Body Mass Index, Waist/Hip Ratio, and Coronary Heart Disease Incidence in African Americans and Whites

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1 American Journal of Epidemiology Copyright 1998 by The Johns Hopkins University School of Hygiene and Public Health All rights reserved Vol. 8, No. 12 Printed in U.S.A. Body Mass Index, Waist/Hip Ratio, and Coronary Heart Disease Incidence in African Americans and Whites Aaron R. Folsom, 1 June Stevens, 2 Pamela J. Schreiner, 1 and Paul G. McGovern 1 for the Atherosclerosis Risk in Communities Study Investigators To study the relation of the amount and distribution of body fat with incident coronary heart disease in two ethnic groups, the authors analyzed prospective data from the Atherosclerosis Risk in Communities Study. Among,040 participants aged years and free of coronary disease at baseline in , we identified 398 events through 1994, an average of 6.2 years of follow-up. Among African-American women, the multivariable-adjusted relative risks of coronary heart disease across quartiles of body mass index were, , and 2.15 (p for trend = 0.27), and those for waist/hip ratio were, 2.07, 2.33, and 4.22 (p for trend = 0.02). Among African-American men, these respective relative risks were,3, 0.83, and 1.20 (p for trend = 0.76) for body mass index and, 8, 1.87, and 1.68 (p for trend = 0.06) for waist/hip ratio. Relative risks for whites were generally similar to those for African Americans. Relative risks were stronger for never smokers than for the overall cohort. Unlike some previous studies, our results suggest that Africa Americans, like whites, are not spared from the coronary heart disease risks accompanying obesity. Am J Epidemiol 1998; 8: blacks; coronary disease; ethnic groups; obesity; weight Extensive prospective epidemiologic research has documented that moderate-to-severe obesity is an important risk factor for coronary heart disease (CHD), either directly or indirectly through intervening risk factors such as hypertension, dyslipidemia, and diabetes (1). Obesity and weight gain arising early in adulthood or occurring over long periods of follow-up seem to convey particularly strong risks of CHD (2-5). African Americans may have a lower weight-related relative risk of cardiovascular disease than do whites (6-11). Independent of overall obesity, a relative preponderance of abdominal fat is associated with increased risk of CHD (12-19). Abdominal (visceral) obesity is strongly associated with metabolic risk factors for CHD. Yet, researchers have not yet fully established the importance of fat distribution for CHD occurrence among African Americans. Received for publication December 1, 1997, and accepted for publication April 20, Abbreviations: ARIC, Atherosclerosis Risk in Communities Study; BMI, body mass index; C-index, conicity index; CHD, coronary heart disease; Cl, confidence interval;, relative risk. 1 Division of Epidemiology, School of Public Health, University of Minnesota, Minneapolis, MN. 2 Departments of Nutrition and Epidemiology, School of Public Health, University of North Carolina, Chapel Hill, NC. Reprint requests to Dr. Aaron R. Folsom, Division of Epidemiology, School of Public Health, University of Minnesota, Suite 300, 00 South Second Street, Minneapolis, MN To study these issues further, we analyzed the relation of anthropometric variables with CHD incidence in a biethnic cohort of middle-aged adults. We hypothesized that a relative preponderance of abdominal fat, as measured by greater waist/hip ratio, would be a risk factor for CHD in both African Americans and whites. MATERIALS AND METHODS Study population The Atherosclerosis Risk in Communities (ARIC) Study (20) is a multicenter prospective investigation of atherosclerotic diseases. One aspect of the study includes examination and follow-up of a biethnic, population-based cohort of those aged years from Forsyth County, North Carolina; Jackson, Mississippi (African Americans only); the northwest suburbs of Minneapolis, Minnesota; and Washington County, Maryland. A baseline home interview in 1987 through 1989 assessed participants' health behaviors, sociodemographic characteristics, and disease histories. A clinic examination included measurements of cardiovascular disease risk factors, a 12-lead electrocardiogram, and a B-mode ultrasound examination of selected arterial sites. Approximately 46 percent of those who were eligible in Jackson and 65 percent in the other three communities completed the clinic visit, yielding a total 1187

