Everyday Discrimination and Kidney Function Among Older Adults: Evidence From the Health and Retirement Study

Abstract

Background

With advancing age, there is an increase in the time of and number of experiences with psychosocial stressors that may lead to the initiation and/or progression of chronic kidney disease (CKD). Our study tests whether one type of experience, everyday discrimination, predicts kidney function among middle and older adults.

Methods

The data were from 10 973 respondents (ages 52–100) in the 2006/2008 Health and Retirement Study, an ongoing biennial nationally representative survey of older adults in the United States. Estimated glomerular filtration rate (eGFR) derives from the Chronic Kidney Disease Epidemiology Collaboration equation. Our indicator of everyday discrimination is drawn from self-reports from respondents. Ordinary Least Squared regression (OLS) models with robust standard errors are applied to test hypotheses regarding the link between everyday discrimination and kidney function.

Results

Everyday discrimination was associated with poorer kidney function among respondents in our study. Respondents with higher everyday discrimination scores had lower eGFR after adjusting for demographic characteristics (B = −1.35, p < .05), and while attenuated, remained significant (B = −0.79, p < .05) after further adjustments for clinical, health behavior, and socioeconomic covariates.

Conclusions

Our study suggests everyday discrimination is independently associated with lower eGFR. These findings highlight the importance of psychosocial factors in predicting insufficiency in kidney function among middle-aged and older adults.

Aging is a complex process, believed to result in significant changes in the structure and function of the kidney (1–5). The ability of the kidney to regulate fluid and electrolytes remains relatively stable through 50 years of age, and then kidney function declines with advancing age (6–8). As individuals age, declining kidney function increase one’s susceptibility to severe cognitive impairment (9,10), loss of muscle strength (11), mobility limitations (12), and chronic kidney disease (CKD), which is defined by the presence of kidney damage (e.g., albuminuria ≥ 30 mg/g) and/or reduced glomerular filtration rate (eGFR) <60 mL/min/1.73m². Data from the United States Renal Data System indicates that the prevalence of CKD among adults aged 60 and older is 32% (13). Other research suggests that two out of every three individuals will develop CKD in their lifetime, and CKD is one of the leading causes of death among older adults in the United States (14). Given the anticipated growth in middle age and older adults in the United States, the number of individuals above the age of 50 with CKD and CKD-related complications is expected to rise (15); which in turn, will lead to greater costs for both families and publicly funded programs.

A number of risk factors are believed to contribute to insufficiency in kidney function among middle age and older adults in the United States. These factors include including hypertension (16), diabetes (17), waist circumference (18), elevated rates of inflammation (19), low socioeconomic status (20), and negative health behaviors (21). There is a reason to believe that experience with chronic stressors such as everyday discrimination (ie, minor or even trivial day-to-day hassles), which are common in late life (22), may also be a risk factor for kidney function among middle age and older adults (23). Stress theory that stressors, which often emerge from interpersonal interactions with others are patterned by social statues such as race/ethnicity, age, and sex (Figure 1). Experiences of everyday discrimination is a perceived chronic stressor that triggers a fight or flight processes that prepares one’s body for action (24). As illustrated by Figure 1 (25), advocates of this perspective contend that greater exposure to everyday discrimination overtaxes cardiovascular and metabolic systems over time, which in turn, may lead to poorer kidney function. Accordingly, a growing body of research has documented links between perceived discrimination and variations in CKD risk factors such as elevated blood pressure (26), waist circumference (27), and inflammation (25). Studies on the association between perceived discrimination and kidney function, however, are rare (28,29,30). One study found that everyday discrimination attributed to one’s gender was associated with lower levels of eGFR among respondents in the Healthy Aging in Neighborhood of Diversity Across the Life Span Study (31). A separate published study finds that a cumulative indicator of life stressors, which includes a measure everyday discrimination was negatively associated with the prevalence of CKD among respondents in the Jackson Heart Study (32).

Figure 1.

Heuristic model of the associations between perceived discrimination, cardiometabolic risk, behavioral factors, and kidney function among older adults.

