Resting metabolic rate in obese, premenopausal black women

In the United States, obesity is more prevalent in black than in non-Hispanic white women. Because low resting metabolic rate (RMR) has been suggested as a risk factor for weight gain, we compared RMR in 22 black and 20 white obese [body mass index (BMI; in kg/m2) range: 28.9-48.6 and 26.9-44.1, respectively], weight-stable, premenopausal, nondiabetic women. RMR was measured on two or three different occasions within a 1-wk period. The black and white groups did not differ significantly in age, degree of fitness, BMI, fat mass, or fat-free mass (FFM). In each group, RMR was predicted independently by FFM but not by age, degree of fitness, body fat mass, or body fat distribution. The slopes of the equations predicting RMR from FFM in black and white groups were not significantly different. However, the black women had significantly lower RMRs than the white women after adjustment for FFM measured by five body-composition models: dual-photon X-ray absorptiometry (DXA), hydrodensitometry, total body water, a three-compartment model, a four-compartment model, as well as for the absolute total-body potassium content as a measure of metabolically active FFM. By each analysis, the black women had significantly lower (P < 0.01) FFM-adjusted RMR than the white women; this difference ranged from 671 to 889 kJ/d depending on the body-composition method used to estimate FFM. This could contribute to the difference in the prevalence of obesity in the populations represented by these groups.

Appendicular skeletal muscle mass: effects of age, gender, and ethnicity

This study tested the hypothesis that skeletal muscle mass is reduced in elderly women and men after adjustment first for stature and body weight. The hypothesis was evaluated by estimating appendicular skeletal muscle mass with dual-energy X-ray absorptiometry in a healthy adult cohort. A second purpose was to test the hypothesis that whole body 40K counting-derived total body potassium (TBK) is a reliable indirect measure of skeletal muscle mass. The independent effects on both appendicular skeletal muscle and TBK of gender (n = 148 women and 136 men) and ethnicity (n = 152 African-Americans and 132 Caucasians) were also explored. Main findings were 1) for both appendicular skeletal muscle mass (total, leg, and arm) and TBK, age was an independent determinant after adjustment first by stepwise multiple regression for stature and weight (multiple regression model r2 = approximately 0.60); absolute decrease with greater age in men was almost double that in women; significantly larger absolute amounts were observed in men and African-Americans after adjustment first for stature, weight, and age; and >80% of within-gender or -ethnic group between-individual component variation was explained by stature, weight, age, gender, and ethnicity differences; and 2) most of between-individual TBK variation could be explained by total appendicular skeletal muscle (r2 = 0.865), whereas age, gender, and ethnicity were small but significant additional covariates (total r2 = 0.903). Our study supports the hypotheses that skeletal muscle is reduced in the elderly and that TBK provides a reasonable indirect assessment of skeletal muscle mass. These findings provide a foundation for investigating skeletal muscle mass in a wide range of health-related conditions.

Body mass index and all-cause mortality among people age 70 and over: the Longitudinal Study of Aging

OBJECTIVES

To assess the relationship between body mass index (BMI; kg/m2) and mortality in a large nationally representative sample of US adults over age 70 years.

DESIGN

Prospective longitudinal cohort study, the Longitudinal Study of Aging (LSOA). Subjects were all those 7260 black and white people (2769 men, 4491 women) initially interviewed in 1984 for whom height and weight were available. These subjects were followed through to 1990.

MEASUREMENTS

Measurements included self-reported height and weight, date of death if subjects died, sex, age, race, measures of socio-economic status, number of living first degree relatives, and responses to questions asking whether the subject had retired due to poor health, had difficulty eating, worried about their health, and felt their health was worse than during the prior year. Smoking status was not assessed.

RESULTS

When analyzed via Cox proportional hazard regression, the relationship between BMI and mortality, represented by means of hazard ratio, was clearly U-shaped for both men and women. The base of the curves was fairly wide suggesting that a broad range of BMIs are well tolerated by older adults. The minimum mortality (estimated from the fitted proportional hazard models) occurred at a BMI of approximately 31.7 for women and 28.8 for men. The results were essentially unchanged, if analyses were weighted, if various disease states were controlled for, and if apparently unhealthy subjects were excluded.

CONCLUSIONS

The finding of the relatively high BMI (27-30 for men, 30-35 for women) associated with minimum hazard in persons older than seventy years supports some previously documented findings and opposes others and, if confirmed in future research, has implications for public health and clinical recommendations.

