Comparison of Visceral Adipose Tissue Mass in Adult African Americans and Whites Daniel J. Hoffman,* ZiMian Wang,† Dympna Gallagher,† and Steven B. Heymsfield†
Abstract HOFFMAN, DANIEL J., ZIMIAN WANG, DYMPNA GALLAGHER, AND STEVEN B. HEYMSFIELD. Comparison of visceral adipose tissue mass in adult African Americans and whites. Obes Res. 2005;13:66 –74. Objective: Previous studies have reported racial differences in the amount of visceral adipose tissue (VAT), a risk factor for metabolic diseases. These results are equivocal and have not controlled for hormonal influences on VAT mass. This study was designed to measure the extent to which race is associated with VAT, controlling for total adipose tissue (TAT) mass and testosterone. Research Methods and Procedures: Using a cross-sectional study design, we measured TAT mass using DXA, VAT and subcutaneous adipose tissue mass using magnetic resonance imaging, and sex hormones using radioimmunoassay in 224 African-American and white men and women. Results: White men had increased VAT mass, even when controlling for TAT and age, compared with African-American men. White women also had a higher VAT mass compared with African-American women, but only when controlling for TAT and age. When multiple linear regression was used to evaluate the racial differences in VAT mass in a subset of subjects (n ⫽ 80), controlling for sex hormones, it was found that white men, but not women, had increased VAT mass compared with their African-American counterparts. Discussion: Based on the results of this study, we conclude that, when controlling for TAT, sex hormone levels, and age, white men, but not women, have more VAT mass than
Received for review November 25, 2003. Accepted in final form November 11, 2004. The costs of publication of this article were defrayed, in part, by the payment of page charges. This article must, therefore, be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. *Department of Nutritional Sciences, Rutgers University, New Brunswick, New Jersey; and †Department of Medicine, Obesity Research Center, St. Luke’s-Roosevelt Hospital Center, Columbia University College of Physicians and Surgeons, New York, New York. Address correspondence to Daniel J. Hoffman, Room 230 Davison Hall, 26 Nichol Avenue, Room 228B, New Brunswick, NJ 08901. E-mail:
[email protected] Copyright © 2005 NAASO
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African-American men and women. Additional studies are needed to explore possible environmental and genetic influences on fat distribution relative to race and sex. Key words: body composition, central adiposity, diabetes, hormones
Introduction As the prevalence of obesity increases in both developed and developing countries (1– 8), there is a parallel increase in the prevalence of chronic disease associated with excess adiposity, such as hypertension, heart disease, and type 2 diabetes (T2D)1 (9 –13). An underlying mechanism behind the association between excess adipose tissue and chronic disease is the distribution of adipose tissue, such as adipose tissue deposited centrally or in the abdominal viscera, visceral adipose tissue (VAT), which has been reported to be a major risk factor for cardiovascular disease and diabetes (1– 4,14 –17). The association between VAT and chronic disease is especially important for African Americans because the risk for both obesity and obesity-associated chronic diseases is higher than for whites. Still, although recent studies have reported differences in the amount of VAT between African-American men and women compared with whites (18 –22), these studies have not investigated the relationship between race and VAT mass using precise body composition techniques and controlling for total adipose tissue (TAT) mass and sex hormone concentrations, the specific aims of the present study. Racial differences in VAT mass have been explored in different racial groups (18,21,23), including Asians (24 – 27), but have often provided equivocal results, especially when considering differences between African-American and white men and women. For example, Hill et al. (21) reported that African-American men had less VAT mass
1 Nonstandard abbreviations: T2D, type 2 diabetes; VAT, visceral adipose tissue; TAT, total adipose tissue; CT, computed tomography; MRI, magnetic resonance imaging; WC, waist circumference; SCAT, subcutaneous adipose tissue; FM, fat mass; FFM, fat-free mass: %BF, percentage body fat; Menop, menopausal status; SHBG, serum hormone-binding globulin.