Despite ongoing interest in the relationship between everyday discrimination and kidney function, we know of no study that examines this relationship among older adults. This is important, given that experiences of everyday discrimination are common among older adults (22), and ongoing evidence that everyday discrimination is a psychosocial stressor that negatively influences the health of older adults (33). Thus, we tested the hypothesis that higher levels of exposure to everyday discrimination would be independently associated with lower levels of kidney function in middle age and older adults.

Materials and Methods

Study Population

Data derives from the Health and Retirement Study (HRS), an ongoing, nationally representative biennial longitudinal study of adults above age 50 in the United States. From its beginning in 1992, HRS oversampled racial/ethnic minority respondents. The HRS is designed, among other things, to monitor changes in the physical, functional, and cognitive health of adults as they age. In 2006, the HRS began collecting biological and psychosocial information from a random half-sample of the non-institutionalized older adults in the core biennial survey (34). Specifically, HRS interviewers collected biomarkers during the in-person interview, while self-administered psychosocial questionnaires were left with HRS respondents at the end of the in-person enhanced core interview and mailed back to the HRS. The other half of the 2006 sample received the biomarker and psychosocial assessments in 2008, and the alternating cycle repeats with the sample who received the assessments each 4 years (35). The present study pooled data from the 2006 and 2008 HRS. Our analytic sample is comprised of respondents with complete demographic information (eg, age, sex, race/ethnicity, and education), biomarker measurements, and psychosocial data were included in the analyses. Complete data was available from 10 973 respondents.

Measures

Everyday discrimination

Everyday discrimination taps into routine and relatively minor experiences of mistreatment and was assessed using the short version of the Everyday Discrimination Scale (36). The HRS asked participants how often any of the following events have happened to them in their daily life: (i) “You are treated with less courtesy or respect than other people”; (ii) “You receive poorer service than other people at restaurants or stores”; (iii) “People act as if they think you are not smart”; (iv) “People act as if they are afraid of you”; and (v) “You are threatened or harassed.” The six-point response scale to each item ranges from “never” (6) to “almost everyday” (1); the items were reverse-coded, rescaled to zero, and averaged over the number of items to produce an everyday discrimination score ranging from zero to five (Cronbach’s alpha = .80). Higher scores reflect more frequent instances of everyday discrimination.

Kidney function

Our outcome, kidney function, derives from Cystatin C. Cystatin C was assayed from dried blood spots collected during the in-home assessment. HRS interviewers collected Cystatin C from respondents by pricking participants’ finger with a sterile lancet, collecting the blood droplets, and placing them on specially treated filter paper. Because of the skewed distribution of this biomarker, the HRS took the natural log to normalize the distribution (34). We calculated eGFR from our measure of Cystatin C (eGFRcysc) using the Chronic Kidney Disease Epidemiology Collaboration Cystatin C equation (37).

Demographics and Comorbidities

The present study controlled for demographic, clinical, socioeconomic, and behavioral factors that could be associated with everyday discrimination and/or kidney function among older adults. Race/ethnicity (white, black, Latino, and Other Race), age, sex, marital status, years of completed education, household income, smoking status, and drinking behaviors derive from self-reports. Our measure of diabetes combined self-reported diabetes, use of prescription diabetic medication, and an HbA1c score of 6.5% and above. Using three sitting blood pressure measurements from an automated blood pressure monitor (Omron HEM-780 Intellisense with ComFit cuff), hypertension was based on clinical outpoints (eg, average of systolic or diastolic blood pressures ≥ 140 mmHg or ≥ 90 mmHg), self-reports of hypertension, or use of anti-hypertensive prescription medication. HRS interviewers measured waist circumference by the nearest 0.1 cm with a measuring tape placed horizontally around the respondent at the narrowest part of the torso. HRS interviewers measured the respondent’s waist circumference by over undergarments or light clothing at the end of a normal exhalation. Respondents who scored in the top 25th percentile of waist circumference were coded as 1, and all else 0.

