Our study shows that the currently used BMI cutoff value for obesity recommended by the NIH (BMI≥30 kg/m2
) may be too high and does not reflect actual body fatness by race/ethnicity among reproductive-aged women. Use of this definition resulted in the misclassification of many obese women when compared to use of WHO reference despite having very good specificity. Similar to our findings, Romero-Corral et al (9
) 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 al (10
) found similar results in white (47.1%) and black (52.6%) postmenopausal women. Several smaller studies have shown similar results (29
). Blew et al (11
) 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 U.S.
Our data driven race/ethnic specific BMI cutoff values to define obesity agree with those of several other U.S. studies which included diverse populations (9
). Evans et al (10
) identified obesity as those with BMI values ≥ 26.9 kg/m2
among white women and ≥28.4 kg/m2
among black women while our study showed BMI values ≥ 25.5, 28.7, and 26.2 kg/m2
for white, black and Hispanic women, respectively. However, Blew et al (11
) observed even lower BMI cutoff values (24.9 kg/m2
) in mostly white postmenopausal women. Romero-Corral et al (9
) 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
). This suggests that the BMI cutoff value should not only be lower than the value currently used, but also should differ by race/ethnicity.
The difference between actual and observed obesity rates in whites (59% vs. 28%) and Hispanic (69% vs. 38%) women could be a threat to the success of obesity awareness and programs in the U.S. NIH –based obesity rate calculations which show that black women have the highest obesity rate was not supported by %BF data in this study. In contrast, Hispanic women had the highest obesity rates based on %BF classified obesity. Thus, there is a need to organize the obesity prevention programs targeting all three race/ethnic groups equally with a special emphasis on Hispanic women. More than two-thirds of Hispanic reproductive-aged women are obese is a serious public health concern.
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 (>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 years old women (29.1%). According to the National Health and Nutrition Examination Survey (NHANES) 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
) compared to 28.0%, 46.5% and 37.7%, respectively in the current study. The similarity of obesity rates between the current study and the NHANES-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 %BF may not be consistent (10
). For example, Fernandez et al (30
) did not observe any difference in %BF between white and black postmenopausal women for a given BMI while Evans et al (10
) observed that white women had 1% higher %BF than black postmenopausal women. In contrast, our study showed that a difference of almost 3%. Aloia et al (32
) also found that at the same %BF black women 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 %BF 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 years 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 women (10
) suggest that similar race/ethnic specific cutoff values might work for other age groups of women residing in the U.S. 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 NHANES-based study (23
) 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 suggest that the currently accepted BMI cutoff value to identify obesity is too high for many reproductive-aged women residing in the US. This suggests that women whose BMI is between 25 and 29.9 kg/m2 (in addition to ≥30 kg/m2), may require additional counseling on how to reduce their body weight in order 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.