VAT in African Americans and Whites, Hoffman et al.
than white men, and women of each group had an opposite relationship, whereas Despre´s et al. (18) reported that women of each group had approximately equal VAT mass. These studies used relatively large sample sizes and advanced body composition techniques, either computed tomography (CT) or magnetic resonance imaging (MRI), but neither controlled for hormonal influences on VAT mass, a factor that has been shown to have differential effects between races. The relative influence of testosterone and estrogen on body fat distribution has been investigated in well-controlled studies of both men and women (28 –33). For men, Bhasin (28) reported that testosterone administration to middle-aged men resulted in a decrease in VAT mass, consistent with a report from Tchernof et al. (34) in which VAT mass in women was found to increase as gonadal androgen levels increased. Tsai et al. (35) reported that low serum testosterone levels predicted VAT mass during an 8-year prospective study. These studies highlight the relative influence of sex hormones on body composition, in general, and VAT mass, in particular. Moreover, there is some suggestion that racial differences exist with respect to sex hormone metabolism. For instance, Manson et al. (36) reported that African-American women had lower levels of estradiol compared with white women. Thus, it is important to consider differences in sex hormone metabolism when exploring racial differences in VAT mass. Therefore, this study was conducted to determine the extent to which racial group is associated with VAT, controlling for TAT mass and hormonal correlates of VAT mass, as measured with MRI, in African-American and white men and women. In addition, the association of sex hormones with VAT was explored to determine the extent to which these hormones are linked with VAT, independently of racial group and sex.
Research Methods and Procedures Experimental Design This cross-sectional cohort study was designed to measure differences in body composition and hormonal profiles in adult African-American and white men and women. Subjects Subjects were 97 men and 127 women (132 white and 92 African American), 18 to 88 years old, recruited from the New York metropolitan area by newspaper and radio advertisements. Each subject received a medical examination with standard blood analyses. Subjects who were free of chronic diseases known to influence body composition (e.g., cancer, hyper- or hypothyroidism, anorexia, or general gastrointestinal disease) were enrolled. Racial group was determined by self-report, and the parents and grandparents of each subject were required to be of the same racial group
as that reported by the subjects. The study was approved by the Institutional Review Board of St. Luke’s-Roosevelt Hospital Center, and all subjects gave a written informed consent to participate in the study. Body Composition Anthropometry. Subjects were weighed in the morning wearing light clothing and to the nearest 0.1 kg using a standard scale (Weight Tronix, New York, NY). Height was measured barefoot to the nearest 0.5 cm using a fixed stadiometer (Holtain, Wales, United Kingdom). Waist circumference (WC) was measured with a flexible measuring tape placed evenly at the narrowest part of the torso between the last rib and iliac crest (37). Hip circumference was measured at the point of greatest protuberance of the buttocks as viewed from the side. MRI. VAT and subcutaneous adipose tissue (SCAT) volumes were measured using whole-body multislice MRI. TAT is defined as the sum of VAT and SCAT. Subjects were placed on the 1.5 Tesla scanner (6X Horizon; General Electric, Milwaukee, WI) with their arms extended above their heads. The protocol involved the acquisition of 40 axial images of 10-cm thickness at 40-cm intervals across the body. All images were analyzed using VECT image analysis software (Martel, Montreal, CA) on a Sun Workstation (Silicon Graphics, Mountain View, CA). Adipose tissue density was assumed to be 0.93 kg/L in converting the estimated MRI volumes to mass. The technical errors for repeated measurements of the same scan by the same investigator of MRI-derived SCAT and VAT volumes in our laboratory are 1.1 ⫾ 1.2% and 1.1 ⫾ 1.5%, respectively (38). DXA. DXA (software version 3.6, Lunar DPX; Lunar Corp., Madison, WI) was used to measure fat mass (FM), fat-free mass (FFM), and percentage body fat (%BF), as previously