Relative overhydration of fat-free mass in postobese versus never-obese subjects

Are methods of estimating fat-free mass loss with energy-restricted diets accurate?

BACKGROUND/OBJECTIVES

Fat-free mass (FFM) often serves as a body composition outcome variable in weight loss studies. An important assumption is that the proportions of components that make up FFM remain stable following weight loss; some body composition models rely on these "constants". This exploratory study examined key FFM component proportions before and following weight loss in two studies of participants with overweight and obesity.

SUBJECTS/METHODS

201 men and women consumed calorie-restricted moderate- or very-low carbohydrate diets leading to 10-18% weight loss in 9-15 weeks. Measured total body fat, lean mass, bone mineral, total body water (TBW), and body weight at baseline and follow-up were used to derive FFM and its chemical proportions using a four-component model.

RESULTS

A consistent finding in both studies was a non-significant reduction in bone mineral and a corresponding increase (p < 0.001) in bone mineral/FFM; FFM density increased significantly in one group of women and in all four participant groups combined (both, p < 0.05). FFM hydration (TBW/FFM) increased in all groups of men and women, one significantly (p < 0.01), and in the combined sample (borderline, p < 0.10). The proportion of FFM as protein decreased across all groups, two significantly (p < 0.05-0.01) and in the combined sample (p < 0.05).

CONCLUSION

FFM relative proportions of chemical components may not be identical before and after short-term weight loss, an observation impacting some widely used body composition models and methods. Caution is thus needed when applying FFM as a safety signal or to index metabolic evaluations in clinical trials when these body composition approaches are used.

Comparison of Bioelectrical Impedance Analysis and Computed Tomography on Body Composition Changes Including Visceral Fat After Bariatric Surgery in Asian Patients with Obesity

PURPOSE

The accuracy of bioelectrical impedance analysis (BIA) in patients with obesity has been controversial. This study aimed to validate the use of BIA in detecting body composition changes, especially for visceral fat, before and after bariatric surgery using computed tomography (CT) as a reference method.

MATERIALS AND METHODS

This retrospective study included Korean patients with a BMI of ≥ 35, or ≥ 30 with metabolic comorbidities. All patients underwent bariatric surgery, and underwent BIA and CT evaluation before and 6 months after the operation. The skeletal muscle index (SMI) and visceral fat index (VFI), variables corrected for height, were compared between BIA and CT.

RESULTS

Forty-eight patients (18 men, 30 women) demonstrated a mean weight loss of 27.5 kg. Significant decreases in both VFI and SMI were observed in both BIA and CT (all p<0.001), with greater VFI change compared to SMI (48.2% vs. 10.4% in CT, respectively). Both pre- and post-operative measurements of VFI and SMI were significantly correlated between BIA and CT (all p<0.05). However, the percent decrease was significantly correlated only with VFI (ρ=0.71, p<0.001). The Bland-Altman analysis showed that BIA underestimated VFI, with a greater degree of underestimation in subjects with higher VFI.

CONCLUSION

Despite the underestimation of BIA in measuring visceral fat, BIA VFI was associated with CT VFI. The SMI values showed significant correlations before and after surgery, but not with the percent decrease. Our results suggest that BIA can be a reliable tool for measuring body composition, especially for visceral fat, after bariatric surgery.

Low serum sodium concentrations in patients with obesity normalizes with weight loss

BACKGROUND & AIMS

Obesity is associated with higher extracellular fluid (ECF) compared to intracellular fluid (ICF) volume and this dysregulation is associated with hypertension and abdominal obesity, associated with metabolic syndrome. As sodium is predominantly an extracellular cation, a higher ECF/ICF ratio will lower serum sodium concentration. The aim of the study was to see whether weight loss, due to dieting and bariatric surgery, had any impact on serum sodium concentrations in patients with severe obesity.

METHODS

Patients with a BMI ≥35 kg/m² admitted for bariatric surgery at Innlandet Hospital Trust, Norway during 2012-14 were included in the study (n = 119). Clinical data and blood samples were recorded at inclusion, after mean six months of dieting, as well as six and 12 months after bariatric surgery.

RESULTS

At inclusion, mean serum sodium was in the lower normal range, 138.3 (SD 2.4) mmol/L, but increased to 141.8 (SD 1.9) mmol/L after weight loss. The increase was significantly correlated to total weight loss (rho: 0.29, p = 0.007). Twelve months after surgery, serum sodium was significantly higher in patients with a normal BMI (<25 kg/m²) compared to patients with overweight.