Statistical Analysis

Sample characteristics were summarized for the entire sample. Ordinary least squares (OLS) models were used to determine the association between everyday discrimination and kidney function. Model 1 examines the everyday discrimination and kidney function among older adults with adjustments for demographic covariates race/ethnicity, age, sex, and marital status. Model 2 examines the relationship between everyday discrimination and kidney function, adjusting for clinical risk factors and the covariates listed in Model 1. Model 3 includes the covariates in Model 2 as well as measures for health behaviors and socioeconomic status. Because potential overlap between race/ethnicity and experiences of everyday discrimination, interaction terms (everyday discrimination × race/ethnicity) were tested in models not shown. In each case, the interactions for blacks (p = .888), Latinos (p = .844), and those respondents of “Other Race” (p = .298) were nonsignificant.

All of these interaction terms were nonsignificant. We used sampling weights and design factors to account for non-response to the psychosocial and biomarker assessments, and for the complex study design of the HRS. All analyses were conducted using STATA 14. P-value less than .05 is considered statistically significant.

Results

Sample characteristics for the full sample are presented in Table 1. The mean eGFRcysc score among respondents in our sample is 76.1 (standard deviation [SD]: 24.6), while the average score on the everyday discrimination scale is 0.68 (SD: 0.7). Our sample is 83% white, 8% black, 6% Latino, and 2% “Other Race.” On average, respondents were 66 years of age (SD = 9.9), while a little more than half of our sample (55%) was comprised of women. Sixty-seven percent of the respondents in our study were married. Most respondents in our sample were hypertensive (66%), diabetic (64%), or have a high-risk waist circumference (61%). Forty-three percent of our sample were former smokers, while 14% currently smoke, and 57% never smoked. Respondents in our sample had an average of 12.9 (SD: 2.9) years of education and a (logged) household income of 10.64 (SD: 1.3).

Table 1.

Participant Characteristics for the Total Sample (n = 10,973)

	Total
eGFR (mean ± SD)	76.1 (24.6)
Everyday discrimination (mean ± SD)	0.68 (0.77)
Demographics
Race/ethnicity, %
White	83
Black	8
Latino	7
Other Race	2
Age (mean ± SD)	66.1 (9.9)
Female, %	55
Currently married, %	67
Clinical risk factors, %
Hypertensive	66
High-risk waist circumference	61
Diabetic	64
Health behaviors, %
Former smoker	43
Current smoker	14
Never smoked	57
Socioeconomic status
Years of education (mean ± SD)	13.0 (3.0)
Mean household income (currency, SD)	72.72 (117.3)
Household income (logged) (mean ± SD)	10.6 (1.3)

Note: CKD = chronic kidney disease; eGFR = estimated glomerular filtration rate. Everyday discrimination ranges from 0 to 5. Source: 2006/2008 Health and Retirement Study.

Everyday Discrimination and Kidney Function

The association between kidney function and everyday discrimination is displayed in Table 2. Results from Model 1 suggest that a one-unit increase in our everyday discrimination scale was associated with poorer kidney function among older adults in our sample (B = −1.35*, standard error [SE = 0.34]). Model 2 shows that adjusting for clinical risk factors reduced but did not fully attenuate the relationship between everyday discrimination and kidney function (B = −0.98*, SE = 0.34). The relationship between everyday discrimination and poorer kidney function remained significant after further controlling for health behaviors and socioeconomic status (Model 3).

Table 2.

Ordinary Least Squared Regressions of Kidney Function on Everyday Discrimination Among Older Adults (n = 10,973)

	Model 1	Model 2	Model 3
Everyday discrimination	−1.35* (0.34)	−0.98* (0.34)	−0.83* (0.34)
Clinical risk factors
Hypertension		-6.11* (0.52)	-5.83* (0.52)
High-risk waist circumference		-6.35* (0.50)	-6.37* (0.51)
Diabetes		-1.58* (0.49)	-1.52* (0.49)
Smoking status
Non-smokers			1.00
Former smoker			-0.63 (0.52)
Current smoker			-4.21* (0.79)
Socioeconomic status
Years of education			0.40* (0.09)
Household income (logged)			0.34 (0.24)
Constant	150.95* (2.00)	150.95* (2.00)	155.16* (2.07)
R2	0.22	0.22	0.26

Notes: All models adjust for race/ethnicity, age, sex, and current marital status. Source: 2006/2008 Health and Retirement Study.

