In this group of older community-dwelling women, there was a significant association between SES and frailty. This association was present irrespective of the measure of SES and remained strong despite controlling for age, race, chronic disease, insurance status, and smoking status. Based on findings of Braveman et al (24
), we analyzed measures of SES separately. In our study, frailty was not related to race which is in contrast to a study by Hirsch et al that examined the independent effect of race on odds of frailty and found that race was a significant frailty predictor independent of SES (25
). One possible reason for this different finding is that the Cardiovascular Health Study has both men and women participants while our study was restricted to women. The association between race and frailty may differ by sex or
their finding could be affected by residual confounding.
In other respects, this study extends the findings of others. Woods et al found that income and education were risk factors for frailty in The Women’s Health Initiative (5
). The current study complements this finding using the objective measures in the Fried frailty definition, which the Women’s Health Initiative was not able to use. Other large U.S. cohorts have examined predictors of frailty. The descriptive tables contained in these studies demonstrate that participants with low education and income are disproportionately represented in the frail groups (6
). However, these studies have not examined the contribution of SES factors to frailty independent of other covariates. In contrast to our findings, Hirsch et al found that neither education nor income was related to frailty in the Cardiovascular Health Study cohort (15
There are several biological mechanisms that could elicit the relationship between low SES and frailty. SES has been linked to inflammation (29
), decreased physical tone (31
), decreased serotonin(32
), and altered biological risk profiles (33
). These same factors may be implicated in the origins of frailty as well. For example, researchers have posited that chronic inflammation may be a key factor in frailty (18
), which has also been suggested to mediate the relationship between SES and morbidity due to chronically sustained psychosocial stressors (11
). SES may also be linked to frailty status through decreased physical activity (34
), which may lead to exhaustion and sarcopenia (35
), which are key features of the frailty syndrome. SES may be linked to frailty through poor nutrition as those of low SES have decreased access to micronutrients (36
) and those with lower levels of micronutrients are more likely to become frail (37
As a second sensitivity analysis, we examined whether neighborhood SES (a composite of median income, wealth, education, and proportion of residents with executive, managerial, or professional specialty occupations) (38
) was associated with frailty status. We used generalized estimating equations to account for the fact that individual’s SES is nested within neighborhood hood SES. Neighborhood SES was a significant but weaker correlate of frailty status when adjusted for age and race (OR of 1.26: 95% CI=1.03, 1.54). Neighborhood SES no longer significantly associated with frailty (OR = 1.18; 95% CI, 0.97 – 1.45) once additional individual level covariates (smoking status, insurance status, disease burden) were added to the model.
Our study has the following limitations. This study is limited by its cross-sectional design. We cannot infer causality due to the cross-sectional design but reverse causation seems unlikely. Education is particularly resistant to reverse causation in older adults as it is usually attained in early life. Our study includes only African-Americans and whites. It is unclear whether these findings might apply to other races or ethnicities. Strengths of the current study include a population-based sample, objective and subjective measures of the frailty components, and three different related measures of socioeconomic status.
The current findings suggest that education and income are related to frailty. Whether the relationship is causal remains to be tested. We also found that the effect of race on frailty is confounded by socioeconomic position. The overall findings are important because the population of older adults with low education is increasing.
What is already known on this subject:
Low socioeconomic status and frailty are both risk factors for illness and mortality in older adults. It has not been known if they are related to each other using objective measures of frailty.
What does this study add?
Odds of frailty are increased for those of low socioeconomic status independent of age, race, insurance or smoking status, and co-morbidities.