2 1188 Folsom et al. of 15,792 participants. A previous report described the participants (21). The ARIC Study reexamined participants in (93 percent return rate) and (86 percent return rate). Baseline measurements Technicians measured height to the nearest centimeter and weight to the nearest pound with participants in scrub suits and without shoes. We calculated body mass index (BMI) (kg/m 2 ). Technicians measured circumferences of the waist (umbilical level) and hip (maximum buttocks) to the nearest centimeter. We calculated both waist/hip ratio and another measure of central adiposity, the conicity index (C-index), which is a height- and weight-standardized measure of abdominal girth (22). Each participant reported his or her weight at age 25 years. We calculated BMI at age 25, using height at baseline, and determined weight change between age 25 and baseline. Participants reported their smoking status and amount smoked per day. They similarly reported their drinking status and the amount of beer, wine, and liquor usually drunk, which was converted to alcohol intake per day. reported their menstrual history and were categorized as premenopausal (current menstruation), perimenopausal (menstruation within the past 2 years, but not currently), postmenopausal (surgical or natural cessation of menstruation more than 2 years ago), or unknown ovarian status. also reported their current or past use of hormonal replacement therapy. We defined prevalent CHD at baseline, for exclusion, as a history of angina pectoris by the Rose questionnaire (23), a self-reported history of a physiciandiagnosed heart attack, evidence of a prior myocardial infarction by electrocardiogram, or a report of prior cardiovascular surgery or coronary angioplasty. We created a score for family history of CHD based on the method described by Hunt et al. (24). Ascertainment of incident events The ARIC Study ascertained CHD incidence by several methods (20, 25). Interviewers contacted participants annually by telephone to identify all hospitalizations and deaths. The ARIC Study staff also surveyed death certificates and discharge lists from local hospitals to detect additional deaths and cardiovascular events. For hospitalized patients, trained abstractors obtained the hospital chart and recorded the presenting signs and symptoms, including chest pain, cardiac enzymes, and related clinical information. They photocopied up to three 12-lead electrocardiograms (the first, the last, and one on approximately the third hospital day). Trained coders at the University of Minnesota visually coded the electrocardiograms using the Minnesota Code (26) and evaluated wave-form evolution using side-by-side comparisons. The ARIC Study investigated out-of-hospital deaths (25) by means of death certificates, interviews with next of kin (in approximately 81 percent of cases), and questionnaires completed by the patient's physicians (in approximately 91 percent of the cases). We abstracted coroner reports and autopsy reports, when available, for use in validation. We defined CHD incidence for this report as a validated definite or probable myocardial infarction or definite CHD death. Two members of the ARIC Morbidity and Mortality Classification Committee reviewed all potential clinical CHD events by using published criteria (25), and the chair of the Committee adjudicated differences between the two reviewers. We defined unrecognized myocardial infarction by the appearance between the first and subsequent ARIC examinations of a major Q wave or a minor Q wave with ischemic ST-T changes or a myocardial infarction by computerized NOVACODE (27) criteria, confirmed by side-by-side visual electrocardiogram comparison. We did not include coronary revascularization in the definition of CHD because we were concerned about differential use of procedures by race, sex, and/or obesity status. Data analysis Of the 15,792 ARIC participants,,040 were free of CHD at baseline. We used analysis of covariance to compute sex-specific, age-, ethnicity-, and ARIC field center-adjusted mean baseline values of the anthropometric variables for participants who did versus those who did not develop an incident CHD event during follow-up. When categorizing anthropometric variables, we used sex-specific quartile or tertile cutting points because the distributions for men and women were quite different. We used Poisson regression to compute age-, ethnicity-, and field center-adjusted incidence rates. We computed length of follow-up for clinically recognized cases as the time lapsed between the baseline examination and the first CHD event. Because we could not determine the exact date of an unrecognized myocardial infarction, we arbitrarily assigned its occurrence as the midpoint between the ARIC examination at which an electrocardiogram revealed it and the immediately prior examination. For noncases, follow-up continued until the date of death, the date of last contact (if lost), or else through December 31, We computed adjusted relative risks of CHD in relation to categorical, or continuous, anthropometric