*p < .05, Robust standard errors in parentheses.

Discussion

Over the past three decades, a growing amount of research has shown that perceived discrimination tends to disfavor the health of older adults (38–42). Though important, scholarship in this area has only begun to explore how everyday discrimination relates to kidney function among older adults (25,30).To address this gap in the literature, the present study examined the interplay between everyday discrimination and kidney function among older adults. Our findings suggest that everyday mistreatment is a risk factor for lower kidney function among older adults.

Our primary goal was to assess how everyday discrimination relates to kidney function among older adults. Prior studies on the relationship between everyday discrimination and kidney function rely on convenience samples of black or white adult population in the United States (30–32). Here, we examined data from a nationally representative sample of middle age and older adults. Based on prior research, we hypothesized that higher levels of exposure to everyday discrimination would be associated with poorer levels of kidney function among older adults and demonstrated that greater exposure to everyday discrimination was associated with lower levels of kidney function among older adults in our sample. In line with the stress process model, results from our study suggest that everyday discrimination is independently associated with poorer kidney function among older adults. The association between everyday discrimination and poorer kidney function among older adults in our study is consistent with the broader research on the psychosocial dimensions of kidney function among younger and middle-aged adults.

Consistent with hypothesis, our study also shows that the association between everyday discrimination and kidney function among respondents in our sample remained significant even after adjusting for established health behaviors, socioeconomic, and even clinical risk factors (25). Across studies, not only are hypertension, diabetes, and high-risk waist circumference are strong correlates of poorer kidney function among older adults, but everyday discrimination is also associated with these clinical risk factors for poor kidney function (29, 43). Other studies have also shown that everyday discrimination predicts higher levels of engagement in unhealthy behaviors such as alcohol consumption and tobacco use (44).

The present study has a number of strengths, including our analyzation of a nationally representative sample of older adults. The large number of biopsychosocial included in the HRS also allowed us to control for many factors that were not addressed in prior research on this topic. Additionally, our use of a Cystatin C measure of kidney function offers an advantage over prior studies, given that Cystatin C is not influenced by diet and muscle mass among older adults and has been shown to play an important role in more accurately identifying kidney function decline across racial/ethnic groups among older adults (45,46). However, our study also has limitations, which include the use of cross-sectional data at one point in time to examine this association, self-report of everyday discrimination, limited data on the extent of risk factor severity/duration, and our inability to capture potential unmeasured confounders including urinary albumin excretion which was not available. Although this work is important, future studies should address the temporality issue that derives from using cross-sectional data by examining how and why the relationship between everyday discrimination and kidney function may vary over time. Second, future work should investigate whether the relationship between everyday discrimination and kidney function persists across different measures of perceived discrimination, such as major discrimination. Finally, given ongoing evidence that the improper management of comorbid conditions among older adults with CKD increases the likelihood of death (15), future studies should examine how everyday discrimination shapes both comorbid conditions and the management of these conditions among older adults.

The increasing population of adults above the age of 50 in the United States will also bring a number of challenges, including a greater number of older adults with CKD and other kidney-related problems. Results from our study indicate that greater exposure to everyday discrimination is associated with poorer kidney function among older adults. These findings highlight the need for creating policy-based interventions that center on alleviating the health-related burden associated with the life of older adults, to delay the expression of abnormal kidney-related conditions that severely affect clinical outcomes among older adults.

Funding

National Center for Advancing Translational Sciences (UL1TR000124); National Institute on Aging (K02AG059140, P30AG021684 T32 AG000037 41); National Institute on Minority Health and Health Disparities (U54MD000214).

Conflict of Interest Statement

None reported.