In this large, representative US population, BMI and waist circumference were more strongly correlated with total fat mass than with percentage of fat mass in the whole body and trunk, respectively. When we used adiposity-related biologic factors as objective references to evaluate the relative validity of BMI or waist circumference in comparison with DXA indices as measures of adiposity, these anthropometric indices were correlated similarly with the biologic markers compared with DXA measurements. Moreover, BMI and waist circumference discriminated between participants with and without the metabolic syndrome equally well in comparison with DXA measurements of fat mass in the whole body and trunk, respectively. These relations were largely consistent across different age, gender, and ethnic groups.
Our results are consistent with the previous observation by Spiegelman et al. (37
) that BMI was more strongly correlated with fat mass than with fat mass percent as measured by densitometry in adults. Similarly, studies conducted in children and adolescents also demonstrated that although BMI was highly correlated with both fat mass and fat mass percent as measured by DXA, a stronger correlation with fat mass was observed (19
). In addition, our observations were also consistent with previous studies that used other reference methods to estimate fat mass and fat mass percent (37
). Interestingly, we observed that waist circumference was also a better measure of fat mass than of fat mass percent in the trunk. Because DXA cannot distinguish visceral fat from subcutaneous fat, we could not examine whether waist circumference was more strongly correlated with fat in a certain compartment of the trunk in the current analysis. Nonetheless, in studies that used CT or MRI to measure visceral and subcutaneous adipose tissue at L4–L5 levels, waist circumference was more strongly correlated with total adipose tissue or subcutaneous fat than with visceral fat (40
). Meanwhile, Kamel et al. (44
) demonstrated that DXA measures of central adiposity were not superior to waist circumference with respect to measuring visceral fat accumulation assessed by MRI in men.
Because measurement error in the biologic factors and adiposity measurements are largely independent, obesity-related biomarkers can serve as objective references in evaluating relative performance of adiposity measurements (45
). Our observation that BMI was correlated with obesity-related factors at least as strongly as DXA estimates of fat mass and fat mass percent was consistent with most of the previous studies that employed a similar study design (15
). Likewise, our observations of the relative validity of waist circumference and trunk fat mass or trunk fat mass percent were also consistent with previous studies (15
). Remarkably, in studies that compared BMI or waist circumference with overall or central adiposity assessed through other reference methods, such as CT, MRI, BIA, ultrasonography, or skinfold thickness, in general BMI or waist circumference correlated with obesity-related factors as well as these reference methods (17
). Our study population consisted of US men and women of various ages and ethnicities, and we obtained similar results in subgroups defined by these characteristics. Previous studies conducted in other populations such as Pima Indian children and adolescents, Caucasian children, and Asians (16
) yielded similar results, suggesting that the validity of BMI or waist circumference with respect to correlations with obesity-related biomarkers is similar across different populations.
Each adiposity assessment method has its own strengths and limitations. CT and MRI can provide the most accurate estimates of body composition. They are especially valuable in that abdominal visceral and subcutaneous adipose tissue can be distinguished by means of these methods (31
). However, high cost, lack of mobility, long measurement duration, and the need for sophisticated technical staff limit the use of these methods in large epidemiologic studies. In contrast, DXA, which directly measures body fatness, is less expensive, relatively mobile, time-efficient, and easy to operate and exposes subjects to much lower levels of radiation than CT scans (31
). However, this method cannot distinguish among fat in different body compartments. In addition, studies have documented some systematic measurement errors in DXA measurements (56
), emphasizing the necessity of calibrating DXA instruments to obtain the most accurate estimates of adiposity for individuals. Of all adiposity assessment methods, BMI and waist circumference are the cheapest, simplest, and most popular indices of body fatness in epidemiologic studies, although these indices have limited ability to distinguishing fat mass from lean mass. Flegal et al. (10
) recently reported that anthropometric indices substantially misclassified body fatness at the individual level, by assuming that percent body fat by DXA is the true measure of fatness. However, when these indices were used to rank subjects according to their body fatness, anthropometric measures were highly accurate; more than 90% of NHANES participants could be correctly classified within 1 category defined by DXA fat mass percent (10
). We further demonstrated that the degree of misclassification was even smaller when using DXA fat mass as the reference measure. These results indicated that simple anthropometric indices could accurately distinguish relatively lean subjects from those with higher body fatness. In addition, because the findings of the current analysis strongly suggest that DXA is not superior to BMI as a measure of body fat, the misclassification suggested by Flegal et al. would have been overstated for anthropometric indices. Therefore, for large epidemiologic studies consisting of thousands of participants, the accuracy of anthropometric measurements is sufficient for ranking participants’ body fatness and evaluating associations between adiposity and disease risk. On the other hand, for studies that require more precise estimates of fat mass in certain compartments of the human body or studies that primarily consist of participants for whom anthropometric measurements are known to perform poorly in assessing adiposity, such as older adults and muscle builders (58
), more accurate methods such as DXA, CT, or MRI should be used.
Our analysis had some limitations. In the current analysis, we could not compare waist circumference with DXA measures of fat in different regions of the trunk because only trunk fat mass data were available. We were unable to compare other anthropometric indices, such as waist-to-hip ratio, with corresponding DXA measures because of lack of hip circumference data. In addition, depending on the validity of imputation models and the assumption regarding missing data, including imputed data may introduce systemic bias into the analysis. However, when we restricted analysis to participants with measured DXA data only, we observed essentially similar results, which argues against this possibility. Strengths of the current study included national representativeness, a large sample size, standardized protocols for DXA and anthropometric measurements, and rich biomarker data allowing comprehensive analysis.
In summary, these data indicate that the validity of simple anthropometric measures such as BMI and waist circumference is comparable to DXA measurements of fat mass and fat mass percent as assessed by their correlations with obesity-related risk factors for cardiovascular disease. Low cost, simplicity, wide availability, and good validity make these anthropometric measures particularly valuable for epidemiologic studies that aim to investigate the role of excess adiposity in the development of chronic diseases in large populations.