This equation shows that for a given BMI, white and Hispanic women will have 2.9% higher percent body fat than black women, which implies that the BMI cutoff value equivalent to 35% body fat will differ in black, white, and Hispanic women. Although the race×BMI interaction was significant for Hispanic women (regression coefficient=−0.181, standard error 0.059, P=.002), the additional contribution was minimal as explained variance improved very little. The interpretation of the significant race×BMI interaction for Hispanic women is as follows: for a given BMI, the difference in percent body fat between black and Hispanic women narrowed with increased BMI.
Of the 555 reproductive-aged women we examined, the BMI cutoff value of 30 kg/m2 suggested by the NIH identified 205 women as obese (percent body fat greater than 35%). Although the NIH-recommended BMI cutoff point for obesity had high specificity (96.8–100%) in different races or ethnicities, the sensitivity was relatively low (47.8–75.0%) (Table 2). The overall sensitivity and specificity of this cutoff value was 57.7% (95% CI 52.5–62.8%) and 98.5% (95% CI 95.8–99.5%), respectively. The sensitivity was significantly higher in black women (75.0%, 95% CI 65.5–82.6%) compared with white (47.8%, 95% CI 38.7–57.0%) and Hispanic women (53.9%, 95% CI 45.7–61.8%, P<.001 both for black compared with white and black compared with Hispanic). It did not differ between white and Hispanic women (P=.335). Specificities (96.7–100%) were statistically similar in different races/ethnicities.
Receiver operating characteristics curve analysis based on our study data showed that the greatest accuracy to identify obesity using 35% body fat corresponded to BMI values of 25.5, 28.7 and 26.2 kg/m2 in white, black, and Hispanic women, respectively. The higher BMI cutoff value for black than white and Hispanic women support the finding based on our regression analysis that for a given BMI, black women have a lower percent body fat than the other two groups of women. The respective area under the curve was 0.967 (95% CI 0.946–0.988), 0.946 (95% CI 0.916–0.977), and 0.927 (95% CI 0.894–0.960). The sensitivity and overall performance of the race/ethnic specific BMI values generated by the current study were improved over those recommended by the NIH, particularly in white and Hispanic women (Table 2). Of the 555 women, race/ethnic specific BMI cutoff values identified 292 obese women out of 350 actually obese women (90 more than that identified with the NIH-based cutoff value). The sensitivity increased to 85.6% from 47.8% (P<.001) in white, 81.3% from 75.0% in black (P=.031), and 83.2% from 57.7% (P<.001) in Hispanic women. The overall sensitivity was significantly improved from 57.7% to 83.4% (P<.001). A greater improvement in sensitivity was observed in white (37.8%) and Hispanic women (29.3%) than among black women (5.3%).
Table 3 shows the obesity rates and 95% CIs based on percent body fat, the NIH definition, and our data using race or ethnic-specific BMI cutoff values. Of the 555 women we examined, 205 (36.9%) were classified as obese based on NIH guidelines (BMI 30 kg/m2 or greater). The obesity rate in black (46.5%) and Hispanic women (37.7%) was significantly higher than that observed in white women (28.0%). However, WHO criterion (percent body fat greater than 35%) classified 350 women as obese (63.1% of total). When percent body fat was used, the obesity rate was highest in Hispanic women (69.1%) and the rates were similar in white (58.7%) and black women (60.4%). When race/ethnic specific BMI cutoff values were used, 311 women were labeled as obese (56.0%) with 52.9% of whites, 52.8% of blacks and 61.4% of Hispanics classified as obese.
Our study shows that the currently used BMI cutoff value for obesity recommended by the NIH (BMI 30 kg/m2 or greater) may be too high and does not reflect actual body fatness by race or ethnicity among reproductive-aged women. Use of this definition resulted in the misclassification of many obese women when compared with use of WHO criteria despite having very good specificity. Similar to our findings, Romero-Corral et al9 also observed that the NIH-based BMI cutoff value to define obesity had low sensitivity (49%) in U.S. women aged 20–80 years. Evans et al10 found similar results in white (47.1%) and black (52.6%) postmenopausal women. Several smaller studies have shown similar results.29,30 Blew et al11 observed even lower sensitivity (25.6%) when this definition was used in mostly white postmenopausal women. Together, these studies provide evidence that the NIH-based BMI cutoff value is not accurate enough to identify obesity among a large number of adult women residing in the United States.