reported (38). Hormonal Evaluations Blood samples were drawn from an antecubital vein before 10 AM after a 12-hour overnight fast, and the serum was stored at ⫺20 °C. Sex hormones were measured by radioimmunoassay for estradiol (Diagnostic Systems Laboratories Inc., Webster, TX) and total testosterone (Diagnostic Products Corporation, Los Angeles, CA). It should be noted that sex hormone levels were evaluated on a subsample of subjects and not in all subjects measured for total body composition. This resulted in a smaller sample size for the regression equations in which we evaluated the effect of race on VAT mass, controlling for confounding variables. Statistical Methods Differences between racial group and sex for physical characteristics were determined using analysis of covariance. All data were normally distributed except for VAT; OBESITY RESEARCH Vol. 13 No. 1 January 2005
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Table 1. Subject characteristics: physical and hormonal characteristics (mean ⫾ SD) Men Variable
White
N 61 Age (years)* 44.0 ⫾ 16.5 Weight (kg)† 83.3 ⫾ 11.8 Height (cm) 178.8 ⫾ 6.9 BMI† 26.1 ⫾ 3.7 %BF† 24.0 ⫾ 7.8 FFM (kg)† 62.7 ⫾ 7.4 FM (kg)† 20.6 ⫾ 8.8 Waist (cm)† 90.2 ⫾ 11.0 Hip (cm)† 101.7 ⫾ 6.6 Waist-to-hip ratio† 0.89 ⫾ 0.06 N 21 Testosterone (ng/mL) 4.9 ⫾ 1.3 Estradiol† (pg/mL) N Menopause status Estradiol† (pg/mL)
Women
African-American
White
African-American
37 36.8 ⫾ 13.2 80.9 ⫾ 12.3 178.0 ⫾ 6.1 25.5 ⫾ 3.4 21.2 ⫾ 6.7 63.2 ⫾ 7.5 17.7 ⫾ 7.4 86.0 ⫾ 10.1 99.12 ⫾ 7.6 0.87 ⫾ 0.07 20 5.7 ⫾ 1.6
73 47.7 ⫾ 17.2 65.7 ⫾ 13.8 163.7 ⫾ 7.6 24.6 ⫾ 5.0 35.2 ⫾ 9.3 41.7 ⫾ 6.1 24.0 ⫾ 10.9 77.6 ⫾ 12.0 98.7 ⫾ 14.8 0.85 ⫾ 0.67 19 0.31 ⫾ 0.14 77.1 ⫾ 48.8
53 48.1 ⫾ 17.2 75.6 ⫾ 14.4 163.1 ⫾ 7.4 28.4 ⫾ 5.2 39.6 ⫾ 9.7 44.6 ⫾ 5.6 31.0 ⫾ 12.2 85.3 ⫾ 12.4 106.5 ⫾ 12.1 0.80 ⫾ 0.07 20 0.32 ⫾ 0.22 39.5 ⫾ 25.4
12 7 9 12 Premenopause Postmenopause Premenopause Postmenopause 90.8 ⫾ 48.3 56.6 ⫾ 42.2 49.4 ⫾ 28.0 30.5 ⫾ 21.3
* p ⬍ 0.05 for men. † p ⬍ 0.05 for women.
thus, log-transformed values of VAT were used in the multiple linear regression analyses. The data were split by sex for multiple regression analysis because significant interactions between racial group and sex were found. Because VAT and SCAT mass are directly related to overall adiposity and change with age, mean values for VAT and SCAT were adjusted for TAT and age using general linear model analysis. Adjusting for TAT is necessary because we aimed to determine the difference in VAT tissue between the races, independent of TAT mass. Given that VAT is a component of TAT, statistically removing the effect of TAT on VAT is necessary to determine the racial influence on VAT mass without the effect of TAT. The influence of racial group on VAT was determined using multiple linear regression analysis, controlling for confounding variables, such as TAT, age, and sex hormones (testosterone or estradiol) in Model 1, BMI, age, and sex hormones in Model 2, and WC, age, and sex hormones in Model 3. Because not all subjects had blood analyses conducted, the use of results from hormone assays in this cohort reduced the number of subjects analyzed in the regression models from 98 men and 126 women to 41 men and 39 women. Special consideration had to be given to the women studied because data on the influence of sex hormones on body fat distribution are limited. Most, but not all, studies 68
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have reported that sex hormones in women protect against central fat deposition (1,31–34,39). We, therefore, wanted to investigate the influence of race on VAT mass, controlling for known confounding factors, of which menopausal status (Menop), estradiol, and testosterone levels are reported to influence VAT mass. Of the women studied, 42% of white and 49% of African-American women were postmenopausal. Initially, age was included in the analyses but was not found to be statistically significant when Menop, estradiol, and testosterone were included in the models, leading us to conclude that Menop was acting as a proxy for age. Based on the fact that the adjusted R2 was consistently higher when these three variables remained in the regression analysis, we decided to leave these variables in the model as representative factors for sex hormone influence on VAT mass. Statistical significance was set at p ⬍ 0.05, and all statistical analyses were conducted using SPSS for Windows version 9.0 (SPSS Inc., Chicago, IL).