CONCLUSION

Obesity and hypertension are associated with body fluid dysregulation affecting serum sodium concentrations. As mild hyponatremia, even within the normal sodium range, is associated with increased total mortality and major cardiovascular disease events, serum sodium might be a potential risk marker in patients with obesity.

Associations of age and body mass index with hydration and density of fat-free mass from 4 to 22 years

BACKGROUND

Most body composition techniques assume constant properties of fat free mass (FFM) (hydration and density) regardless of nutritional status, which may lead to biased values.

AIM

To evaluate the interactive associations of age and body mass index (BMI) with hydration and density of FFM.

METHODS

Data from subjects aged between 4 and 22 years old from several studies conducted in London, UK were assessed. Hydration (H_FFM) and density (D_FFM) of FFM obtained from the four-component model in 936 and 905 individuals, respectively, were assessed. BMI was converted in to z-scores, and categorised into five groups using z-score cut-offs (thin, normal weight, overweight, obese, and severely obese). Linear regression models for H_FFM and D_FFM were developed using age, sex, and BMI group as predictors.

RESULTS

Nearly 30% of the variability in H_FFM was explained by models including age and BMI groups, showing increasing H_FFM values in heavier BMI groups. On the other hand, ∼40% of variability in D_FFM was explained by age, sex, and BMI groups, with D_FFM values decreasing in association with higher BMI group.

CONCLUSION

Nutritional status should be considered when assessing body composition using two-component methods, and reference data for H_FFM and D_FFM is needed for higher BMI groups to avoid bias. Further research is needed to explain intra-individual variability in FFM properties.

Fat-Free Mass and Skeletal Muscle Mass Five Years After Bariatric Surgery

OBJECTIVE

This study investigated changes in fat-free mass (FFM) and skeletal muscle 5 years after surgery in participants from the Longitudinal Assessment of Bariatric Surgery-2 trial.

METHODS

A three-compartment model assessed FFM, and whole-body magnetic resonance imaging (MRI) quantified skeletal muscle mass prior to surgery (T0) and 1 year (T1), 2 years (T2), and 5 years (T5) postoperatively in 93 patients (85% female; 68% Caucasian; age 44.2 ± 11.6 years) who underwent gastric bypass (RYGB), sleeve gastrectomy, or adjustable gastric band. Repeated-measures mixed models were used to analyze the data.

RESULTS

Significant weight loss occurred across all surgical groups in females from T0 to T1. FFM loss from T0 to T1 was greater after RYGB (mean ± SE: -6.9 ± 0.6 kg) than adjustable gastric band (-3.5 ± 1.4 kg; P < 0.05). Females with RYGB continued to lose FFM (-3.3 ± 0.7 kg; P < 0.001) from T1 to T5. A subset of males and females with RYGB and MRI-measured skeletal muscle showed similar initial FFM loss while maintaining FFM and skeletal muscle from T1 to T5.

CONCLUSIONS

Between 1 and 5 years following common bariatric procedures, FFM and skeletal muscle are maintained or decrease minimally. The changes observed in FFM and muscle during the follow-up phase may be consistent with aging.

Body Composition Tools for Assessment of Adult Malnutrition at the Bedside: A Tutorial on Research Considerations and Clinical Applications

Because of the key role played by the body's lean tissue reserves (of which skeletal muscle is a major component) in the response to injury and illness, its maintenance is of central importance to nutrition status. With the recent development of the Academy of Nutrition and Dietetics/American Society for Parenteral and Enteral Nutrition diagnostic framework for malnutrition, the loss of muscle mass has been recognized as one of the defining criteria. Objective methods to evaluate muscle loss in individuals with acute and chronic illness are needed. Bioimpedance and ultrasound techniques are currently the best options for the clinical setting; however, additional research is needed to investigate how best to optimize measurements and minimize error and to establish if these techniques (and which specific approaches) can uniquely contribute to the assessment of malnutrition, beyond more subjective evaluation methods. In this tutorial, key concepts and statistical methods used in the validation of bedside methods to assess lean tissue compartments are discussed. Body composition assessment methods that are most widely available for practice and research in the clinical setting are presented, and clinical cases are used to illustrate how the clinician might use bioimpedance and/or ultrasound as a tool to assess nutrition status at the bedside. Future research needs regarding malnutrition assessment are identified.

