Evaluating the utility of the body adiposity index in adolescent boys and girls

Abstract

Objectives

This study examines the ability of the body adiposity index to estimate percent body fat in a mixed sample of adolescents. Additionally, the sensitivity of the body adiposity index to identify adolescents with increased health risk (i.e., high waist circumference) was determined and compared to other anthropometric indicators of health (skinfolds and body mass index).

Design

Cross-sectional.

Methods

Anthropometric data (height, weight, waist circumference, and skinfolds) were collected on 913 adolescents (50% boys; mean age: 15.1 yrs). Percent body fat estimates were determined using skinfold equations and the body adiposity index, while body mass index was calculated using weight and height (wt/ht²). Age- and gender-specific thresholds for waist circumference, body fat, and body mass index were utilized to create categorical values. Correlation coefficients, t-tests, and kappa statistics were used to describe the relationship between anthropometric variables.

Results

Correlation coefficients revealed an association between percent body fat estimates from skinfolds and body adiposity index (boys 0.77; girls 0.67); however, the body adiposity index systematically overestimated percent body fat at lower levels of adiposity, primarily in boys. Higher mean percent body fat estimates were reported using the body adiposity index compared to skinfolds in boys (25.1% vs. 17.8%), while the opposite relationship was found in girls (body adiposity index 28.5% vs. skinfolds 29.6%). Among boys and girls, correlations to waist circumference and also sensitivity values were higher for body mass index compared to either skinfolds or body adiposity index.

Conclusions

No clear advantage exists in using the body adiposity index to estimate percent body fat or to identify adolescents with increased health risk. The results support the continued use of the body mass index to identify adolescents with increased health risk.

Introduction

The body mass index (BMI) is used extensively in adult and youth populations as a screening tool to identify weight-related problems. Adults are considered overweight when BMI values ≥ 25 and obese when BMI values ≥ 30. In contrast, gender- and age-specific percentiles are used among children and adolescents to define overweight (≥85 percentile) and obesity (≥95 percentile).¹ Epidemiologic studies have documented several health consequences associated with a classification of overweight or obese, including increased risk for diabetes, hypertension, dyslipedemia, and stroke.

The BMI offers a practical approach to categorize individuals based on anthropometric measures (body mass and height), but it is unable to distinguish between fat mass and fat-free mass. Therefore, the use of this index assumes that individuals with a similar BMI will also have a similar body composition, which is an invalid premise provided that body composition at a given BMI depends on age, sex, and ethnicity.2, 3, 4 Because excess adiposity, rather than excess body mass for height, increases the risk for several obesity-related comorbidities, reporting regional and global estimates of body fat is important. Abdominal obesity, estimated via waist circumference (WC), has been used to define and diagnosis the metabolic syndrome in adolescents and, compared to BMI, is a better indicator of visceral fat⁵ and cardiovascular disease risk.⁶ Similar to BMI, age- and gender-specific WC thresholds exist to identify at-risk youth.⁷ Prediction equations have been developed to estimate percent body fat (%BF) using BMI, gender, and age⁸; however, other approaches (e.g., dual-energy X-ray absorptiometry (DXA), bioelectrical impedance, and skinfold thickness) are more widely utilized to estimate %BF in youth. Skinfolds are strongly related to chronic disease risk factors⁹ and have been used extensively to estimate %BF in school-based fitness programs, such as FITNESSGRAM^®, and also in national surveillance studies (e.g., National Health and Nutrition Examination Survey). Body composition standards are available for skinfolds using static¹⁰ and, more recently, dynamic¹¹ thresholds that account for normal growth and maturation during adolescence.

Collecting body dimensions (e.g., WC, height, and body mass) is a straightforward process that requires few tools, whereas techniques to assess body composition may be inaccessible or too costly for routine use in certain locations. As an alternative option to estimate %BF, Bergman and colleagues developed the body adiposity index (BAI = (hip (cm)/height (m)^1.5) − 18) and reported a high correlation to %BF values derived from DXA (r = 0.85).¹² The utility of the BAI to estimate %BF has also been reported in other adult samples. One study reported correlations > 0.70 between BAI and DXA across an ethnically-mixed sample of men and women,¹³ whereas an additional study reported a mean %BF difference of nearly 7% between mean %BF values from BAI (22%) and DXA (15%) among participants with measured adiposity levels ≤ 20%.¹⁴ Few studies have directly compared associations between %BF_BAI and traditional anthropometric indicators, such as WC and BMI, to health outcomes. Among adults, Schulze et al.,¹⁵ found that WC was a better predictor of diabetes compared to BMI and %BF_BAI. The utility of the BAI has been questioned among adolescents¹² but, to date, research in this area does not exist. Evaluating the BAI within a large, mixed sample of adolescents addresses a gap in the literature regarding the value of this equation. Others have examined the ability of the BAI to estimate %BF across varying ethnic populations,13, 14 but comparable studies among varying age groups are not available.

This study examines the ability of the BAI to estimate %BF in a mixed sample of adolescent boys and girls (mean age: 15 yrs). Given the interest in identifying adolescents with increased health risk, the sensitivity of the BAI to identify at-risk adolescents (based on WC) will also be examined and compared to other anthropometric indicators of health (skinfolds and BMI). The inclusion of DXA as a criterion measure offers clear advantages in interpreting %BF estimates from skinfolds and the BAI; however, its use extends beyond the capabilities of the current study. Skinfolds are widely used under field conditions to estimate %BF, and therefore were viewed as an acceptable standard.