摘要：Objectives. We explored how income and skin color interact to influence the blood pressure of African American adults enrolled in the longitudinal Coronary Artery Risk Development in Young Adults (CARDIA) Study. Methods. Data were derived from 1893 African American CARDIA year-15 participants who had undergone skin reflectance assessments at year 7. We adjusted for age, gender, body mass index, smoking status, and use of antihypertensive medication to examine whether year-15 self-reported family incomes, in interaction with skin reflectance, predicted blood pressure levels. Results. Mean systolic and diastolic blood pressure levels were 117.1 (±16.07) and 76.9 (±12.5) mm Hg, respectively. After adjustment, the interaction between skin reflectance and income was significantly associated with systolic blood pressure ( P < .01). Among lighter-skinned African Americans, systolic pressure decreased as income increased (b= −1.15, P <.001); among those with darker skin, systolic blood pressure increased with increasing income (b=0.10, P =.75). Conclusions. The protective gradient of income on systolic blood pressure seen among African Americans with lighter skin is not observed to the same degree among those with darker skin. Psychosocial stressors, including racial discrimination, may play a role in this relationship. In the United States, marked racial disparities in morbidity and mortality exist across a wide range of health indicators. 1 , 2 Gaps in health between African Americans and White Americans are particularly pronounced for cardiovascular diseases and their related risk factors 3 , 4 and persist despite considerable advances in clinical treatment and care. 5 Rates of hypertension (blood pressure above 140/90 mm Hg) are nearly twice as high among African Americans as among Whites, and the condition tends to be more severe and to have an earlier onset among African Americans. 6 This racial disparity in hypertension is also the largest contributor to broader all-cause mortality differentials between African and White Americans. 7 Racial disparities in health are complex in terms of their underlying causes and are not completely understood, with a range of social–environmental factors implicated in the poorer cardiovascular health of African Americans. One such factor is socioeconomic status (SES), which is associated with cardiovascular disease risk in a gradient fashion in the United States and other developed nations and is lower on average among African Americans than among Whites. 8 , 9 However, evidence has increasingly suggested that race and class interact in complex ways to influence health, and the magnitude of association between SES and health varies according to race. 10 – 15 Although the relationship has been less consistent, skin color is another socially significant factor that has shown a graded association with cardiovascular health, particularly blood pressure, among African Americans. 16 – 18 Given the social significance of skin color in United States society, 19 – 21 it may constitute an additional factor along which SES effects on health vary. The well-documented SES gradient in health, wherein incremental increases in economic standing are associated with improved health across the entire SES range, suggests that psychosocial stress may be an important contributor to health disparities. 9 , 22 Because African Americans are overrepresented at the bottom of the SES ladder, some social–environmental models of health disparities have posited that economic factors are largely responsible for race gaps in health status. 12 However, it is now well demonstrated that SES alone does not account for racial health disparities, and the effects of SES on health are not uniform across race groups. 10 , 11 , 13 , 23 For example, Farmer and Ferraro 13 recently demonstrated that although the SES gradient is strong for White Americans, economic success does not carry the same health benefits for African Americans. They observed a pattern of “diminishing returns” 13 (p192) in general health status among African Americans of higher economic standing. A similar pattern was recently observed with respect to body mass index (BMI; weight in kilograms divided by height in meters squared), an important cardiovascular risk factor. Lewis et al. 14 found that, among women, racial differences in BMI were most pronounced at higher levels of education, with White women having lower BMIs than African American women. These findings highlight the complexity of interactions among race and class and suggest that additional social–environmental factors should be considered in attempts to explain racial health disparities. An example of such a social–environmental factor is skin color. 19 , 24 A skin color gradient in blood pressure, such that darker skin color is associated with higher average blood pressure levels, has long been observed among African Americans and other African diaspora populations. 16 , 19 , 25 In the first observations of this gradient, during the 1970s, the relationship was largely attributed to genetic or unknown physiological causes. 16 , 25 However, skin color has been shown to be an imperfect marker of proportional genetic ancestry in multiracial societies. 26 , 27 In addition, genetic components seem to explain relatively little population-level prevalence rates of common, complex diseases such as hypertension. 28 Thus, skin color is not likely to be a significant marker of differences in gene-based disease susceptibility. 27 Skin color has long been recognized to carry considerable social significance in United States society, however, and many studies have noted the potential confounding effects of psychosocial factors on the skin color gradient in blood pressure. 17 – 19 , 29 , 30 Relative to African Americans with lighter skin, it has been shown that, on average, African Americans with darker skin are employed in occupations with lower incomes and less prestige. 31 – 33 Studies examining the potential confounding effects of economic status on the skin color gradient in blood pressure have produced mixed findings. 16 – 18 Harburg et al. found that the relationship between skin color and blood pressure is independent of SES, 16 whereas Keil et al. suggested that skin color differences in blood pressure are contingent on SES. 17 Klag et al. 18 found a moderating effect of SES on the association between skin color and blood pressure, with only individuals in lower socioeconomic tiers exhibiting the skin color gradient. The findings of these studies indicate that more research is needed regarding the interactive effects of skin color and SES on blood pressure among African Americans. Recently, Gravlee et al. 24 , 34 reported that, among adults in Puerto Rico, socially ascribed “color,” of which skin tone is a dimension, moderates the SES gradient in blood pressure. They found higher SES to be associated with lower blood pressure only among those of lighter ascribed color. A similar pattern may exist for skin pigmentation in the United States; whereas skin color is associated with SES, it is also a marker of other social realities and experiences that may affect health status, including discrimination and low social standing. 19 , 35 , 36 Thus, skin color may represent an additional dimension of social variation along which health varies in the United States and, as such, may complicate the effects of SES on the health of African Americans. Similar to recent findings showing that the health benefits of higher SES are more limited for African Americans than they are for Whites, 13 these benefits may also be more limited depending on one’s skin color. We explored whether skin color is a significant moderator of the effects of SES on blood pressure among African Americans taking part in the Coronary Artery Risk Development in Young Adults (CARDIA) Study. We sought to answer the following question: does the SES gradient in blood pressure vary among African Americans of darker skin color and those of lighter skin color?
