John C. Thornton

Relation of BMI to fat and fat-free mass among children and adolescents

OBJECTIVE:Although the body mass index (BMI, kg/m2) is widely used as a surrogate measure of adiposity, it is a measure of excess weight, rather than excess body fat, relative to height. We examined the relation of BMI to levels of fat mass and fat-free mass among healthy 5- to 18-y-olds.METHODS AND PROCEDURES:Dual-energy X-ray absorptiometry was used to measure fat and fat-free mass among 1196 subjects. These measures were standardized for height by calculating the fat mass index (FMI, fat mass/ht2) and the fat-free mass index (FFMI, fat-free mass/ht2).RESULTS:The variability in FFMI was about 50% of that in FMI, and the accuracy of BMI as a measure of adiposity varied greatly according to the degree of fatness. Among children with a BMI-for-age ≥85th P, BMI levels were strongly associated with FMI (r=0.85–0.96 across sex–age categories). In contrast, among children with a BMI-for-age <50th P, levels of BMI were more strongly associated with FFMI (r=0.56–0.83) than with FMI (r=0.22–0.65). The relation of BMI to fat mass was markedly nonlinear, and substantial differences in fat mass were seen only at BMI levels ≥85th P.DISCUSSION:BMI levels among children should be interpreted with caution. Although a high BMI-for-age is a good indicator of excess fat mass, BMI differences among thinner children can be largely due to fat-free mass.

C-Terminal octapeptide of cholecystokinin decreases food intake in obese men

Six of eight obese men ate significantly less food during an intravenous infusion of the C-terminal octapeptide of cholecystokinin (CCK-8, 4 ng . kg-1 . min-1) than during a saline infusion in a double blind experimental paradigm. Subjects stopped eating sooner during CCK-8. CCK-8 did not change the rate of eating. No overt side effects were reported or observed. This is the first report of the satiety effect of CCK-8 in obese humans and it suggests that the therapeutic potential of CCK-8 for the treatment of obesity deserves investigation.

Anthropometry in Body Composition: An Overview

Abstract: Anthropometry is a simple reliable method for quantifying body size and proportions by measuring body length, width, circumference (C), and skinfold thickness (SF). More than 19 sites for SF, 17 for C, 11 for width, and 9 for length have been included in equations to predict body fat percent with a standard error of estimate (SEE) range of ±3% to ±11% of the mean of the criterion measurement. Recent studies indicate that not only total body fat, but also regional fat and skeletal muscle, can be predicted from anthropometrics. Our Rosetta database supports the thesis that sex, age, ethnicity, and site influence anthropometric predictions; the prediction reliabilities are consistently higher for Whites than for other ethnic groups, and also by axial than by peripheral sites (biceps and calf). The reliability of anthropometrics depends on standardizing the caliper and site of measurement, and upon the measuring skill of the anthropometrist. A reproducibility of ± 2% for C and ± 10% for SF measurements usually is required to certify the anthropometrist.

Body fat and water changes during pregnancy in women with different body weight and weight gain

Objective To determine the fat deposited during pregnancy in women gaining according to recommendations of the Institute of Medicine and the relationship of weight gain to fat gain in women of different starting weights (classified by their body mass index). Methods A cohort study of healthy, nonsmoking women, 18–36 years of age, identified during prenatal visits at three hospital clinics and one birthing center in New York City. From a pool of 432 eligible volunteers who signed a consent form, body composition measurements were performed on 200 women at weeks 14 (−2) and 37+ of pregnancy, and bone mineral mass was measured at 2–4 weeks postpartum. Body fat was estimated with a model that used total body water, weight, and density and bone mineral mass. Results In women gaining as recommended by the Institute of Medicine, fat gains during pregnancy for women underweight, normal weight, overweight, or obese before pregnancy were 6.0 ± 2.6 kg, 3.8 ± 3.4 kg, 3.5 ± 4.1 kg, and −0.6 ± 4.6 kg, respectively. Higher weight gain increased fat gain. Body water gain was not different among the four prepregnancy weight groups. Conclusion Recommended weight gain should not cause obesity in any weight group. Underweight women will normalize their body composition if they gain as recommended, whereas obese women will have little or no change in body fat. A majority of women do not gain as recommended during pregnancy.

Classification of body fatness by body mass index-for-age categories among children.

OBJECTIVE To examine the ability of various body mass index (BMI)-for-age categories, including the Centers for Disease Control and Preventions 85th to 94th percentiles, to correctly classify the body fatness of children and adolescents. DESIGN Cross-sectional. SETTING The New York Obesity Research Center at St Lukes-Roosevelt Hospital from 1995 to 2000. PARTICIPANTS Healthy 5- to 18-year-old children and adolescents (N = 1196) were recruited in the New York City area through newspaper notices, announcements at schools and activity centers, and word of mouth. MAIN OUTCOME MEASURES Percent body fat as determined by dual-energy x-ray absorptiometry. Body fatness cutoffs were chosen so that the number of children in each category (normal, moderate, and elevated fatness) would equal the number of children in the corresponding BMI-for-age category (<85th percentile, 85th-94th percentile, and > or =95th percentile, respectively). RESULTS About 77% of the children who had a BMI for age at or above the 95th percentile had an elevated body fatness, but levels of body fatness among children who had a BMI for age between the 85th and 94th percentiles (n = 200) were more variable; about one-half of these children had a moderate level of body fatness, but 30% had a normal body fatness and 20% had an elevated body fatness. The prevalence of normal levels of body fatness among these 200 children was highest among black children (50%) and among those within the 85th to 89th percentiles of BMI for age (40%). CONCLUSION Body mass index is an appropriate screening test to identify children who should have further evaluation and follow-up, but it is not diagnostic of level of adiposity.