Results Subjects Subject characteristics are summarized in Table 1. White men were older than African-American men (44.0 ⫾ 16.5 vs. 36.8 ⫾ 13.2 years, respectively). There were no statis-
VAT in African Americans and Whites, Hoffman et al.
Table 2. Adipose tissue components in men and women by racial group (mean ⫾ SD) Males
Females
Variable
White
African American
White
African American
N VAT (kg)* SCAT (kg)† VAT to SCAT* SCAT to TAT* VAT to TAT*
61 2.79 ⫾ 2.08 17.65 ⫾ 7.11 0.15 ⫾ 0.08 0.87 ⫾ 0.06 0.12 ⫾ 0.06
37 1.49 ⫾ 1.42 16.03 ⫾ 6.65 0.09 ⫾ 0.08 0.91 ⫾ 0.06 0.08 ⫾ 0.06
73 1.42 ⫾ 1.30 22.49 ⫾ 9.88 0.06 ⫾ 0.04 0.94 ⫾ 0.03 0.05 ⫾ 0.03
53 1.62 ⫾ 1.10 29.32 ⫾ 11.44 0.05 ⫾ 0.03 0.95 ⫾ 0.02 0.05 ⫾ 0.02
* p ⬍ 0.05 for men. † p ⬍ 0.05 for women.
tically significant differences between men in the two ethnic groups with respect to the other variables studied. For testosterone levels, there was also no significant difference between white and African-American men (4.9 ⫾ 1.3 vs. 5.7 ⫾ 1.6 ng/mL, p ⫽ 0.121, respectively). Finally, the concentration of serum hormone-binding globulin (SHBG) was not statistically different between the white and African-American men (90.3 ⫾ 43.3 vs. 85.4 ⫾ 29.7 nM, p ⫽ 0.680, respectively). For women, the white women weighed less, had lower BMI, %BF, FFM, FM, and lower WC and hip circumference compared with African-American women (65.7 ⫾ 13.8 kg, 24.6 ⫾ 5.0, 35.2 ⫾ 9.3%, 41.7 ⫾ 6.1 kg, 24.0 ⫾ 10.9 kg, 77.6 ⫾ 12.0 cm, and 98.7 ⫾ 14.8 cm vs. 75.6 ⫾ 14.4 kg, 28.4 ⫾ 5.2, 39.6 ⫾ 9.7%, 44.6 ⫾ 5.6 kg, 31.0 ⫾ 12.2 kg, 85.3 ⫾ 12.4 cm, and 106.5 ⫾ 12.1 cm, respectively). Hormonal profiles of a subsample showed that white women had significantly lower estradiol concentrations compared with African-American women (39.5 ⫾ 25.4 vs. 77.1 ⫾ 48.8 pg/mL, respectively). When split by pre- and post-Menop, there were no significant differences in estradiol between the postmenopausal women of each racial group. However, for the premenopausal women, the white women still had a significantly lower level of estradiol compared with African-American women (49.4 ⫾ 28.0 vs. 90.8 ⫾ 48.3 pg/mL, p ⫽ 0.034). Body Composition Subcompartments of adipose tissue measured by MRI were analyzed by racial group and sex and are summarized in Table 2. White men had a significantly greater VAT mass compared with African-American men (2.79 ⫾ 2.08 vs. 1.49 ⫾ 1.42 kg, respectively, p ⬍ 0.001). White men also had a greater ratio of VAT to SCAT, a lower ratio of SCAT to TAT, and a greater ratio of VAT to TAT compared with
African-American men (0.15 ⫾ 0.08 vs. 0.09 ⫾ 0.08, 0.87 ⫾ 0.06 vs. 0.91 ⫾ 0.06, and 0.12 ⫾ 0.06 vs. 0.08 ⫾ 0.06, respectively, p ⬍ 0.05). For women, white women had significantly less SCAT mass compared with African-American women (22.49 ⫾ 9.88 vs. 29.32 ⫾ 11.44 kg, respectively, p ⫽ 0.001). There were no significant differences with respect to the other body fat compartments, including VAT mass, or ratios between compartments. Because VAT mass is associated with overall