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Abstract Background The Forbes equation relating fat-free mass (FFM) to fat mass (FM) has been used to predict longitudinal changes in FFM during weight change but has important limitations when paired with a one dimensional energy balance differential equation. Direct use of the Forbes model within a one dimensional energy balance differential equation requires calibration of a translate parameter for the specific population under study. Comparison of translates to a representative sample of the US population indicate that this parameter is a reflection of age, height, race and gender effects. Results We developed a class of fourth order polynomial equations relating FFM to FM that consider age, height, race and gender as covariates eliminating the need to calibrate a parameter to baseline subject data while providing meaningful individual estimates of FFM. Moreover, the intercepts of these polynomial equations are nonnegative and are consistent with observations of very low FM measured during a severe Somali famine. The models preserve the predictive power of the Forbes model for changes in body composition when compared to results from several longitudinal weight change studies. Conclusions The newly developed FFM-FM models provide new opportunities to compare individuals undergoing weight change to subjects in energy balance, analyze body composition for individual parameters, and predict body composition during weight change when pairing with energy balance differential equations.

Abstract Background To compare the relationship of skeletal muscle mass with bone mineral content in an ethnically diverse group of 6 to 18 year old boys and girls. Methods 175 healthy children (103 boys; 72 girls) had assessments of body mass, height, and Tanner stage. Whole body bone mineral content, non-bone lean body mass (nbLBM), skeletal muscle mass, and fat mass were assessed using dual-energy X-ray absorptiometry (DXA). Muscle mass was estimated from an equation using appendicular lean soft tissue measured by DXA, weight and height. Estimates of skeletal muscle mass and adipose tissue were also assessed by whole body multi-slice magnetic resonance imaging (MRI). Linear regression was used to determine whether skeletal muscle mass assessed by DXA or by MRI were better predictors of bone mineral content compared with nbLBM after adjusting for sex, age, race or ethnicity, and Tanner stage. Results Greater skeletal muscle mass was associated with greater bone mineral content (p < 0.001). The skeletal muscle mass assessed by MRI provided a better fitting regression model (determined by R2 statistic) compared with assessment by DXA for predicting bone mineral content. The proportion of skeletal muscle mass in nbLBM was significantly associated with greater bone mineral content adjusted for total nbLBM. Conclusions This study is among the first to describe and compare the relationship of skeletal muscle to bone using both MRI and DXA estimates. The results demonstrate that the use of MRI provides a modestly better fitting model for the relationship of skeletal muscle to bone compared with DXA. Skeletal muscle had an impact on bone mineral content independent of total non-bone lean body mass. In addition, Hispanics had greater bone mineral content compared to other race and ethnic groups after adjusting for sex, age, adipose tissue, skeletal muscle mass, and height.

Abstract Background Dual-energy x-ray absorptiometry (DXA) provides an affordable and practical assessment of multiple whole body and regional body composition. However, little information is available on the assessment of changes in body composition in top-level athletes using DXA. The present study aimed to assess the accuracy of DXA in tracking body composition changes (relative fat mass [%FM], absolute fat mass [FM], and fat-free mass [FFM]) of elite male judo athletes from a period of weight stability to prior to a competition, compared to a four compartment model (4C model), as the criterion method. Methods A total of 27 elite male judo athletes (age, 22.2 ± 2.8 yrs) athletes were evaluated. Measures of body volume by air displacement plethysmography, bone mineral content assessed by DXA, and total-body water assessed by deuterium dilution were used in a 4C model. Statistical analyses included examination of the coefficient of determinant (r2), standard error of estimation (SEE), slope, intercept, and agreement between models. Results At a group level analysis, changes in %FM, FM, and FFM estimates by DXA were not significantly different from those by the 4C model. Though the regression between DXA and the 4C model did not differ from the line of identity DXA %FM, FM, and FFM changes only explained 29%, 36%, and 38% of the 4C reference values, respectively. Individual results showed that the 95% limits of agreement were -3.7 to 5.3 for %FM, -2.6 to 3.7 for FM, and -3.7 to 2.7 for FFM. The relation between the difference and the mean of the methods indicated a significant trend for %FM and FM changes with DXA overestimating at the lower ends and underestimating at the upper ends of FM changes. Conclusions Our data indicate that both at group and individual levels DXA did not present an expected accuracy in tracking changes in adiposity in elite male judo athletes.

