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The Journals of Gerontology Series A: Biological Sciences and Medical Sciences 61:284-290 (2006)
© 2006 The Gerontological Society of America

Association of Inflammatory Markers With Socioeconomic Status

Annemarie Koster, Hans Bosma, Brenda W. J. H. Penninx, Anne B. Newman, Tamara B. Harris, Jacques Th. M. van Eijk, Gertrudis I. J. M. Kempen, Eleanor M. Simonsick, Karen C. Johnson, Ronica N. Rooks, Hilsa N. Ayonayon, Susan M. Rubin, Stephen B. Kritchevsky, for the Health ABC Study

1 Department of Health Care Studies, Section Medical Sociology, Universiteit Maastricht, The Netherlands.
2 Department of Psychiatry, VU University Medical Center, Amsterdam, The Netherlands.
3 Department of Epidemiology, Graduate School of Public Health, University of Pittsburgh, Pennsylvania.
4 Laboratory of Epidemiology, Demography and Biometry, National Institute on Aging, Bethesda, Maryland.
5 Clinical Research Branch, National Institute on Aging, Baltimore, Maryland.
6 Division of Geriatric Medicine and Gerontology, Department of Medicine, Johns Hopkins School of Medicine, Baltimore, Maryland.
7 Department of Preventive Medicine, University of Tennessee Health Science Center, Memphis.
8 Department of Sociology, Kent State University, Kent, Ohio.
9 Department of Epidemiology and Biostatistics, University of California, San Francisco.
10 Sticht Center on Aging, Section on Gerontology and Geriatric Medicine, Wake Forest University School of Medicine, Winston-Salem, North Carolina.

Address correspondence to Annemarie Koster, PhD, Universiteit Maastricht, Department of Health Care Studies, Section of Medical Sociology, P.O. Box 616, 6200 MD Maastricht, The Netherlands. E-mail: a.koster{at}zw.unimaas.nl


   Abstract
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 Abstract
Methods
 Results
 Discussion
 References
 
Background. This study examines the association between socioeconomic status (SES) and inflammatory markers in well-functioning older adults and seeks to determine whether any association remains after adjusting for biomedical and behavioral factors typically related to elevated serum levels of inflammatory markers.

Methods. Data were obtained from 3044 men and women, aged 70–79 from Pittsburgh, Pennsylvania and Memphis, Tennessee participating in the Health, Aging and Body Composition study. Three indicators of SES were used: education, income, and ownership of financial assets. Serum levels of interleukin-6, C-reactive protein, and tumor necrosis factor-{alpha} were measured.

Results. Low SES was associated with significantly elevated levels of interleukin-6, C-reactive protein, and tumor necrosis factor-{alpha} compared to high SES. Behavioral factors (including smoking, drinking, obesity) explained a substantial part of the inverse association between SES and inflammatory markers. Adjustment for prevalent diseases (including heart diseases, lung disease, and diabetes) associated with inflammation explained less of the association.

Conclusions. This study suggests that interventions to improve health behaviors, even in old age and especially in low SES groups, may be useful in reducing risks associated with inflammation.

LOW socioeconomic status (SES) has recently been related to higher levels of inflammatory markers (1–4). This is important, as inflammation—being a biological response of the immune system—has been associated with increased morbidity and mortality across the age span, including in old age (5–7). Two recent studies (1,2) showed that persons with low SES have higher serum levels of C-reactive protein (CRP) than do persons with high SES. Another study (3) found lower levels of interleukin-6 (IL-6) and tumor necrosis factor-{alpha} (TNF-{alpha}) in high SES groups, but the effect was nonlinear and found in women only. Evidence relating inflammatory markers to SES is, however, limited at present especially regarding to what extent this association might be mediated by other health and behavioral factors commonly associated with low SES.

Increased levels of inflammatory markers, such as IL-6 and CRP, are observed in the presence of acute and chronic conditions including atherosclerosis (8), cerebrovascular disease (9,10), coronary heart disease (11–14), congestive heart failure (15), and arthritis (16). Recent studies (17,18) also show that depression and cognitive impairment are associated with elevated levels of inflammatory markers. These health conditions are also more common in low SES groups and may thus contribute to the association between low SES and higher levels of inflammatory markers. Similarly, behavioral factors may also explain an association between low SES and increased inflammatory marker levels. Low SES is related to many adverse behavioral factors, such as smoking, excessive alcohol use, obesity, and physical inactivity (19). These factors are also related to higher levels of inflammatory markers (20–22).