Adaptive Thermogenesis in Resistance to Obesity Therapies: Issues in Quantifying Thrifty Energy Expenditure Phenotypes in Humans

Dieting and exercise are likely to remain the core approaches in the management of obesity in the foreseeable future despite their well-documented failures for achieving long-term weight loss. Explanations for such poor prognosis are centered on patient's self-regulatory failure and lack of compliance to the prescribed diet or exercise regimen. While a role for physiological adaptations leading to diminished rates of heat production has also been advocated, there are considerable uncertainties about the quantitative importance of such regulated heat production (i.e., adaptive thermogenesis) to the less-than-expected weight loss and ease for weight regain. This paper first reviews the most compelling evidence of what is often considered as weight loss-induced adaptive thermogenesis in various compartments of daily energy expenditure. It then discusses the major limitations and issues in quantifying such thrifty energy expenditure phenotypes and underscores the plausibility of diminished core temperature as a thrifty metabolic trait in resistance to weight loss. Although an accurate quantification of adaptive thermogenesis will have to await the applications of deep body composition phenotyping and better discrimination of physical activity energy expenditures, the magnitude of diminished energy expenditure in response to weight loss in certain individuals is large enough to support the concept that adaptive thermogenesis contribute importantly to their resistance to obesity therapies.

Adaptive thermogenesis in human body weight regulation: more of a concept than a measurable entity?

According to Lavoisier, 'Life is combustion'. But to what extent humans adapt to changes in food intake through adaptive thermogenesis--by turning down the rate of heat production during energy deficit (so as to conserve energy) or turning it up during overnutrition (so as to dissipate excess calories)--has been one of the most controversial issues in nutritional sciences over the past 100 years. The debate nowadays is not whether adaptive thermogenesis exists or not, but rather about its quantitative importance in weight homoeostasis and its clinical relevance to the pathogenesis and management of obesity. Such uncertainties are likely to persist in the foreseeable future primarily because of limitations to unobtrusively measure changes in energy expenditure and body composition with high enough accuracy and precision, particularly when even small inter-individual variations in thermogenesis can, in dynamic systems and over the long term, be important in the determining weight maintenance in some and obesity and weight regain in others. This paper reviews the considerable body of evidence, albeit fragmentary, suggesting the existence of quantitatively important adaptive thermogenesis in several compartments of energy expenditure in response to altered food intake. It then discusses the various limitations that lead to over- or underestimations in its assessment, including definitional and semantics, technical and methodological, analytical and statistical. While the role of adaptive thermogenesis in human weight regulation is likely to remain more a concept than a strictly 'quantifiable' entity in the foreseeable future, the evolution of this concept continues to fuel exciting hypothesis-driven mechanistic research which contributes to advance knowledge in human metabolism and which is bound to result in improved strategies for the management of a healthy body weight.

Comparison of Methods to Assess Body Composition Changes during a Period of Weight Loss

OBJECTIVE

To assess the accuracy of body composition measurements by air displacement plethysmography and bioelectrical impedance analysis (BIA) compared with DXA during weight loss.

RESEARCH METHODS AND PROCEDURES

Fifty-six healthy but overweight participants, 34 women and 22 men (age, 52 +/- 8.6 years; weight, 92.2 +/- 11.6 kg; BMI, 33.3 +/- 2.9 kg/m(2)) were studied in an outpatient setting before and after 6 months of weight loss (weight loss, 5.6 +/- 5.5 kg). Subjects were excluded if they had initiated a new drug therapy within 30 days of randomization, were in a weight loss program, or took a weight loss drug within 90 days of randomization. Subjects were randomly assigned either to a self-help program, consisting of two 20-minute sessions with a nutritionist and provision of printed materials and other self-help resources, or to attendance at meetings of a commercial program (Weight Watchers). Body composition was examined by each of the methods before and after weight loss.

RESULTS

BIA (42.4 +/- 5.8%) underestimated percentage fat, whereas the BodPod (Siri = 51.7 +/- 6.9%; Brozek = 48.5 +/- 6.5%) overestimated percentage fat compared with DXA (46.1 +/- 7.9%) before weight loss. Correlation coefficients for detecting changes in body composition between DXA and the other methods were relatively high, with Brozek Deltafat mass (FM; r(2) = 0.63), Siri FM (r(2) = 0.65), tetrapolar BIA percentage fat (r(2) = 0.57), and Tanita FM (r(2) = 0.61) being the highest.