adiposity and increases with age, the values of VAT and SCAT were adjusted for TAT and age and compared between the two racial groups of each sex (Table 3). White men had significantly greater VAT mass and lower SCAT mass compared with African-American men (2.51 ⫾ 0.13 and 16.82 ⫾ 0.13 vs. 1.94 ⫾ 0.17 and 17.39 ⫾ 0.17 kg, respectively, p ⫽ 0.011 and 0.010). White women also had significantly greater VAT mass and lower SCAT mass compared with African-American women (1.63 ⫾ 0.09 and 25.24 ⫾ 0.09 vs. 1.34 ⫾ 0.11 and 25.53 ⫾ 0.11 kg, respectively, p ⫽ 0.048 and 0.045). Prediction Models Separate multiple linear regression models to evaluate the differences between the racial groups for VAT mass were evaluated (Tables 4 and 5). Each model, by sex, is described below. Racial Group, Age, TAT, and Sex Hormones. The first equation evaluated the influence of racial group on VAT controlling for age, TAT, and sex hormones (Tables 4A and 5A). Racial group was a significant predictor of VAT when controlling for age, TAT, and testosterone in men (p ⫽ 0.003), such that African-American men had less VAT compared with white men. For women, there was no significant influence of race on VAT, even when controlling for menopause, estradiol, or testosterone levels. OBESITY RESEARCH Vol. 13 No. 1 January 2005
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Table 3. Adipose tissue components in men and women adjusted for age and TAT mass by racial group (mean ⫾ SE) Men
Women
Variable
White
African American
White
African American
N VAT (kg)*† SCAT (kg)*‡
61 2.51 ⫾ 0.13 16.82 ⫾ 0.13
37 1.94 ⫾ 0.17 17.39 ⫾ 0.17
73 1.63 ⫾ 0.09 25.24 ⫾ 0.09
53 1.34 ⫾ 0.11 25.53 ⫾ 0.11
* p ⫽ 0.01 for men. † p ⫽ 0.048 for women. ‡ p ⫽ 0.045 for women.
Racial Group, Age, BMI, and Sex Hormones. VAT mass was predicted using racial group, age, BMI, and sex hormones. African-American men had significantly less VAT than white men when controlling for age, BMI, and testosterone (p ⫽ 0.007) (Table 4B). The influence of race on VAT mass in African-American and white women was not statistically significant (p ⫽ 0.771) (Table 5B). Racial Group, WC, and Sex Hormones. Finally, because WC is a clinical measurement to assess central adiposity, it was of interest to see whether WC predicted VAT differently between the two racial groups. WC significantly predicted VAT in both men and women (p ⬍ 0.05), suggesting that a larger WC is associated with increased mass, but there was a statistically significant effect of race for men only (p ⫽ 0.003) (Table 4C) and not for women (p ⫽ 0.388) (Table 5C).
Discussion As the prevalence of obesity increases throughout the United States, it has become common knowledge that ex-
Table 4A. Multiple linear regression Model 1 for VAT mass in men
Constant Racial group Age TAT Testosterone (ng/mL)
B ⴞ SE
p
0.723 ⫾ 0.219 ⫺0.250 ⫾ 0.077 0.012 ⫾ 0.003 0.041 ⫾ 0.004
0.002 0.003 0.000 0.000
⫺0.096 ⫹ 0.024
0.000
Adjusted R2 ⴝ 0.893
Racial group: 0, white; 1, African American. B, regression coefficient.