The prediction of body fatness by BMI and skinfold thicknesses among children and adolescents

Background: Although the body mass index (BMI, kg m−2) is widely used as a measure of adiposity, it is a measure of excess weight, rather than excess body fat. It has been suggested that skinfold thicknesses be measured among overweight children to confirm the presence of excess adiposity. Objective: The present study examined the additional information provided by skinfold thicknesses on body fatness, beyond that conveyed by BMI-for-age, among healthy 5- to 18-years old (n = 1196). Methods and procedures: Total body dual-energy X-ray absorptiometry (DXA) provided estimates of % body fat, and the sum of two skinfolds (triceps and subscapular) was used as an indicator of the overall skinfold thickness. Results: As assessed by the multiple R2s and the residuals of various regression models, information on the skinfold sum significantly ( p < 0.001) improved the prediction of body fatness beyond that obtained with BMI-for-age. For example, the use of the skinfold sum, in addition to BMI-for-age, increased the multiple R2s for predicting % body fat from 0.81 to 0.90 (boys), and from 0.82 to 0.89 (girls). The use of the skinfold sum also reduced the overall prediction errors (absolute value of the residuals) for % body fat by 20–30%, but these reductions varied substantially by BMI-for-age. Among overweight children, defined by a BMI-for-age ≥95th percentile, the skinfold sum reduced the predication errors for % body fat by only 7–9%. Conclusions: Although skinfold thicknesses, when used in addition to BMI-for-age, can substantially improve the estimation of body fatness, the improvement among overweight children is small.

Racial/ethnic Differences in Body Fatness Among Children and Adolescents

Background: Although the BMI is widely used as a measure of adiposity, it is a measure of excess weight, and its association with body fatness may differ across racial or ethnic groups.

Do Skinfold Measurements Provide Additional Information to Body Mass Index in the Assessment of Body Fatness Among Children and Adolescents

OBJECTIVES. The purpose of this work was to validate the performance of age- and gender-specific BMI, triceps, and subscapular skinfold for the classification of excess of body fat in children and adolescents and to examine how much additional information these 2 skinfold measurements provide to BMI-for-age. METHODS. The receiver operating characteristic curve was used to characterize the sensitivity and specificity of these 3 indices in classifying excess body fat. Percentage of body fat was determined by dual-energy radiograph absorptiometry. Both ≥85th and ≥95th percentile of percentage of body fat were used to define excess body fat. Data from the New York Pediatric Rosetta Body Composition Project were examined (n = 1196; aged 5–18 years). RESULTS. For children aged 5 to 18 years, BMI-for-age, triceps skinfold-for-age, and subscapular skinfold-for-age each performed equally well alone in the receiver operating characteristic curves in the identification of excess body fat defined by either the 85th or 95th percentile of percentage of body fat by dual-energy radiograph absorptiometry. However, if BMI-for-age was already known and was >95th percentile, the additional measurement of skinfolds did not significantly increase the sensitivity or specificity in the identification of excess body fat. CONCLUSIONS. In contrast to the recommendations of expert panels, skinfold measurements do not seem to provide additional information about excess body fat beyond BMI-for-age alone if the BMI-for-age is >95th percentile.

Reproducibility of Pediatric Whole Body Bone and Body Composition Measures by Dual-Energy X-Ray Absorptiometry Using the GE Lunar Prodigy

The use of dual-energy X-ray absorptiometry (DXA) in pediatrics is increasing. It is safe, readily available, and easily performed, but there is little information on reproducibility. The aim of this study is to evaluate the reproducibility of whole body DXA scans in children. Total and regional bone mineral density, bone mineral content, nonbone, lean fat mass, and percent fat were measured twice by whole body DXA (GE Lunar Prodigy) in 49 subjects (5 to 17 yr). Within each subject, between subjects, and reading standard deviations for each body component were evaluated as well as intraclass correlations (IC) and coefficients of variation (CV). Total body measurements had better IC and CV than regional results from the whole body scan, with legs and arms better than trunk and spine. IC values were >or=0.989 for total body, >or=0.976 for legs and arms, and >or=0.875 for trunk and spine. CV values ranged 0.18 to 1.97% for total body, and 0.96 to 6.91% for regional measures. These values confirm that body composition and bone mass by DXA are highly reproducible among pediatric subjects. The results of this study can be used by clinicians and researchers for interpretation of longitudinal observations and for power calculations.

The body adiposity index (hip circumference ÷ height1.5) is not a more accurate measure of adiposity than is BMI, waist circumference, or hip circumference

Based on cross‐sectional analyses, it was suggested that hip circumference divided by height1.5 −18 (the body adiposity index (BAI)), could directly estimate percent body fat without the need for further correction for sex or age. We compared the prediction of percent body fat, as assessed by dual‐energy X‐ray absorptiometry (PBFDXA), by BAI, BMI, and circumference (waist and hip) measurements among 1,151 adults who had a total body scan by DXA and circumference measurements from 1993 through 2005. After accounting for sex, we found that PBFDXA was related similarly to BAI, BMI, waist circumference, and hip circumference. In general, BAI underestimated PBFDXA among men (2.5%) and overestimated PBFDXA among women (4%), but the magnitudes of these biases varied with the level of body fatness. The addition of covariates and quadratic terms for the body size measures in regression models substantially improved the prediction of PBFDXA, but none of the models based on BAI could more accurately predict PBFDXA than could those based on BMI or circumferences. We conclude that the use of BAI as an indicator of adiposity is likely to produce biased estimates of percent body fat, with the errors varying by sex and level of body fatness. Although regression models that account for the nonlinear association, as well as the influence of sex, age, and race, can yield more accurate estimates of PBFDXA, estimates based on BAI are not more accurate than those based on BMI, waist circumference, or hip circumference.