mass. Social isolation tended to correlate with measures of low socioeconomic status. Although educational level, our most robust socioeconomic status measure, remained significant for the total cohort in our multivariate models, it was not significant among any of the race-ethnic groups (all p>0.20) when social isolation was also in the model. Socioeconomic status has been shown to be a determinant of left ventricular mass.4
It is possible that social isolation is a partial mediator of the relationship between socioeconomic status and left ventricular mass but this remains to be further studied.
Although the prevalence of social isolation was not significantly different among the race-ethnic groups, social isolation was associated with increased left ventricular mass among Hispanics but not in the other race-ethnic groups. Our analyses suggest that race-ethnicity may be a proxy for differences in the strength of social resources. Indeed, the strong social support systems among Hispanics who are poor but maintain traditional kin groupings24
may account for the lower mean left ventricular mass in the not socially isolated group. Hispanics may be more likely to stress family values over educational attainment: maintaining traditional kinship values that include living in extended family units and taking care of the sick and elderly. This is expressed in the core Hispanic belief of familism
. Familism is having strong bonds with nuclear and extended family members thus extending a high level of perceived family support. Most studies on familism suggest that it has salutary effects and may explain the better-than-expected health outcomes observed in Hispanics.44
For example, family members are often a source of financial and emotional support which can facilitate access to health services,45
and better prevention practices including medical adherence.46, 47
Non-Hispanic blacks typically have not retained traditional kin norms. Despite levels of poverty similar to Hispanics, the elimination of traditional norms among non-Hispanic blacks has likely resulted in population level stresses, isolation of family values, and mistrust of both medical and other community resources. It may be that familism enhances the social friendship network among Hispanics in a manner that is not found in other race-ethnic groups studied.
That there was a non-significant trend towards an inverse relationship of social isolation with left ventricular mass among non-Hispanic blacks was surprising in light of the evidence that social support is beneficial in reducing stress among non-Hispanic blacks.48
However, prior studies used a different assessment tool to determine social isolation which may account for the different results. Alternatively, it our results may suggest that a more extensive social network is burdensome among non-Hispanic blacks for unclear reasons. The standard error of the regression coefficient for non-Hispanic blacks was the largest among the race-ethnic groups, likely the result of the limited number of non-Hispanic blacks in our cohort. The β coefficient for social isolation was non-significant in univariate or multivariate median regression which argues against outliers influencing the relation seen between social isolation and LVM among non-Hispanic blacks.
As for the relationship among non-Hispanic whites, although having no friends is clearly worse than having the highest number of friends, there is no significant trend seen. It seems as if among non-Hispanic whites, LVM has a non-linear relationship with social isolation. However, there is no way we could either prove or disprove this because the number of friends were categorized as ordinal variables. It is possible that what matters is not so much increasing isolation but a shift in the form and type of social connection. This will require further study.
Study Strengths and Limitations
A major strength of these analyses includes the use of a community based multiethnic cohort. Limitations of the study with regard to social support include the inability to capture individual participation in religious activities, senior centers, and other community-based organizations. Further, the data collected are limited in their ability to characterize the mechanisms by which social isolation impacts on left ventricular mass. Our study was cross-sectional in nature and thus causality cannot be inferred nor temporality be established. Unmeasured variables and residual confounding may account for some of the observed differences. Our interaction term was only significant when looking at Hispanics vs. non-Hispanics and not significant (p>0.20) when looking at Hispanics, whites, and blacks separately. Because of the limited number of participants in non-Hispanic race-ethnic groups and a power issue within each strata, our study was underpowered to detect statistically significant small to moderate interactions. Using values for mean left ventricular mass in those with and without social isolation and a two-sided type-I error of 0.05, the current sample had only 55% power to detect the 2.5 gm/m2.7 difference in left ventricular mass and would have much less power for within-strata analysis. Nevertheless, socially isolated Hispanics were at a significantly higher risk of increased left ventricular mass than the other groups. Lastly, our findings may not be generalizable to all Hispanics; 88% of our Hispanic sample consists of Caribbean-Hispanics from the Dominican Republic, Cuba, and Puerto Rico. Confirmation by other investigators using other Hispanic populations is required.