In a racially and socioeconomically diverse urban population of adults, we observed that individual-level poverty (low SES) was associated with prevalent CKD among African Americans, but not among whites. Among African Americans, low SES was independently associated with a nearly 2-fold greater risk of CKD when compared to higher SES. Further, we found that poverty had no statistically significant relationship with CKD among whites; however there was a trend towards a negative association. Our major finding was robust to adjustment for several risk factors for CKD and to the use of stricter eGFR cutoffs. We found that, in general, further adjustment for potential confounders strengthened the association between poverty and CKD; however this was in part due to negative confounding (for example, both age and health insurance were positively associated with CKD but were negatively associated with poverty).
Few studies have reported on the relationship between SES and pre-ESRD CKD, and among these, variable associations with race have been reported. Shoham et al. found in the Atherosclerosis Risk in Communities (ARIC) Study that working class membership in the life-course was more strongly associated with CKD among African Americans than among whites, even independent of hypertension and diabetes2
. However, in an earlier report from the same study population, Merkin et al. noted that living in a low SES area was associated with progressive CKD only among white men7
. In studies examining SES, race and CKD in participants with specific diseases, significant contributions of SES to racial disparities in CKD have been observed among persons with diabetic19
There are several reasons why the results of our study may differ in some cases from previous reports in the literature. First, although more balanced than many studies of racial and SES differences, our study did include more African Americans than whites, and in general, whites in our study were of higher income than African Americans. This may have contributed to the differential findings by race that we observed. Additionally, the sole use of the poverty threshold as our measure of SES may not have been appropriate for whites in our study. We did, for example, find that lack of employment was associated with CKD among both African Americans and whites.
The major implication of our study is that poverty may impact African Americans differently than whites in the development of CKD. Poverty may exert its differential effect on African Americans via several mechanisms. Plausible biological mechanisms include the increased prevalence of low birth weight observed among African Americans, a condition associated with poverty. Low birth weight is a risk factor for ESRD, and is thought to be a contributor to the racial disparities seen in ESRD32
. Also, the gene encoding non–muscle myosin heavy chain type II isoform A (MYH9
) has been associated with non-diabetic ESRD in African Americans, but not in whites33
. Poverty and its consequences (i.e. toxic environmental exposures such as heavy metals) may play a role in the probable gene-environment interactions that lead to ESRD in these individuals. It has also been reported that living in low socioeconomic status neighborhoods is more strongly associated with greater cumulative biological risk profiles (defined using nine indicators of increased risk) among African Americans than among whites.34
Many of these indicators, such as blood pressure and waist-to-hip ratio, are also associated with increased prevalence of CKD.
Poverty may also differentially impact health beliefs and behaviors among African Americans as compared to whites, which could lead to increased risk of CKD and its progression. A recent report from the Americans’ Changing Lives Survey, for example, found a positive association between number of unhealthy behaviors and number of chronic conditions among African Americans but not among Whites. They postulated that these behaviors may serve as a coping mechanism for those living in chronically stressful environments.35
Also, life stressors commonly encountered in poverty, such as unemployment and discrimination, may impact African Americans differently than whites. Notably, a positive association between blood pressure and acceptance of unfair treatment has been shown in a population of working-class African Americans36
. As hypertension is an important risk factor for CKD, the stress of discrimination may serve as a mediator of the relationship between poverty and CKD in African Americans.
Our study has certain limitations. As an observational study, the possibility of selection bias is of concern, as is participant drop-out and failure to complete study measures. We noted that those participants who completed laboratory assessments necessary for this analysis differed from the non-completers on a number of potentially relevant covariates, including age, gender and employment status. An additional limitation is that the cross-sectional analyses performed do not allow for determination of causality. Therefore, although very improbable, reverse causality (CKD causing poverty) is a possibility. Prior longitudinal studies, however, have supported the notion that poverty may often precede the development of progressive CKD and ESRD6, 7, 20
. There were also some limitations to the definition of poverty (low SES) used in our study. We were primarily restricted to a self-report of falling above or below poverty level as reported during the initial household survey. Only 76% of participants included in our analysis gave detailed information regarding their actual household income, and we were lacking other important measures of SES, such as inherited wealth and life-course SES which may have impacted our findings. Finally, because our study was conducted in an urban setting, our findings may not be generalizable to non-urban populations.
The potential role of poverty in the greater burden of advanced kidney disease seen among African Americans is worthy of further investigation. Future studies should focus on specific factors related to poverty that may account for the strong differential influence it appears to have on African Americans in the development of kidney disease.