DISCUSSION

In conclusion, all of the methods were relatively accurate for assessing body composition compared with DXA, although there were biases. Furthermore, each of the methods was sensitive enough to detect changes with weight loss.

Validation of BIA in obese children and adolescents and re-evaluation in a longitudinal study

Decrease in fat mass (FM) is a one of the aims of pediatric obesity treatment; however, measurement techniques suitable for routine clinical assessment are lacking. The objective of this study was to validate whole-body bioelectrical impedance analysis (BIA; TANITA BC-418MA) against the three-component (3C) model of body composition in obese children and adolescents, and to test the accuracy of our new equations in an independent sample studied longitudinally. A total of 77 white obese subjects (30 males) aged 5-22 years, BMI-standard deviation score (SDS) 1.6-3.9, had measurements of weight, height (HT), body volume, total body water (TBW), and impedance (Z). FM and fat-free mass (FFM) were calculated using the 3C model or predicted from TANITA. FFM was predicted from HT(2)/Z. This equation was then evaluated in 17 other obese children (5 males) aged 9-13 years. Compared to the 3C model, TANITA manufacturer's equations overestimated FFM by 2.7 kg (P < 0.001). We derived a new equation: FFM = -2.211 + 1.115 (HT(2)/Z), with r(2) of 0.96, standard error of the estimate 2.3 kg. Use of this equation in the independent sample showed no significant bias in FM or FFM (mean bias 0.5 +/- 2.4 kg; P = 0.4), and no significant bias in change in FM or FFM (mean bias 0.2 +/- 1.8 kg; P = 0.7), accounting for 58% (P < 0.001) and 55% (P = 0.001) of the change in FM and FFM, respectively. Our derived BIA equation, shown to be reliable for longitudinal assessment in white obese children, will aid routine clinical monitoring of body composition in this population.

Usefulness of different techniques for measuring body composition changes during weight loss in overweight and obese women

The objective was to compare measures from dual-energy X-ray absorptiometry (DXA), bioelectrical impedance analysis (BIA) and anthropometry with a reference four-compartment model to estimate fat mass (FM) and fat-free mass (FFM) changes in overweight and obese women after a weight-loss programme. Forty-eight women (age 39.8 ± 5.8 years; weight 79·2 ± 11·8 kg; BMI 30·7 ± 3·6 kg/m2) were studied in an out-patient weight-loss programme, before and after the 16-month intervention. Women attended weekly meetings for the first 4 months, followed by monthly meetings from 4 to 12 months. Body composition variables were measured by the following techniques: DXA, anthropometry (waist circumference-based model; Antrform), BIA using Tanita (TBF-310) and Omron (BF300) and a reference four-compartment model. Body weight decreased significantly ( − 3·3 (sd3·1) kg) across the intervention. At baseline and after the intervention, FM, percentage FM and FFM assessed by Antrform, Tanita, BF300 and DXA differed significantly from the reference method (P ≤ 0·001), with the exception of FFM assessed by Tanita (baselineP = 0·071 and afterP = 0·007). DXA significantly overestimated the change in FM and percentage FM across weight loss ( − 4·5v. − 3·3 kg;P < 0·001 and − 3·7v. − 2·0 %;P < 0·001, respectively), while Antrform underestimated FM and percentage FM ( − 2·8v. − 3·3 kg;P = 0·043 and − 1·1v. − 2·0 %;P = 0·013) compared with the four-compartment model. Tanita and BF300 did not differ (P>0·05) from the reference model in any body composition variables. We conclude that these methods are widely used in clinical settings, but should not be applied interchangeably to detect changes in body composition. Furthermore, the several clinical methods were not accurate enough for tracking body composition changes in overweight and obese premenopausal women after a weight-loss programme.

What is the required energy deficit per unit weight loss?