Using data from a large sample of community-dwelling older adults, this study examined the association between SES and several markers of inflammation (CRP, IL-6, and TNF-{alpha}). In addition, the present study sought to determine whether any association remained after adjusting for biomedical and behavioral factors typically related to elevated serum levels of inflammatory markers.


   METHODS
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 Methods
Results
 Discussion
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Study Population
The Health, Aging and Body Composition (Health ABC) study is a longitudinal cohort study consisting of 3075 well-functioning, 70- to 79-year-old, black and white men and women. Participants were identified from a random sample of white Medicare beneficiaries and all age-eligible community-dwelling black residents in designated ZIP codes surrounding Memphis, Tennessee, and Pittsburgh, Pennsylvania. Participants were eligible if they reported no difficulty in either walking one quarter of a mile, going up 10 steps without resting, or performing any basic activity of daily living. Participants were excluded if they reported a history of active treatment for cancer in the prior 3 years, planned to move out of the study area in the next 3 years, or were currently participating in a randomized trial of a lifestyle intervention. Data, collected between April 1997 and June 1998, included an in-person interview and a clinic-based examination, with evaluation of body composition, clinical and subclinical diseases, and physical functioning.

Data on inflammatory markers were missing for 30 participants, and SES data were missing for one participant, leaving 3044 participants for the present analyses. All participants signed informed written consent forms approved by the institutional review boards of the clinical sites.

Measures
SES.-- Three indicators of SES were used: education, family income, and ownership of financial assets (23). Different SES indicators were used because these variables represent different dimensions of SES (24). Education is the sociocultural dimension of SES and represents social class in an early stage of the life course (compared to income, which represents a later life stage). Ownership of financial assets, as a proxy for wealth, describes availability of material resources accumulated over the life course. Categories for education were: less than 12 years, 12 years, and more than 12 years. Family income was defined as: wages, salaries, social security or retirement benefits, help from relatives, and rent from property. Five categories of family income from the year prior to interview were distinguished: less than $10,000, between $10,000 and less than $25,000, between $25,000 and less than $50,000, greater than or equal to $50,000, and missing (n = 371, 12.2%). The number of assets a person reported was used as a third SES measure. Assets included money market accounts, savings bonds or treasury bills, home ownership or investment property or housing, a business or farm, stock or stock mutual funds, an individual retirement account (IRA) or KEOGH account, and other investments. Three categories were created: none, one or two, and three to seven (23).

Inflammatory markers.-- Measures for the cytokines IL-6 and TNF-{alpha} and for CRP were obtained from frozen stored plasma or serum. Fasting blood samples were obtained in the morning, and after processing, the specimens were aliquoted into cryovials, frozen at –70°C, and shipped to the Health ABC Core Laboratory at the University of Vermont. Cytokines were measured in duplicate by enzyme-linked immunosorbent assay (ELISA) kits from R&D Systems (Minneapolis, MN). The detectable limit was 0.10 pg/mL for IL-6 (by HS600 Quantikine Kit) and 0.18 pg/mL for TNF-{alpha} (by HSTA50 kit). Serum levels of CRP were also measured in duplicate by ELISA based on purified protein and polyclonal anti-CRP antibodies (Calbiochem, San Diego, CA). The CRP assay was standardized according to the World Health Organization First International Reference Standard with a sensitivity of 0.08 µg/mL. Assays of blind duplicates collected for 150 participants showed an average interassay coefficient of variation of 10.3% for IL-6, 8.0% for CRP, and 15.8% for TNF-{alpha}.

Covariates.-- Sociodemographics included age, sex, race (black or white), study site (Memphis or Pittsburgh), and marital status (never married, previously married, married). For the analyses with family income, we also controlled for whether the household included anyone in addition to the participant and his or her spouse. Behavioral factors included smoking status (current, former, never smoker), cigarette use in pack-years, alcohol use during the past year (0, <1, 1–7, >7 drinks per week), physical activity (total kilocalories per kilogram per week), and body mass index (BMI; weight in kilograms divided by height in meters squared [kg/m2]). Abdominal visceral fat (centimeters squared) was quantified by analyses of computed tomography (CT) scans by General Electric 9800 Advantage (in Pittsburgh) and a Siemens Somatron and Picker PQ2000S (in Memphis). Presence of lung, heart, and cerebrovascular disease, diabetes mellitus, osteoarthritis, and cancer was determined using standardized algorithms considering self-report, use of specific medications, and clinical assessments. Depressed mood was assessed with the Center for Epidemiologic Studies Depression (CES-D) scale. A cutoff score of 16 was used as a criterion for major depressive symptoms (25). Cognitive impairment was defined as a Modified Mini-Mental State Examination (3MS) score less than 78 (26). Medications taken in the previous 2 weeks were recorded and coded according to the Iowa Drug Information System (27). By use of this system, the use of anti-inflammatory drugs and corticosteroids were assessed.