3 Body Mass Index, Waist/Hip Ratio, and Coronary Disease 1189 indices using proportional hazards regression. We tested trends in relative risks across quartiles in regression models using a quartile variable coded 1 through 4. We chose to adjust in various models for age (continuous), ethnicity (African American, white), cigarette smoking (never, former, current; and packyears), alcohol intake status (never, former, current), and family history of CHD (24) score (<-0.5, -0.5 to 0.5, >0.5, or unknown). We tested five other covariates that proved not to be confounding factors of the association between body size and incident CHD; these were dropped because the number of events to covariates was small. The covariates were ARIC field center, education level (less than high school, high school graduate, post-high school), grams of alcohol per day, and, in women, menopausal status (pre-, peri-, or postmenopausal, or unknown ovarian status), and hormone replacement therapy (never, former, current). Although the ARIC Study measured plasma lipids, blood pressure, and serum glucose, we did not adjust for these potential mediators of obesity effects, since doing so would be overadjustment because of their position on the causal pathway between obesity and CHD (28). RESULTS In the ARIC Study cohort free of prevalent CHD at baseline, 273 men and 125 women developed CHD over an average of 6.2 years of follow-up. Table 1 shows that men and women who developed CHD had a higher average waist circumference, waist/ hip ratio, and C-index at baseline than did those who remained free of CHD. In women, but not in men, mean weight and BMI at baseline and BMI at age 25 also were higher in those who developed CHD versus those who did not. Mean values of height, change between weight reported at age 25 years and that measured at baseline, and hip circumference were not different (p > 0.05) in those who developed CHD versus those who did not. The C-index was highly correlated with waist/hip ratio (r = 0.81) and waist circumference (r = 0.80), and the waist circumference was highly correlated with BMI (r = 0.88) and waist/hip ratio (r = 0.73). Therefore, we focused most of our attention on waist/ hip ratio and BMI (which were correlated, r = 0.43). Figure 1 shows that the age-, ethnicity-, and field center-adjusted incidence rate of CHD increased at least two-fold across quartiles of the waist/hip ratio in both sexes. In contrast, the incidence rate increased 1.1-fold in men and 1.8-fold in women across quartiles of BMI (figure 2). Adjusted for potential confounders (table 2), the relative risks of CHD overall rose 1.80-fold across quartiles of waist/hip ratio in men (p for trend = 0.002) and 2.10-fold in women (p for trend = 0.008). Modeled as a continuous variable, the relative risk of CHD per increment (1 standard deviation) of waist/hip ratio was 1.44 in men and 1.32 in women (both p < 0.01), and quadratic terms were not significant. For BMI, relative risks increased 1.31-fold across quartiles in men {p for trend = 0.061) and 1.98-fold in women {p for trend = 0.015). Per 5.3 kg/m 2 (1 standard deviation), relative risks were 1.11 in men (p = 0.16) and 1.22 in women (p = 0.011), and quadratic terms were not significant. When waist/ hip ratio and BMI were both included as continuous variables in sex-specific models, only waist/hip ratio was statistically significant. Relative risks across quartiles of waist circumference increased 1.51-fold in men (p for trend = 0.007) and 2.53-fold in women {p for trend = 0.005) (not shown). TABLE 1. Sex-specific, age-, ethnicity-, and center-adjusted mean values of anthropometric variables in participants who did versus those who did not develop CHDt in ARIC,t Variable Height (cm) Weight (kg) BMIf (kg/m 2 ) BMI at age 25 (kg/m 2 ) Weight change (kg) Waist circumference (cm) Hip circumference (cm) Waist/hip ratio C-index Approximate SDf Yes (n = 273) No (n = 5,915) * * 1.31* Incident CHD Yes (n=125) No (n = 7,727) * 27.7* 22.1* * * 1.30* * p < 0.05 for sex-specific difference between incident cases and noncases. t CHD, coronary heart disease; ARIC, Atherosclerosis Risk in Communities; SD, standard deviation; BMI, body mass index.