Abstract Background Despite increasing research and clinical significance, limited information is available on how the visceral and subcutaneous adipose tissue (VAT and SAT) compartments develop during growth and maturation and then vary in volume across the adult lifespan. The present study aimed at exploring how adipose tissue compartments partition across the lifespan. Methods Total body VAT and SAT were quantified in an ethnically-diverse cross-sectional sample of healthy subjects ages 5 – 88 yrs [children (5–17 years): males n = 88, BMI percentile (X ± SD), 61.9 ± 27.1; females, n = 59, BMI percentile, 60.0 ± 28.4; adults (≥ 18 yrs): males, n = 164, BMI, 25.6 ± 3.7 kg/m2, and females, n = 188, BMI, 25.5 ± 5.4 kg/m2]. Subjects completed a whole-body magnetic resonance imaging scan and images were then segmented for VAT and SAT; total compartment volumes were calculated from respective slice areas. Sex and age distributions were evaluated by generating quadratic and cubic smoothing lines fitted to the data. Plots were developed with and without adjustment for total adipose tissue, ethnicity, and menopausal status in women. VAT and SAT volumes were both larger with greater age. Results In adulthood, VAT was larger in males than in females with and without adjustment. In contrast, SAT volume was larger in females than in males after entering puberty and sex differences remained, with and without adjustment, across the remaining lifespan. Conclusion Based on observations made in this cross-sectional sample, VAT and SAT volumes were variably larger with greater age across most of the human lifespan, although the relatively small number of children warrants future larger scale studies to validate our observations. Moreover, the pattern and magnitude of adipose tissue "growth" differed between males and females, with the mechanistic basis of this sexual dimorphism only partially understood. These descriptive observations in a large cross-sectional cohort provide an initial foundation for future longitudinal and cohort studies.

Abstract Correction to Levitt DG, Heymsfield SB, Pierson Jr RN, Shapses SA, Kral JG: Physiological models of body composition and human obesity. Nutrition & Metabolism 2007, 4:19

Abstract Background Body volume expands with the positive energy balance associated with the development of adult human obesity and this "growth" is captured by two widely used clinical metrics, waist circumference and body mass index (BMI). Empirical correlations between circumferences, BMI, and related body compartments are frequently reported but fail to provide an important common conceptual foundation that can be related to key clinical observations. A two-phase program was designed to fill this important gap: a geometric model linking body volume with circumferences and BMI was developed and validated in cross-sectional cohorts; and the model was applied to the evaluation of longitudinally monitored subjects during periods of voluntary weight loss. Concepts emerging from the developed model were then used to examine the relations between the evaluated clinical measures and body composition. Methods Two groups of healthy adults (n = 494 and 1499) were included in the cross-sectional model development/testing phase and subjects in two previous weight loss studies were included in the longitudinal model evaluation phase. Five circumferences (arm, waist, hip, thigh, and calf; average of sum, C), height (H), BMI, body volume (V; underwater weighing), and the volumes of major body compartments (whole-body magnetic resonance imaging) were measured. Results The evaluation of a humanoid geometric model based a cylinder confirmed that V derived from C and H was highly correlated with measured V [R2 both males and females, 0.97; p < 0.001). Developed allometric models confirmed model predictions that C and BMI (represented as V/H) are directly linked as, C = (V/H)0.5. The scaling of individual circumferences to V/H varied, with waist the highest (V/H~0.6) and calf the lowest (V/H~0.3), indicating that the largest and smallest between-subject "growth" with greater body volume occurs in the abdominal area and lower extremities, respectively. A stepwise linear regression model including all five circumferences2 showed that each contributed independently to V/H. These cross-sectional observations were generally confirmed by analysis of the two longitudinal weight loss studies. The scaling of circumference ratios (e.g., waist/hip) to V/H conformed to models developed on the scaling of individual circumferences to V/H, indicating their relations to BMI are predictable a priori. Waist, hip, and arm/calf circumferences had the highest associations with whole-body visceral adipose tissue, subcutaneous adipose tissue, and skeletal muscle volumes, respectively. Conclusion These observations provide a simple geometric model relating circumferences with body size and composition, introduce a conceptual foundation explaining previous empirical observations, and reveal new clinical insights.

