ZiMian Wang

Organ-tissue mass measurement allows modeling of REE and metabolically active tissue mass.

Investigators have expressed interest in the associations between resting energy expenditure (REE) and body mass for over a century. Traditionally, descriptive models using regression analysis are applied, linking REE with metabolically active compartments such as body cell mass (BCM) and fat-free body mass (FFM). Recently developed whole body magnetic resonance imaging (MRI) and echocardiography methods now allow estimation of all major organs and tissue volumes in vivo. Because measured values are available for REE, BCM, and FFM content of individual organs and tissues, it should now be possible to develop energy expenditure-body composition estimation models based on MRI-measured organ-tissue volumes. Specifically, the present investigation tested the hypothesis that in vivo estimation of whole body REE, BCM, and FFM is possible using MRI- and echocardiography-derived organ volumes combined with previously reported organ-tissue metabolic rates and chemical composition. Thirteen subjects (5 females, 8 males) had REE, BCM, and FFM measured by indirect calorimetry, whole body40K counting, and dual-energy X-ray absorptiometry, respectively. Models developed from estimated and measured variables were highly correlated, with no significant differences between those estimated and measured [e.g., calculated vs. measured REE: r = 0.92, P < 0.001; (mean ± SD) 6,962 ± 1,455 and 7,045 ± 1,450 kJ/day, respectively ( P = not significant)]. Strong associations were observed between REE, individual or combined organ weights, BCM, and FFM that provide new insights into earlier observed metabolic phenomona. The present approach, the first to establish an energy expenditure-body composition link with a mechanistic model in vivo, has the potential to greatly expand our knowledge of energy expenditure-body size relationships in humans.Investigators have expressed interest in the associations between resting energy expenditure (REE) and body mass for over a century. Traditionally, descriptive models using regression analysis are applied, linking REE with metabolically active compartments such as body cell mass (BCM) and fat-free body mass (FFM). Recently developed whole body magnetic resonance imaging (MRI) and echocardiography methods now allow estimation of all major organs and tissue volumes in vivo. Because measured values are available for REE, BCM, and FFM content of individual organs and tissues, it should now be possible to develop energy expenditure-body composition estimation models based on MRI-measured organ-tissue volumes. Specifically, the present investigation tested the hypothesis that in vivo estimation of whole body REE, BCM, and FFM is possible using MRI- and echocardiography-derived organ volumes combined with previously reported organ-tissue metabolic rates and chemical composition. Thirteen subjects (5 females, 8 males) had REE, BCM, and FFM measured by indirect calorimetry, whole body 40K counting, and dual-energy X-ray absorptiometry, respectively. Models developed from estimated and measured variables were highly correlated, with no significant differences between those estimated and measured [e.g., calculated vs. measured REE: r = 0.92, P < 0. 001; (mean +/- SD) 6,962 +/- 1,455 and 7,045 +/- 1,450 kJ/day, respectively (P = not significant)]. Strong associations were observed between REE, individual or combined organ weights, BCM, and FFM that provide new insights into earlier observed metabolic phenomona. The present approach, the first to establish an energy expenditure-body composition link with a mechanistic model in vivo, has the potential to greatly expand our knowledge of energy expenditure-body size relationships in humans.

Six-compartment body composition model: inter-method comparisons of total body fat measurement.

OBJECTIVE: To compare 16 currently used total body fat methods to a six-compartment criterion model based on in vivo neutron activation analysis.DESIGN: Observational, inter-method comparison study.SUBJECTS: Twenty-three healthy subjects (17 male and 6 female).MEASUREMENTS: Total body water (TBW) was measured by tritium dilution; body volume by underwater weighing (UWW); total body fat and bone mineral by dual-energy X-ray absorptiometry (DXA), total body potassium (TBK) by whole-body 40K counting; total body carbon, nitrogen, calcium, phosphorus, sodium and chlorine by in vivo neutron activation analysis; skinfolds/circumferences by anthropometry (Anth); and resistance by single-frequency bioimpedance analysis (BIA).RESULTS: The average of total body fat mass measurements by the six-compartment neutron activation model was 19.7±10.2 kg (mean±s.d.) and comparable estimates by other methods ranged from 17.4–24.3 kg. Although all 16 methods were highly correlated with the six-compartment criterion model, three groups emerged based on their comparative characteristics (technical error, coefficient of reliability, Bland-Altman analysis) relative to criterion fat estimates, in decreasing order of agreement: 1. multi-compartment model methods of Baumgartner (19.5±9.9 kg), Heymsfield (19.6±9.9 kg), Selinger (19.7±10.2 kg) and Siri-3C (19.6±9.9 kg); 2. DXA (20.0±10.8 kg), Pace-TBW (18.8±10.1 kg), Siri-2C (20.0±9.9 kg), and Brozek-UWW (19.4±9.2 kg) methods; and 3. Segal-BIA (17.4±7.2 kg), Forbes-TBN (21.8±10.5 kg), Durnin-Anth (22.1±9.5 kg), Forbes-TBK (22.9±11.9 kg), and Steinkamp-Anth (24.3±9.5 kg) methods.CONCLUSION: Relative to criterion fat estimates, body composition methods can be organized into three groups based on inter-method comparisons including technical error, coefficient of reliability and Bland-Altman analysis. These initial groupings may prove useful in establishing the clinical and research role of the many available fat estimation methods.

