Socioeconomic status and C-reactive protein levels in the US population: NHANES IV

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Abstract

C-reactive protein (CRP), a marker of inflammation, has been identified as a risk factor for cardiovascular disease and mortality. Using data on adults aged 20 and over from the fourth National Health and Nutrition Examination Survey, a nationally representative cross-sectional survey, we examined the association between socioeconomic status and CRP in US adults (N = 7634). Socioeconomic variation in CRP occurred only at very high levels of CRP (>10.0 mg/L). There was no significant difference in the prevalence of moderate (1.1–3.0 mg/L) or high values of CRP (3.1–10.0 mg/L) by socioeconomic status; however, among those with family income at or below the poverty level, 15.7% had very high levels of CRP (greater than 10.0 mg/L), compared to only 9.1% of those in families above the poverty level. Logistic regression results indicate that acute illness, chronic conditions, and differential health behaviors account for about two-thirds of this association. African Americans, Hispanics, and women were more likely to have high levels of CRP. Obesity was the largest risk factor for every level of CRP above normal. Results suggest that differences in very high CRP may be due to factors beyond acute illness and may also reflect chronic health, behavioral and disease processes associated with low socioeconomic status.

Introduction

Socioeconomic status (SES) represents one of the most important risk factors for chronic disease, disability, and mortality. Low SES individuals are at greater risk for infectious illness (Cohen, 1999) and have a higher prevalence of sub-clinical markers of disease risk (Seeman et al., 2004) and chronic health conditions (Hayward et al., 2000). All-cause mortality is higher among persons of lower SES, and there is a marked socioeconomic gradient in mortality due to coronary heart disease, stroke, and diabetes (Steenland et al., 2004).

C-reactive protein (CRP) is an acute phase protein produced as part of the immune response to acute infection or injury and is a useful general marker of systemic inflammation. In healthy individuals, CRP levels return to normal after immune activation subsides. However, some persons exhibit chronically elevated levels of CRP (Danesh et al., 2004). This type of chronic inflammation has emerged as an important mechanism in the development of atherosclerosis (Fahdi et al., 2003) and as a risk factor for cardiovascular disease (Danesh et al., 2000, Koenig et al., 1999, Ridker et al., 1998), diabetes mellitus (Pradhan et al., 2001), and mortality (Harris et al., 1999, Reuben et al., 2002). CRP has been shown to be clinically useful, particularly in the prediction of cardiovascular disease and cardiac event outcomes (Smith et al., 2004).

Several studies have found associations between CRP levels and SES. CRP levels have been found to be lower among those with more years of education (Koenig et al., 1999, Panagiotakos et al., 2004, Pankow et al., 2001, Wu et al., 2002) and with higher family incomes (Jousilahti et al., 2003). Lower SES, as indexed by job status and job control, has also been inversely related to CRP levels in a British sample, such that those with the lowest status occupations have the highest levels of inflammation (Hemingway et al., 2003).

Socioeconomic differences in levels of inflammation could result from a number of processes. A long history of research shows that socioeconomic status is related to differences in both the physical and social environments and in the capacity to respond to stressful life experiences (Siegrist and Marmot, 2004). Lower SES individuals have greater exposure to infection, are more susceptible to infections (Cohen, 1999), and have a greater risk of developing chronic diseases (Crimmins et al., 1994), but they are also more likely to lack access to health care and treatment (Adler et al., 1993). Thus, differences in susceptibility, exposure, prevalence, and treatment create an environment in which individuals from low SES backgrounds are more likely to experience both acute and chronic health conditions and to suffer from them for longer durations. Individuals from low SES backgrounds are also more likely to engage in risky health behaviors and to experience higher levels of psychological stress, but on average have less information and fewer social resources with which to cope (Pincus and Callahan, 1995).

To date, research has not addressed the extent to which the broad range of factors known to affect CRP (see de Maat and Kluft, 2001, for a review), including acute health problems, chronic conditions, and health behaviors, might account for socioeconomic differences in CRP levels. Furthermore, researchers have not assessed the relationship between CRP and SES in a sample representative of the socioeconomic diversity of the United States, and most previous analyses of socioeconomic variation in CRP have neglected the importance of recognized clinical CRP categories (Pearson et al., 2003), focusing instead on continuous measurement of CRP (Hemingway et al., 2003, Koenig et al., 1999, Panagiotakos et al., 2004, Pankow et al., 2001, Wu et al., 2002). Clinical CRP categories represent different levels of cardiovascular risk and may be related to different sources of inflammation (acute vs. chronic) with varied implications. Blood CRP levels less than 1 mg/L are considered normal; levels between 1 and 3 mg/L indicate moderately increased risk for cardiovascular disease; levels greater than 3 mg/L are considered high risk for cardiovascular disease and cardiac events; and levels greater than 10 mg/L are typically considered indicative of ongoing infection, acute illness, or injury (Pearson et al., 2003). Although these very high levels of CRP are generally considered temporary elevations indicative of acute inflammation (in contrast to levels between 1 and 10 mg/L, which are generally considered indicative of chronic inflammation), levels of CRP above 10 mg/L have also been shown to be associated with increased cardiovascular risk (Ridker and Cook, 2004) and increased risk of mortality following a stroke (Muir et al., 1999).

The purpose of this study was to determine socio-economic variation in CRP in a US population-based sample, focusing on socioeconomic differences in clinical CRP categories and the mechanisms through which SES may increase CRP levels.

Section snippets

Participants

Data were drawn from the fourth wave of the National Health and Nutritional Examination Surveys (NHANES), collected between 1999 and 2002. The NHANES are nationally representative, cross-sectional surveys of the non-institutionalized US population, including interview, clinical examination, and laboratory test data. Briefly, NHANES IV surveyed a stratified multi-stage probability sample of US households, with an oversample of older persons, blacks, and Hispanics (predominantly Mexican

Unadjusted descriptive results

Table 1 provides characteristics of the study population by poverty status. Subjects in poverty had higher mean levels of CRP and blood leukocyte count. They were younger, more likely to be female, and more likely to be non-white. Those in poverty exhibited a higher self-reported prevalence of recent illness, asthma, chronic bronchitis, and rheumatoid arthritis. The prevalence of obesity, current smoking, and heavy drinking were higher among those in poverty, while the prevalence of recent

Discussion

The purpose of this analysis was to examine socioeconomic variation in CRP with a focus on clinical CRP categories and the mechanisms through which SES may increase CRP levels. Three main findings deserve further consideration. First, results suggest that the relationship between socioeconomic status and inflammation is not linear. Rather, differences are evident only at very high levels of CRP (>10 mg/L). People in poverty were significantly more likely to have very high levels of CRP often

Conclusion

In conclusion, socioeconomic status is related to higher CRP, but this effect is greatest at very high CRP levels (>10 mg/L). Recent illness and immune activation, chronic conditions, and health behaviors explain much, but not all, of this association. This research provides another piece of information about the relationships between socioeconomic status and inflammation, an important risk factor for poor health outcomes in late life.

Acknowledgments

Support for this project was provided by the National Institutes of Health (NIH), Grant Nos. P30 AG17265, R01 AG023347, K12 AG01004, and T32 AG00037. Preliminary results were presented at the annual meeting of the Population Association of America, Philadelphia, PA, March 31, 2005.

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