One of the most pervasive weight loss rules is that a cumulative energy deficit of 3500 kcal is required per pound of body weight loss, or equivalently 32.2 MJ kg(-1). Under what conditions is it appropriate to use this rule of thumb and what are the factors that determine the cumulative energy deficit required per unit weight loss? Here, I examine this question using a modification of the classic Forbes equation that predicts the composition of weight loss as a function of the initial body fat and magnitude of weight loss. The resulting model predicts that a larger cumulative energy deficit is required per unit weight loss for people with greater initial body fat-a prediction supported by published weight loss data from obese and lean subjects. This may also explain why men can lose more weight than women for a given energy deficit since women typically have more body fat than men of similar body weight. Furthermore, additional weight loss is predicted to be associated with a lower average cumulative energy deficit since a greater proportion of the weight loss is predicted to result from loss of lean body mass, which has a relatively low energy density in comparison with body fat. The rule of thumb approximately matches the predicted energy density of lost weight in obese subjects with an initial body fat above 30 kg but overestimates the cumulative energy deficit required per unit weight loss for people with lower initial body fat. International Journal of Obesity (2008) 32, 573-576; doi:10.1038/sj.ijo.0803720; published online 11 September 2007.

The altered fluid distribution in obesity may reflect plasma hypertonicity

OBJECTIVE

This study explored whether the increased extracellular relative to intracellular fluid (ECF/ICF) ratio in obesity might reflect osmotic effects of elevated plasma solute concentrations.

DESIGN

Cross-sectional, epidemiological survey.

SETTING AND SUBJECTS

The present analysis used nationally representative data from the Third National Health and Nutrition Examination Survey on community-dwelling adults (aged 40-59 years) in the US without evidence of glucose dysregulation or chronic disease (n=1285).

INTERVENTION

Body mass index (BMI) was estimated from measured height and weight. Total body reactance, an index of body fluid distribution, was determined by bioelectrical impedance analysis. Plasma tonicity (the cumulative index of osmotically effective plasma solute) was estimated from plasma glucose, sodium and potassium. Sex-specific relative odds of lower reactance (<or=50 Omega/m for women, <or=40 Omega/m for men) and plasma hypertonicity (tonicity >or=295 mmol/l) associated with overweight (25<or=BMI <30) and obesity (BMI>or=30) were estimated using logistic regression models that controlled for sociodemographic variables, smoking, leisure-time physical activity, total energy intake, serum creatinine, plasma insulin and glucose. Multinomial logistic regression models tested for associations between weight status and specific serum solute.

RESULTS

Independent of covariates, in men and women, overweight and obesity were associated with increased odds of lower reactance and hypertonicity. Overweight and obese individuals with lower reactance had significantly higher serum sodium than normal weight individuals.

CONCLUSIONS

Elevated plasma solute concentrations are associated with obesity in free-living adults. Physicians and researchers should be alert to a possible link between hypertonicity and obesity.

SPONSORSHIP

Grants from the NIH, Nestle Waters.

Body composition in normal weight, overweight and obese children: matched case–control analyses of total and regional tissue masses, and body composition trends in relation to relative weight

BACKGROUND

Childhood obesity is defined on the basis of weight and height, using body mass index (BMI). There is little detailed information on the body composition characteristic of overweight and obesity.

OBJECTIVE

To evaluate total and regional body composition in overweight, obese and control children aged 7-14 years.

DESIGN

Body composition was measured by the four-component model and dual X-ray absorptiometry in 38 age- and sex-matched pairs of obese and control children. Body composition trends were also evaluated by quintile of BMI standard deviation score (SDS) in these and 31 other children (n=107; BMI SDS range -1.0 to 4.3).

RESULTS

Obese children were taller than controls (Delta=0.6 SDS; P=0.01) and had greater hydration of fat-free mass (FFM) (Delta=1.8 %, P<0.0001). After adjusting for these variables, obese children had greater FFM, fat mass (FM) and mineral (P<0.0001). Regional analyses showed that these differences were apparent in the arm, leg and trunk, but the three tissues had different proportional distributions of the excess. Fat was primarily in the trunk, but mineral in the leg. FM, FFM, hydration and mineral mass all increased across BMI SDS quintiles (P<0.0001), but the trend for FM was much the steepest.

DISCUSSION

The greater weight of obese children is due to excess FFM including mineral as well as excess fatness. Increasing weight has a strong continuous relationship with increasing FM across the whole spectrum of weight.

Four-compartment cellular level body composition model: comparison of two approaches

OBJECTIVE

The aim of this study was to develop and compare two DXA-based four-compartment [body weight=body cell mass (BCM)+extracellular fluid (ECF)+extracellular solids (ECS)+fat] cellular level models.