Abstract Background The body mass index (BMI) is the standard parameter for predicting body fat fraction and for classifying degrees of obesity. Currently available regression equations between BMI and fat are based on 2 or 3 parameter empirical fits and have not been validated for highly obese subjects. We attempt to develop regression relations that are based on realistic models of body composition changes in obesity. These models, if valid, can then be extrapolated to the high fat fraction of the morbidly obese. Methods The analysis was applied to 3 compartment (density and total body water) measurements of body fat. The data was collected at the New York Obesity Research Center, Body Composition Unit, as part of ongoing studies. A total of 1356 subjects were included, with a BMI range of 17 to 50 for males and 17 to 65 for females. The body composition model assumes that obese subjects can be represented by the sum of a standard lean reference subject plus an extra weight that has a constant adipose, bone and muscle fraction. Results There is marked age and sex dependence in the relationship between BMI and fat fraction. There was no significant difference among Caucasians, Blacks and Hispanics while Asians had significantly greater fat fraction for the same BMI. A linear relationship between BMI and fat fraction provides a good description for men but overestimates the fat fraction in morbidly obese women for whom a non-linear regression should be used. New regression relations for predicting body fat just from experimental measurements of body density are described that are more accurate then those currently used. From the fits to the experimental BMI and density data, a quantitative description of the bone, adipose and muscle body composition of lean and obese subjects is derived. Conclusion Physiologically realistic models of body composition provide both accurate regression relations and new insights about changes in body composition in obesity.

A correction to the article "Physiological models of body composition and human obesity" that was published in the 2007 issue is presented.

Background: Despite increasing research and clinical significance, limited information is available on how the visceral and subcutaneous adipose tissue (VAT and SAT) compartments develop during growth and maturation and then vary in volume across the adult lifespan. The present study aimed at exploring how adipose tissue compartments partition across the lifespan. Methods: Total body VAT and SAT were quantified in an ethnically-diverse cross-sectional sample of healthy subjects ages 5 - 88 yrs [children (5-17 years): males n = 88, BMI percentile (X ± SD), 61.9 ± 27.1; females, n = 59, BMI percentile, 60.0 ± 28.4; adults (= 18 yrs): males, n = 164, BMI, 25.6 ± 3.7 kg/m², and females, n = 188, BMI, 25.5 ± 5.4 kg/m²]. Subjects completed a whole-body magnetic resonance imaging scan and images were then segmented for VAT and SAT; total compartment volumes were calculated from respective slice areas. Sex and age distributions were evaluated by generating quadratic and cubic smoothing lines fitted to the data. Plots were developed with and without adjustment for total adipose tissue, ethnicity, and menopausal status in women. VAT and SAT volumes were both larger with greater age. Results: In adulthood, VAT was larger in males than in females with and without adjustment. In contrast, SAT volume was larger in females than in males after entering puberty and sex differences remained, with and without adjustment, across the remaining lifespan. Conclusion: Based on observations made in this cross-sectional sample, VAT and SAT volumes were variably larger with greater age across most of the human lifespan, although the relatively small number of children warrants future larger scale studies to validate our observations. Moreover, the pattern and magnitude of adipose tissue "growth" differed between males and females, with the mechanistic basis of this sexual dimorphism only partially understood. These descriptive observations in a large cross-sectional cohort provide an initial foundation for future longitudinal and cohort studies. [ABSTRACT FROM AUTHOR]