Statistical Analyses
Analyses were performed using SPSS (version 12.0; SPSS Inc., Chicago, IL). Characteristics of the study population were stratified by race. Differences in main characteristics between black and white persons were determined using chi-square tests for categorical variables and t test statistics or nonparametric Mann–Whitney statistics for continuous variables. A nonparametric test for differences in inflammatory marker levels was used because levels of inflammatory markers were not normally distributed. To compare serum levels of inflammatory markers across SES groups, a Kruskal–Wallis test was used; median values of inflammatory markers with interquartile ranges were reported.

Linear regression analysis was used to test the association between SES and serum levels of inflammatory markers. Log-transformed values of inflammatory markers were used as dependent variables in the regression models. Three models were fitted, the first model adjusted for sociodemographics. The second adjusted for sociodemographics and behavioral factors, and the third model adjusted for sociodemographics and diseases, anti-inflammatory drugs, and corticosteroids.


   RESULTS
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 Methods
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Discussion
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Characteristics of the study population are shown in Table 1. The mean age of the study sample was 74.2 years, 51.4% were women, and 41.5% were black. Of the 3044 participants, 25.1% had less than 12 years of education, 11.6% had an income less than $10,000, and 16.5% had no assets. Because the Health ABC study includes one of the largest samples of older African Americans, the main characteristics are also shown for blacks and whites separately. Black participants were less well-educated, had lower income, and had fewer assets compared with white participants. Levels of IL-6 and CRP were higher in blacks than in whites, and TNF-{alpha} levels were higher in whites than in blacks. The association between SES and inflammatory markers, however, was not different between the two groups (data not shown). This finding was confirmed by the nonsignificant interactions between each indicator of SES and race; therefore, subsequent results are shown for both together. Interaction terms between SES and sex were also not statistically significant (all p >.05; data not shown). Table 2 presents median values of IL-6, CRP, and TNF-{alpha} in each SES group. People with low SES had significantly higher serum levels of IL-6 and CRP than did people in high SES groups (p <.001). Median values of TNF-{alpha} were highest in the middle SES groups of education and income, and results were not statistically significant for the number of assets.


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  		Table 1. Main Characteristics of the Study Population.

 

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  		Table 2. Median (Interquartile Range) of IL-6, CRP, and TNF-{alpha} by Education, Income, and Number of Assets.

 
Regression coefficients for the association of each indicator of SES with serum levels of IL-6, CRP, and TNF-{alpha} are shown in Tables 3, 4, and 5, respectively. Table 3 shows that low SES was associated with significantly higher levels of IL-6 compared with high SES groups in the model that adjusted for age, sex, race, study site, and marital status (Model 1). When behavioral variables were added to the first model (Model 2), regression coefficients decreased from 42% to 66% (without taking into account the missing category for income), and most results did not remain statistically significant. When prevalent diseases were added to the model with demographic variables, only the regression coefficients decreased from 18% to 32%. There still was a statistically significant association between low income and elevated levels of IL-6 after adjustment. Low SES was also associated with significantly higher levels of CRP, but this was not statistically significant for assets (Table 4). The role of behavioral factors and diseases was similar to that seen for IL-6. Behavioral factors decreased the regression coefficients substantially (56% on average), whereas control for diseases decreased the regression coefficients much less (28% on average). There was a nonlinear association of education and assets with TNF-{alpha}; the highest levels of TNF-{alpha} were found in the middle SES groups compared with the highest SES groups (Table 5). The largest reduction in regression coefficients related to SES was also found in Model 2 in which behavioral factors were added. In additional analyses, it was determined which behavioral factors most reduced the regression coefficient of SES (data no shown). BMI and abdominal visceral fat most reduced the regression coefficients for all three inflammatory markers. Smoking and drinking were further able to reduce the regression coefficients; physical activity was least important.


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  		Table 3. Regression Coefficients (With SE) of Interleukin-6 (Log Transformed)* by Education, Income, and Number of Assets.