Our data-driven race or ethnic-specific BMI cutoff values to define obesity agree with those of several other U.S. studies that included diverse populations.9–11 Evans et al10 identified obesity as those with BMI values 26.9 kg/m2 or greater among white women and 28.4 kg/m2 or greater among black women while our study showed BMI values 25.5 or greater, 28.7, and 26.2 kg/m2 for white, black, and Hispanic women, respectively. However, Blew et al11 observed even lower BMI cutoff values (24.9 kg/m2) in mostly white postmenopausal women. Romero-Corral et al9 found that the cut off value should be 25.5 kg/m2 among multiethnic women. Moreover, sensitivities of the revised BMI cutoff values generated in our study are also similar to previously published studies.9–11 This suggests that the BMI cutoff value should not only be lower than the value currently used, but it also should differ by race or ethnicity.
The difference between actual and observed obesity rates in whites (59% compared with 28%) and Hispanic (69% compared with 38%) women could be a threat to the success of obesity awareness and programs in the United States. The NIH-based obesity rate calculations, which show that black women have the highest obesity rate, were not supported by percent body fat data in this study. In contrast, Hispanic women had the highest obesity rates based on percent body fat classified obesity. Thus, there is a need to organize the obesity prevention programs targeting all three race or ethnic groups equally with a special emphasis on Hispanic women. It is a serious public health concern that more than two-thirds of Hispanic reproductive-aged women are obese.
Moreover, obesity rates based on NIH guidelines in white and Hispanic women are severely underestimated, which needs to be corrected. The current BMI cutoff value results in about half of women with actual obesity (greater than 35% body fat) being labeled as normal or overweight. Thus, the opportunity to reduce body weight by appropriate intervention in this group of people is missed. It is possible that the improvement in sensitivity in white and Hispanic women using race/ethnic specific BMI cutoff values will result in labeling a few women as obese who are not, causing them additional stress. However, considering that fewer women will be misclassified by the revised cutoff values and the myriad public health implications of obesity, any potential harm would be outweighed by the benefit of identifying an increased number of actually obese women.
Our finding that the NIH classified obesity rate was 36.9% among reproductive-aged women is consistent with population based reports of its prevalence in 20–39-year-old women (29.1%). According to the National Health and Nutrition Examination Survey 1999–2002 data,23 24.9% of non-Hispanic whites, 46.6% of non-Hispanic blacks, and 31.2% of Hispanic women between the age of 20 and 39 years were obese (BMI 30 kg/m2 or greater) compared with 28.0%, 46.5% and 37.7%, respectively, in the current study. The similarity of obesity rates between the current study and the National Health and Nutrition Examination Survey–based study increases the external validity of our study results.
Published studies show that the influence of race/ethnicity on the relationship between BMI and percent body fat may not be consistent.10,22,31,32 For example, Fernandez et al30 did not observe any difference in percent body fat between white and black postmenopausal women for a given BMI while Evans et al10 observed that white women had 1% higher percent body fat than black postmenopausal women. In contrast, our study showed a difference of almost 3%. Aloia et al32 also found that black women at the same percent body fat had significantly higher BMI than white perimenopausal women. Differences in age distribution could be the reason for these discrepancies. However, further studies on age-related changes in percent body fat based on 10-year increments by race/ethnicity are needed to shed more light on this issue.
This study has several limitations. First, we examined diagnostic performance of BMI in only 20–33-year-old women, so we don't know whether similar findings would be observed in other age groups. However, similar findings in studies of postmenopausal white and black women10 suggest that similar race/ethnic specific cutoff values might work for other age groups of women residing in the United States. Second, our study is not based on a random sample, and thus, our sample may not be representative of all white, black, and Hispanic women. However, similar obesity rates in the current study and National Health and Nutrition Examination Survey–based study23 increase the external validity of the study. Third, the Hispanic women in our study were predominantly of Mexican descent, so extension of these data to Hispanic women of other origins should be done with caution. Finally, use of a single site could limit the generalizability of our findings.
In conclusion, our findings show that the currently accepted BMI cutoff value to identify obesity is too high for many reproductive-aged women residing in the United States. This suggests that women whose BMI is between 25 and 29.9 kg/m2 (in addition to 30 kg/m2 or greater), may require additional counseling on how to reduce their body weight to avoid obesity-related morbidity. Furthermore our data suggest that race-specific BMI classifications need to be established to more accurately identify reproductive-aged women who are obese so they can be counseled appropriately. Substantial increases in sensitivity in white (38% increase) and Hispanic women (30% increase) make the BMI cutoff values generated in this study reasonable to consider for reproductive-aged women. As a validation measure, however, these proposed criteria and their relationship to cardiovascular risk factors need to be further examined using an independent nationally representative sample.
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