4 1190 Folsometal Quartile of Waist-to-Hip Ratio FIGURE 1. Sex-specific age-, ethnicity-, and field center-adjusted incidence of coronary heart disease according to quartiies of waist/hip ratio, the ARIC Study, Quartile of Body Mass Index (kg/m 2 ) FIGURE 2. Sex-specific age-, ethnicity-, and field center-adjusted incidence of coronary heart disease according to quartiies of BMI, the ARIC Study, In the quartile analysis, the associations of BMI with CHD appeared generally similar in whites and African Americans (table 2), and tests of BMI by ethnicity interaction in continuous variable models were not significant (p < 0.25). The association of waist/ hip ratio quartiies with CHD was equally strong in African-American and white men, but appeared stronger in African-American than in white women. However, again a waist/hip ratio by ethnicity interaction was not significant. Among the participants who had never smoked, the magnitudes of the relative risks were basically similar for waist/hip ratio and BMI (the 95 percent confidence intervals overlapped), but relative risks for nonsmokers were higher than those for the overall cohort (suggesting that relative risks for the overall cohort were affected by some residual confounding by smoking). Subgroup associations for waist circumference and CHD (not shown) were similar to those for waist/hip ratio. We also determined the joint relative risk of CHD by tertiles of waist/hip ratio and those of BMI simultaneously (figure 3). Compared with those in the lowest tertiles of both variables and adjusted for the same covariates as in table 2, the relative risk of CHD was elevated most for those in the highest tertile of waist/ hip ratio who also had a BMI in the second tertile

5 Body Mass Index, Waist/Hip Ratio, and Coronary Disease 1191 TABLE 2. Sex-specific relative risks* (95% confidence intervals) of coronary heart disease in relation to quartiles of waist/hip ratio and body mass index, ARICt Study, Waist/hip ratio < >0 p for trend < >0.954 p for trend Overall t t African-American White Never smokers Overall African-American White Never smokers < > p for trend BMIt (kg/m 2 ) < > p for trend Adjusted (except when stratified) for age, ethnicity, smoldng status and amount, alcohol drinking status, and family history of coronary heart disease, t ARIC Atherosclerosis Risk In Communities;, relative risk; Cl, confidence Interval; BMI, body mass index. (relative risk () = 2.1, 95 percent confidence interval (CI) for men; = 1.7, 95 percent CI for women) or the third tertile ( = 1.4, 95 percent CI for men; = 1.8, 95 percent CI for women) and for those in the highest tertile of BMI who also had a waist/hip ratio in the second tertile ( = 1.5, 95 percent CI for men; = 1.6, 95 percent CI for women). DISCUSSION This prospective study of middle-aged adults showed overall that waist/hip ratio and BMI were both moderately strongly associated with incident CHD in women, whereas in men, waist/hip ratio showed a somewhat stronger positive association with CHD than did BMI. These associations between body size were somewhat stronger among never smokers than in the total cohort. This is consistent with the argument made by Manson et al. (28) that smoking is such a strong confounder of obesity-disease relations that it should be examined among never smokers separately. More uniquely, we found that the associations of BMI and waist/hip ratio with CHD did not differ greatly between African Americans and whites. Admittedly, statistical power was sufficient to rule out only large differences between ethnic groups. Our ethnic-specific findings for CHD incidence differ from those of several previous reports based on

6 1192 Folsom et al. MEN Relative Risk BMI Tertiies Waist/Hip Ratio Tertiies WOMEN Relative Risk 1 BMI Tertiies Waist/Hip Ratio Tnrti I ac- FIGURE 3. Sex-specific relative risks of coronary heart disease adjusted for age, ethnicity, smoking status and amount, alcohol drinking status, and family history of CHD according joint tertiies of BMI and waist/hip ratio, the ARIC Study, mortality in cohorts from the 1940s to 1960s. Researchers studying cohorts assembled in the early 1960s in Evans County, Georgia (6), and Charleston, South Carolina (8), found little association of BMI, or chest, abdominal, and arm circumferences with CHD mortality among African-American women. In contrast, measures of obesity were associated positively with CHD mortality in at least some subgroups of white women (8, 9). In the Charleston cohort, BMI was not associated with CHD mortality in white men, but showed a U-shaped association in AfricanAmerican men (7). In the American Cancer Society Prevention Study I, begun in , BMI was associated positively with CHD mortality in white women, but there was no association in African-American women (9). A study of male US Army veterans dis- charged in found a positive association of BMI with cardiovascular mortality in African Americans, similar to that for whites (10). We previously reported that odds ratios between BMI and CHD prevalence at baseline in the ARIC Study were approximately equal in African Americans and whites (29). Differences in the ethnic-specific findings of these studies versus the ARIC Study may relate to different eras, in particular, because weight has increased markedly in recent years in the United States (30). In the past 15 years, research has implicated abdominal visceral adiposity as the most hazardous to cardiovascular health. Yet, the number of prospective studies of CHD in relation to waist/hip ratio is relatively small (12-15, 18). In most of these studies (12-15), waist/hip ratio was positively associated with Am J Epidemiol Vol. 8, No. 12,