RESEARCH METHODS AND PROCEDURES

Total body potassium (TBK) model: BCM from TBK by whole-body counting-ECF(TBK)=LST-[BCM(TBK)+0.73 x osseous mineral (Mo)]. Bromide model: ECF from sodium bromide dilution-BCM(BROMIDE)=LST-(ECF(BROMIDE)+0.73xMo); Mo and LST measurements came from DXA. The two approaches were evaluated in 99 healthy men and 118 women.

RESULTS

BCM estimates were highly correlated (r=0.97, p<0.001), as were ECF estimates (r=0.87, p<0.001); a small statistically significant mean difference was present (mean+/-SD; BCM(TBK) model, 30.4+/-8.9 kg; BCM(BROMIDE), 31.4+/-9.3 kg; Delta=1.0+/-2.8 kg; p<0.001; ECF(TBK), 18.5+/-4.2 kg; ECF(BROMIDE), 17.5+/-3.6 kg; Delta=1.0+/-2.8 kg; p<0.001). A high correlation (r=0.97, p<0.001) and good agreement (38.9+/-9.5 vs. 38.9+/-9.5 kg; Delta=0.0+/-2.4 kg; p=0.39) were present between TBW, derived as the sum of intracellular water from TBK and ECW from bromide, and measured TBW by 2H2O dilution.

DISCUSSION

Two developed four-compartment cellular level DXA models, one of which is appropriate for use in most clinical and research settings, provide comparable results and are applicable for BCM and ECF estimation of subject groups with hydration disturbances.

Composition of the fat-free mass in obese and nonobese children: matched case-control analyses

OBJECTIVE

Most body composition techniques assume constant properties of the fat-free mass (FFM), such as hydration, density and mineralisation. Previous studies suggested that FFM composition may change in childhood obesity; however, this issue has not been investigated in detail.

AIM

To compare FFM composition in obese and nonobese children.

DESIGN

Observational matched case-control analyses.

SUBJECTS

A total of 28 obese children (13 boys, 15 girls) and 22 nonobese children (10 boys, 12 girls) aged 7-14 y. Obesity was defined as body mass index centile >95.

METHODS

Measurements were made of weight, height, total body water, and body volume. Bone mineral content was estimated in a subsample. Body composition was calculated using three- and four-component models.

RESULTS

According to the three-component model (n=22 matched pairs), obese children had greater hydration (P<0.05), and reduced density (P=0.057) of FFM. According to the four component model (n=11 pairs), obese children had greater hydration (P<0.01) and reduced density (P<0.002) of FFM. The mineralisation of FFM was increased, but not significantly so.

CONCLUSION

The greater hydration and reduced density of FFM of obese children should be taken into account if body composition is to be measured with optimum accuracy during treatment programmes. These differences may be addressed by using multicomponent rather than two-component models of body composition. Although the greater mineralisation of FFM in obese children was not significant in the present study, the four-component model is best able to address the combined differences in hydration and mineralisation that occur in childhood obesity.

Body composition assessment in extreme obesity and after massive weight loss induced by gastric bypass surgery

Body composition methods were examined in 20 women [body mass index (BMI) 48.7 +/- 8.8 kg/m(2)] before and after weight loss [-44.8 +/- 14.6 (SD) kg] after gastric bypass (GBP) surgery. The reference method, a three-compartment (3C) model using body density by air displacement plethysmography and total body water (TBW) by H(2)18O dilution (3C-H(2)18O), showed a decrease in percent body fat (%BF) from 51.4 to 34.6%. Fat-free mass hydration was significantly higher than the reference value (0.738) in extreme obesity (0.756; P < 0.001) but not after weight reduction (0.747; P = 0.16). %BF by H(2)18O dilution and air displacement plethysmography differed significantly from %BF by 3C-H(2)18O in extreme obesity (P < 0.05) and 3C models using (2)H(2)O or bioelectrical impedance analysis (BIA) to determine TBW improved mean %BF estimates over most other methods at both time points. BIA results varied with the equation used, but BIA better predicted %BF than did BMI at both time points. All methods except BIA using the Segal equation were comparable to the reference method for determining changes over time. A simple 3C model utilizing air displacement plethysmography and BIA is useful for clinical evaluation in this population.

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 3,500 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 x weight (P<0.001) and age x weight (P<0.001) interactions; total model r(2) and s.e.e. were 0.89 and 107.3 MJ, respectively. The model's 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 (approximately 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, approximately 21.8-23.8 MJ/kg. The predicted energy content of weight change increases by about 10% in older (age approximately 70 y) vs younger (approximately 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.