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cess adiposity is positively associated with heart disease, stroke, hypertension, and T2D (9 –13). Moreover, central adiposity, specifically VAT mass, is a major risk factor for diabetes and heart disease in men and women of different racial groups (1– 4,14,15,40,41). This association is especially important for persons of African descent because the risk for both obesity and obesity-associated chronic diseases is higher than for whites (5,42). Still, although many studies have reported relative differences in the amount of VAT in African-American men and women compared with whites (18 –22), few have used whole-body MRI in large, carefully controlled samples in which hormonal profiles were measured. We found that white men, but not women, had a greater VAT mass and smaller SCAT mass compared with AfricanAmerican men and women, respectively. These results differ from those reported by others using the same or similar techniques to measure body composition. For example, Conway et al. (20) reported that, in a sample of 18 women, African-American women had 23% less VAT mass compared with white women when VAT was measured using single-slice CT. Lovejoy et al. (43), also using single-slice CT to measure VAT content, reported that 37 AfricanAmerican women, matched for age and body mass to 22
Table 4B. Multiple linear regression Model 2 for VAT mass in men
Constant Racial group Age (years) BMI Testosterone (ng/mL)
B ⴞ SE
p
⫺0.941 ⫾ 0.500 ⫺0.296 ⫾ 0.103 0.017 ⫾ 0.004 0.083 ⫾ 0.014
0.068 0.007 0.000 0.000
⫺0.068 ⫾ 0.035
0.057
Adjusted R2 ⴝ 0.806
VAT in African Americans and Whites, Hoffman et al.
Table 4C. Multiple linear regression Model 3 for VAT mass in men
Table 5B. Multiple linear regression Model 2 for VAT mass in women
Adjusted R2 ⴝ 0.870
Adjusted R2 ⴝ 0.760
Constant Racial group Age (years) Waist (cm) Testosterone (ng/mL)
B ⴞ SE
p
⫺1.910 ⫾ 0.468 ⫺0.268 ⫾ 0.085 0.008 ⫾ 0.003 0.037 ⫾ 0.004
0.000 0.003 0.020 0.000
⫺0.040 ⫾ 0.029
0.176
Constant Racial group BMI Menop Estradiol Testosterone (ng/mL)
white women, had significantly less VAT, 98.0 and 117.3 cm2, respectively. Perry et al. (22), using MRI, reported that white women had a higher volume of VAT compared with African-American women. Although these studies provide evidence of a racial difference in fat distribution, specifically in the deposition of VAT, the sample sizes were small, used single-slice measurements of VAT, or did not control for TAT or hormonal influences on VAT. In our study sample, we found only a borderline statistically significant difference in VAT mass between the women studied and only when we controlled for TAT and age, but not when we included hormonal status. Still, it is important to note that only a subsample of 39 subjects in our study had complete sex hormone profiles, compared with our total sample size of 126 subjects. This could account for some of the discrepancy found in our results comparing men and women of each racial group. The results presented suggest that sex differences in VAT mass may be partly explained by the apparent difference in
Table 5A. Multiple linear regression Model 1 for VAT mass in women
Constant Racial group TAT Menop Estradiol Testosterone (ng/mL)
B ⴞ SE
p
0.67 ⫾ 0.124 ⫺0.051 ⫾ 0.079 0.029 ⫾ 0.004 0.331 ⫾ 0.087 ⫺0.003 ⫾ 0.001
0.000 0.523 0.000 0.001 0.003
⫺0.742 ⫾ 0.197
0.001
Adjusted R2 ⴝ 0.842
Racial group: 0, white; 1, African American; Menop: 0, premenopausal; 1, postmenopausal. B, regression coefficient.