Body composition in children and adults by air displacement plethysmography

Objectives: Air displacement plethysmography (ADP) may provide a partial alternative to body density (Bd) and therefore body composition measurement compared to conventional hydrodensitometry (Hd) in children. As there are no evaluation studies of ADP in children, this study had a two-fold objective: to compare Bd estimates by ADP and Hd; and to compare fat estimates by both ADP and Hd to fat estimates by another reference method, dual energy X-ray absorptiometry (DXA).Setting: Obesity Research Center, St. Luke’s/Roosevelt Hospital, New York, USA.Subjects: One hundred and twenty subjects (66 females/54 males) who ranged in age from 6–86 y and in body mass index (BMI, kg/m2) from 14.1–40.0  kg/m2 met study entry criteria.Study Design: Cross-sectional study of healthy children (age≤19 y) and adult group for comparison to earlier studies. Each subject completed ADP, Hd, and DXA studies on the same day. Only subjects with subjectively-judged successful Hd studies were entered into the study cohort.Results: There was a high correlation between Bd by ADP and Hd (Bd Hd=0.11+0.896×Bd ADP; r=0.93, SEE=0.008 g/cm3, P<0.0001), although the regression line slope and intercept differed significantly from 1 and 0, respectively. Additional analyses localized a small-magnitude Bd bias in the child (n=48) subgroup. Both ADP and Hd%fat estimates were highly correlated (r>0.9, P<0.0001) with%fat by DXA in child and adult subgroups. Bland–Altman analyses revealed no significant%fat bias by either ADP or Hd vs DXA in either children or adults, although a bias trend (P=0.11) was detected in the child subgroup.Conclusion: With additional refinements, the air displacement plethysmography system has the potential of providing an accurate and practical method of quantifying body fat in children as it now does in adults.Sponsorship: This study was in-part supported by NIH Grants RR00645, NIDDK 42618 and NIDDK 37352.

Techniques used in the measurement of body composition: an overview with emphasis on bioelectrical impedance analysis.

The study of human body composition is now a distinct research area consisting of three interconnected parts: the five-level model and associated rules that govern the relations between components, body-composition methodology, and biological factors that influence body composition. In this overview we summarize fundamental concepts that relate to the five-level model and body-composition methods. We show how these concepts can be used to outline the essential features needed to critically evaluate the bioelectrical impedance analysis method. Body-composition research is a rapidly expanding area and in-depth systematic evaluation of new methods is a vital aspect of the fields growth.

Truncal fat in relation to total body fat: influences of age, sex, ethnicity and fatness

Objective:To investigate the influence of age, sex, ethnicity and total fatness on central obesity in four ethnic populations.Design:Cross-sectional analysis of study subjects enrolled from 1993 to 2005.Subjects:A multi-ethnic (Caucasian (CA), African-American (AA), Hispanic-American (HA) and Asian (As)) convenience sample of 604 men and 1192 women (aged 18–96 years, body mass index 15.93–45.80 kg/m2).Measurements:Total body fat (TBF) and truncal fat were measured by dual-energy X-ray absorptiometry. General linear regression models were used to test for independent associations with log10-transformed truncal fat.Results:For all ethnicities, men had a lower percent body fat and more truncal fat than women. Log10-transformed truncal fat increased with TBF approximately as a square root function. At older ages, there was a greater amount of truncal fat in CA, HA and As men (∼0.20–0.25 kg/decade) with the effect more pronounced in AA men (∼0.33 kg/decade). For women, the increment of truncal fat per decade was reduced in CA and AA women (∼0.07 kg) compared with As and HA women (∼0.33 kg). Adjusted for mean values of covariates in our sample, AA had less truncal fat than As.Conclusion:The accumulation of truncal fat is strongly related to age, ethnicity and total fatness in both men and women.

Sexual dimorphism in the energy content of weight change

BACKGROUND: The energy content of weight change is assumed to be sex- and age-neutral at 3500 kcal/pound or 32.2 MJ/kg.OBJECTIVES: As sexual dimorphism in body composition generally exists in mammals, the primary hypothesis advanced and tested was that the energy content of weight change differs between men and women.DESIGN: The energy content of 129 adult men and 287 women was measured by neutron activation analysis. Cross-sectional energy content prediction models were developed and then evaluated in two longitudinal samples: one that used the same methods in 26 obese women losing weight; and the other a compilation of 18 previously reported weight change–body composition studies.RESULTS: Multiple regression modeling identified weight, sex, age and height as total energy content predictor variables with significant sex×weight (P<0.001) and age×weight (P<0.001) interactions; total model r2 and s.e.e. were 0.89 and 107.3 MJ, respectively. The models predictive value was supported in both longitudinal evaluation samples. Model calculations using characteristics of representative adults gaining or losing weight suggested that the energy content of weight change in women (∼30.1–32.2 MJ/kg) is near to the classical value of 32.2 MJ/kg and that in men the value is substantially lower, ∼21.8–23.8 MJ/kg. The predicted energy content of weight change increases by about 10% in older (age ∼70 y) vs younger (∼35 y) men and women.CONCLUSIONS: Sexual dimorphism and age-dependency appears to exist in the estimated energy content of weight change and these observations have important clinical and research implications.