7 Body Mass Index, Waist/Hip Ratio, and Coronary Disease 1193 the occurrence of CHD more strongly than was BMI. This was also true in three of four sex-ethnicity groups in this study, reinforcing the hypothesis that a preponderance of abdominal fat is most risky. Data for African Americans relating fat distribution with cardiovascular disease risk are extremely limited. In contrast to our results, the only previous prospective study found a strong association between waist/hip ratio and cardiovascular mortality in white men, but not in African-American men (10). Gillum (19) created two fat distribution indices to approximate waist/ hip ratio and found them nonsignificantly higher in prevalent CHD cases than in noncases in a US national survey, with little ethnic heterogeneity apparent. The ARIC Study found similarly elevated odds ratios for prevalent CHD in relation to waist/hip ratio (odds ratio range, ) in African Americans and whites and in men and women (29). Studies of fat distribution and CHD risk factors do not show consistent differences between African Americans and whites. For example, the Coronary Artery Risk Development in Young Adults (CARDIA) investigators reported similar associations between waist/hip ratio and metabolic CHD risk factors in African Americans and whites (31). The Insulin Resistance and Atherosclerosis Study found that African Americans, Hispanics, and non-hispanic whites had a similar association of insulin sensitivity with abdominal obesity, although the association of fasting insulin with abdominal obesity was slightly weaker in African Americans than in non-hispanic whites (32). Yet, Dowling and Pi-Sunyer (11) concluded that metabolic risk factors were less related to waist/hip ratio, controlled for total body fatness, in African Americans than in whites. Strengths of this study include its carefully standardized measurements of anthropometry and CHD incidence in population-based samples. Limitations include the relatively short follow-up, yielding a modest number of events so far, and a narrow cohort age range. In addition, waist/hip ratio only approximates the distribution of adipose tissue. Other measures, such as computerized tomography, magnetic resonance imaging, and dual energy x-ray absorptiometry, can more accurately assess visceral adipose mass (33), but these are impractical for field epidemiology. Others have argued that waist circumference, not waist/ hip ratio, should be used to assess these relations (34). However, waist circumference is highly correlated with (r = 0.88) and essentially reflects BMI, whereas waist/hip ratio seems to capture additional predictive information. Regardless of which index of adiposity is most predictive of CHD, concern is growing that the prevalence of obesity in the United States is rising markedly (30). Moderate-to-severe obesity is a risk factor not only for CHD, demonstrated once again in these data, but also for other chronic diseases (1). Our data do not suggest that African Americans are spared more than whites from the CHD risks accompanying obesity. This observation is especially important because in African Americans the prevalence of obesity (and, thus, its attributable risk for CHD) is high. Clearly, continued and more-effective population-wide efforts to prevent and treat obesity are needed. ACKNOWLEDGMENTS The ARIC Study was funded by contracts N01-HC , N01-HC-55016, N01-HC-55018, NO1-HC-55O19, N01-HC-55020, N01-HC-55021, and N01-HC from the US National Heart, Lung, and Blood Institute. The authors thank the following ARIC staff for important contributions: Phyllis Johnson, Marilyn Knowles, and Catherine Paton from the University of North Carolina, Chapel Hill, NC; Jeannette Bensen, Kay Burke, Wilhelmenia Cheeks, and Revitha Cook from the University of North Carolina, Forsyth County, NC; Agnes Hayes, Jane Johnson, Penney Lowery, and Patricia Martin from the University of Mississippi Medical Center, Jackson, MS; Nancy MacLennan, Gail Murton, Linda Neal, and Marilyn Nelson from the University of Minnesota, Minneapolis, MN; Holly Healy, Joel Hill, Mary Hurt, and Joan Nelling from The Johns Hopkins University, Baltimore, MD; Valarie Stinson, Pam Pfile, Hoang Pham, and Teri Trevino from the University of Texas Medical School, Houston, TX; Wanda Alexander, Doris Harper, Charles Rhodes, and Selma Soyal from the Atherosclerosis Clinical Laboratory, Methodist Hospital, Houston, TX; Kathy Joyce, Mary Lauffer, Suzanne Pillsbury, and Tiffany Robertson from the Ultrasound Reading Center, Bowman-Gray School of Medicine, Winston-Salem, NC; and Doris Jones, C. 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