B ⴞ SE
p
⫺0.042 ⫾ 0.286 ⫺0.029 ⫾ 0.098 0.061 ⫾ 0.012 0.285 ⫾ 0.119 ⫺0.004 ⫾ 0.001
0.884 0.771 0.000 0.023 0.002
⫺0.904 ⫾ 0.253
0.001
the effect of sex hormones on fat distribution. Several studies have reported a significant relationship between decreased testosterone levels and increased central fat deposition in men (28 –30). Likewise, it has been reported in that postmenopausal women experience an increase in visceral fat deposition compared with premenopausal women (31–33). Moreover, this differential deposition of fat centrally may be dependent on the abundance of specific hormones (1,32,35,44). For example, higher testosterone and estrogen concentrations tend to be associated with a decreased quantity of VAT in men and women, respectively (31,35). The fact that testosterone was a significant predictor of VAT for men in only one of three prediction models, in addition to the observation that race remained a significant predictor, suggests that some unexplained factor associated with race may have a more powerful influence on the deposition of VAT in white men compared with AfricanAmerican men. It is not likely that differences in SHBG resulted in differential misclassification of testosterone levels because there were no significant differences in SHBG between the racial groups studied. It should also be noted that the sample size for the regression equations was smaller
Table 5C. Multiple linear regression Model 3 for VAT mass in women
Constant Racial group Waist (cm) Menop Estradiol Testosterone (ng/mL)
B ⴞ SE
p
⫺0.603 ⫾ 0.336 ⫺0.081 ⫾ 0.093 0.025 ⫾ 0.004 0.302 ⫾ 0.105 ⫺0.003 ⫾ 0.001
0.082 0.388 0.000 0.007 0.040
⫺0.741 ⫾ 0.225
0.002
Adjusted R2 ⴝ 0.796
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than our overall study sample and that the ages of the white and African-American men were different, but having statistically controlled for age, it is reasonable to anticipate a difference if one actually exists. Nonetheless, our results are consistent with others and suggest that racial differences in the deposition of VAT do, indeed, exist. How this difference explains the difference in risk for chronic disease has yet to be explored. With respect to TAT mass, because TAT is a predictor of VAT, it is of interest that Hill et al. (21) reported that, after adjusting for TAT, both white men and women had significantly greater VAT mass than African-American men and women, respectively. The results from our study support those from Hill et al. when adjusting for TAT and age. However, results from our regression analyses, in which we used different variables that are predictive of VAT, such as sex hormone levels, suggest that race has a significant influence on VAT mass for men only. A potential cause for the difference between our results and those of Hill et al. may be that they analyzed VAT mass in adults who were ⬍30 years old, whereas we studied middle-aged adults. Because age of the subjects was not reported in that paper, it is not possible to say exactly how different our cohort was in terms of age. Mechanisms to explain the racial difference in VAT content are lacking, but it is important to note that racial group is a socially defined variable and has yet to be considered a physical characteristic with distinct physiological differences (45). Although our results show that African Americans, compared with whites, may, in fact, have less VAT mass relative to TAT, the risk for chronic metabolic diseases associated with VAT is higher in African Americans compared with whites (21,40,46 – 48). One potential explanation for this discrepancy may be that African Americans are exposed to a different milieu of dietary and environmental risk factors that increase their risk for cardiovascular disease, independently of VAT mass, compared with whites (40,49,50). Such risk factors include dietary intake, physical activity, and body composition, essentially VAT content (51,52). This observation is not unlike that made in a well-studied population of individuals at extremely high risk for developing T2D, Pima Indians living in Arizona, who were reported to have less VAT compared with white subjects (23). Because African-American persons seem to have less VAT compared with white persons, one could infer that they are exposed to greater environmental risk factors for chronic disease, such as a diet high in fat or processed carbohydrates or low levels of physical activity. It should also be stressed that the relationship between central adiposity and risk for chronic diseases is complicated by many factors, such as alcohol intake, physical activity, smoking, stress, and dietary habits, any one of which may influence disease risk independently of body composition. Controlling for such confounding variables is 72
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a challenge to any research study and emphasizes the need for more research into the precise mechanism underlying the relationship between body composition and risk for metabolic disorders.
Acknowledgments We thank Gerald B. Phillips for his assistance in the evaluation and interpretation of the hormone analyses and in the writing of this manuscript, the men and women who volunteered to participate in this study, and the staff of the Body Composition Unit at the New York Obesity Research Center. This work was supported by NIH Grant DK-42618.
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