Low physical activity levels of modern Homo sapiens among free-ranging mammals

Obesity prevalence rates are increasing worldwide and one prevailing hypothesis is that physical activity levels of modern humans are markedly reduced compared to those of our Paleolithic ancestors. We examine this hypothesis by deriving relative activity energy expenditure from available doubly labeled water and indirect calorimetry data in free-ranging non-human mammals. Our results, given the constraints posed by limited data availability, suggest that a low physical activity level, much less than that observed in free-ranging non-human mammals or highly active humans, is present in modern adult humans living within advanced settings. Our observations lend support to the hypothesis that low activity-related energy expenditure levels contribute to the rising worldwide prevalence of obesity.

Extracellular water across the adult lifespan: reference values for adults

Extracellular water (ECW) is a large and clinically important body compartment that varies widely in volume both in health and disease. Interpretation of ECW measurements in the clinical setting requires consideration of potential influencing factors such as age, race, sex and other variables that influence fluid status. An important gap in physiological research is a lack of normative ECW values against which to reference perturbations in fluid homeostasis. The current studys aim was to develop conditional quantile equations for ECW based on weight, height, age, sex and race using a large (n = 1538, 854 females and 684 males) healthy adult multi-ethnic (African American, Asian, European American, Hispanic) sample. ECW was derived from total body water and potassium measured by isotope dilution and whole-body 40K counting, respectively. Quantile regression methods were used to identify five percentile levels (10th, 25th, 50th, 75th, 90th). Weight and height were significant variables at each quantile in both males and females; age made a significant contribution in the male but not the female sample. These regression equations provide ECW quantile reference values based on a large multi-ethnic adult population that should not only prove useful in clinical settings and physiological research, but serve as a model approach for developing body composition normative ranges.

Assessment by bioimpedance of forearm cell mass: a new approach to calibration.

Objective: Changes in skeletal muscle mass are involved in several important clinical disorders including sarcopenia and obesity. Unlike body fat, skeletal muscle is difficult to quantify in vivo, particularly without highly specialized equipment. The present study had a two-fold aim: to develop a regional 40K counter for non-invasively estimating cell mass in the arm, mainly skeletal muscle cell mass, without radiation exposure; and to test the hypothesis that cell mass in the arm is highly correlated with electrical impedance after adjusting for the arms length.Methods: Forearm cell mass was estimated using a rectangular lead-shielded 40K counter with 4-NaI crystals; impedance of the arm was measured at multiple frequencies using a segmental bioimpedance analysis (BIA) system. The systems within- and between-day coefficient of variation (CV) for 40K-derived elemental potassium averaged 1.8±1.3 and 5.8±1.2%, respectively. The corresponding BIA systems CVs were 1.0±0.4 and 2.1±1.0%, respectively.Subjects and results: Participants in the study were 15 healthy adults (eight females, seven males; age 39±2.8 y, BMI 22.9±4.5 kg/m2). The right arms K (5.2±1.7 g) was highly correlated with length-adjusted impedance (r2=0.81, 0.82, and 0.83 for 5, 50 and 300 kHz, respectively; all P<0.001); multiple regression analysis showed no additional improvement by adding age or sex to the prediction models.Conclusion: These results demonstrate the feasibility of calibrating BIA-measured electrical properties of the arm against estimates of arm cell mass, mainly of skeletal muscle, obtained by regional 40K counting. This simple and practical approach should facilitate the development of BIA-based regional cell mass prediction formulas.Sponsorship: National Institutes of Health grants RR00645 and NIDDK 42618.

Measurement of intramyocellular lipid levels with 2-D magnetic resonance spectroscopic imaging at 1.5 T

Abstract.Intramyocellular lipid (IMCL) plays an important role in the study of metabolism in vivo. Magnetic resonance spectroscopy (MRS) studies of IMCL are usually performed with clinical 1.5-T magnetic resonance imaging (MRI) systems and have employed the single-voxel MRS technique. The present study reports the results of our systematic evaluation of the ability of single- and multi-voxel MRS to yield high-quality, contamination-free IMCL levels from the tibialis anterior (TA) muscle. A clinical, 1.5-T, whole-body MRI scanner was used to measure IMCL with a standard knee coil, head coil, or a 3-cm receive-only surface coil with a body coil transmit. Excellent IMCL spectra were obtained in healthy males in only 8 min from multiple 0.25-cm3 voxels using the surface coil receive/body coil transmit in conjunction with the standard PRESS spectroscopic imaging (SI) technique. This approach provided the spatial resolution and voxel placement flexibility permitting optimal separation of IMCL and extramyocellular lipid. Our findings demonstrate the potential of the SI approach.