 

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  		Table 4. Regression Coefficients (With SE) of C-Reactive Protein (Log Transformed)* by Education, Income, and Number of Assets.

 

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  		Table 5. Regression Coefficients (With SE) of Tumor Necrosis Factor-{alpha} (Log Transformed)* by Education, Income, and Number of Assets.

 
We further excluded 1437 persons with diseases that were strongly associated with elevated levels of inflammatory markers (heart disease, lung disease, cerebrovascular disease, and diabetes mellitus). The association between SES and all three inflammatory markers remained similar and significant (data not shown). In addition, interaction terms between SES and those diseases were not statistically significant, which confirmed that the association between SES and inflammatory markers was similar for people with and without prevalent disease.


   DISCUSSION
Top
 Abstract
 Methods
 Results
 Discussion
References
 
In well-functioning older adults, low SES was associated with elevated levels of inflammatory markers, compared with high SES. The most consistent linear associations were found for IL-6 and CRP. There also was a linear association of income with TNF-{alpha} levels. It remains unclear why there was a nonlinear association of education and assets with levels of TNF-{alpha}; highest levels were found in the middle SES groups rather than in the lowest SES groups. A study by Steptoe and colleagues (3) also shows the highest levels of TNF-{alpha} in the middle SES groups. A substantial portion of the inverse association between SES and inflammatory markers was explained by behavioral factors. Adjustment for prevalent diseases associated with inflammation explained less of the association. Both behavioral factors and diseases failed to explain completely the association between SES and inflammatory markers.

The prevalence of many diseases including heart disease, diabetes mellitus, cognitive impairment, and depression is higher in low SES groups (28–32). However, the results suggest that the inverse association between SES and inflammatory markers is largely independent of prevalent disease despite these diseases being related to increased levels of inflammatory markers (6,17,18). In contrast, behavioral factors explained a substantial part of the SES differences in inflammatory marker levels. An unhealthy lifestyle, including smoking, drinking, and obesity, is more prevalent in low SES groups (19) and is also related to higher levels of inflammatory markers (20,21). It is not surprising that BMI and abdominal visceral fat, both of which are inversely related to SES, are important in explaining SES differences in inflammatory marker levels because IL-6 and TNF-{alpha} are produced within adipose tissue (33). There is an indirect effect of abdominal fat and CRP. Because IL-6 is a primary stimulus of CRP production in the liver, higher levels of IL-6 could result in higher levels of CRP (34).

This study has some limitations. First, this study was cross-sectional. Consequently, it is impossible to draw conclusions regarding the causal pathways. It is, however, unlikely that high levels of inflammatory markers could have lowered SES, especially education, as the latter indicator is less sensitive to change during adulthood. In this study, similar results were found across all SES indicators, including education. Second, only limited information on severity of diseases was available. We cannot exclude the possibility that a better measurement of the disease burden would have resulted in a stronger contribution of diseases to the association between SES and inflammatory markers. In particular, subclinical cardiovascular disease is extremely common in older adults and is associated with high IL-6 levels (35). Finally, there may be other factors which could also be important in explaining SES differences in inflammatory marker levels. These factors may include psychological stress that is related to both low SES and increased levels of inflammatory markers (perhaps through adverse behavioral factors; 3,36). This study also has several strengths. To our knowledge, this is the first study that examined the association between SES and levels of inflammatory markers in a large sample of black and white older adults. Furthermore, a large number of potential confounders and mediators were considered in the association between SES and inflammatory markers.

Conclusion
Low SES was associated with elevated levels of inflammatory markers. Rather than by a higher prevalence of diseases in low SES groups, the elevated levels of inflammatory markers in low SES groups were to a larger extent based on a higher prevalence of unhealthy behaviors (especially obesity, smoking, and drinking) in these groups; abdominal visceral fat is, however, less amenable to modification. This suggests that interventions to improve health behaviors, even in old age and especially in low SES groups, may be useful in reducing risks associated with inflammation.


   Acknowledgments
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 Methods
 Results
 Discussion
 References
 
This study was supported by National Institute on Aging (NIA) contracts N01-AG-6-2101, N01-AG-6-2103, and N01-AG-6-2106. This research was supported (in part) by the Intramural Research Program of the National Institutes of Health, NIA.


   Footnotes
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 Discussion
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Decision Editor: Luigi Ferrucci, MD, PhD

Received June 27, 2005

Accepted November 30, 2005


   References
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 Discussion
 References
 

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