Density of fat-free body mass: relationship with race, age, and level of body fatness

The two-compartment body composition method assumes that fat-free body mass (FFM) has a density of 1.100 kg/l. This study tested the hypothesis that FFM density is independent of race, age, and body fatness. Subjects were 703 black and white subjects, ages 20-94 yr, with body mass index (BMI) 17-35 kg/m2. Body composition was assessed using a four-compartment model based on tritium dilution volume, body density by underwater weighing, bone mineral by dual-energy X-ray absorptiometry, and body weight. No relationship was observed between FFM density and race or BMI. A tendency was observed for a lower FFM density only in older white women. The difference in percent body fat (delta fat) between the four-compartment model and underwater weighing was < 2% for all groups. Race, age, and BMI explained only 2.3 (women) and 1.4% (men) of the variance in delta fat, whereas the total body water fraction of FFM explained 77%. In contrast to current thinking, these results show that the assumption of constant FFM density is valid in black, elderly, and obese subjects.

A multi-center comparison of dual energy X-ray absorptiometers: in vivo and in vitro soft tissue measurement

OBJECTIVE

To assess intra- and inter-site soft tissue variability by dual energy X-ray absorptiometry (DXA).

DESIGN

Cross-sectional trial.

SETTING

Three medical research institutions.

SUBJECTS

Five humans (in vivo) and four phantoms (in vitro), configured from two whole body phantoms with artificial skeletons and thickness overlays.

INTERVENTIONS

Duplicate total-body DXA scans were performed on all subjects at each institution within a 15 d period.

RESULTS

All intra-site coefficients of variation (CV) were < 0.5% for total tissue mass, but in vitro and in vivo Cvs were 7.2% and 2.3% for fat mass (FM) and 2.5% and 0.9% for lean mass (LM), respectively. Several total-body and regional FM and LM measurements were significantly different between sites (P < 0.05), with percent differences between sites ranging from 2.6-13.3% for FM and from 1.6-13.6% for LM. Site 2 was consistently lower for FM and Site 3 was consistently lower for LM.

CONCLUSIONS

These results stress the need for both rigorous and standardized cross-calibration procedures for soft tissue measurement by DXA.

Bioimpedance analysis: evaluation of leg-to-leg system based on pressure contact footpad electrodes

Conventional single frequency bioimpedance analysis (BIA) systems require technician placement of arm and leg gel electrodes, a suitable location for recumbent measurements, and a separate measurement of body weight. The aim of this study was to evaluate a new single frequency 50 KHz leg-to-leg bioimpedance analysis (BIA) system combined with a digital scale that employs stainless steel pressure-contact foot pad electrodes for standing impedance and body weight measurements. Healthy adults were evaluated for 1) electrode validity and 2) potential for body component estimation. Pressure-contact foot-pad electrode measured impedance was highly correlated with (N = 9, r = 0.99, P < 0.001) impedance measured using conventional gel electrodes applied to the plantar surface of both lower extremities; mean (+/-SD) impedance was systematically higher by about 15 ohms for pressure contact electrodes (526 +/- 56 ohms vs 511 +/- 59 ohms; P < 0.001). Second, the relationship between stature-adjusted leg-to-leg impedance (H2/Z) measured by the new system and two body composition components (total body water by 3H2O dilution (N = 144); and fat-free body mass, by underwater weighing and dual x-ray absorptiometry (N = 231)) was modeled using multiple regression analysis. Correlation coefficients for H2/Z alone versus body composition components were lower for leg-to-leg BIA than for arm-to-leg BIA; correlation coefficients and SEEs became similar for the leg-to-leg and arm-to-leg BIA systems with addition of three covariates (age, gender, and waist/hip circumference ratio) to regression models. The leg-to-leg pressure contact electrode BIA system has overall performance characteristics for impedance measurement and body composition analysis similar to conventional arm-to-leg gel electrode BIA and offers the advantage of increased speed and ease of measurement.

Electrocardiographic QTC interval: short-term weight loss effects

OBJECTIVE

Prolongation of the QTc interval is a risk-factor for ventricular arrhythmias and sudden death. There is a correlation between body mass index (BMI) and QTc, with longer intervals observed in obese subjects. Dieting and weight loss on selected diets are also associated with prolongation of the QTc interval, although the mechanisms and risk factors of this syndrome are not clearly established. The aim of the current investigation was to determine if short-term weight loss over seven days while living on a metabolic ward influences cardiac repolarization as indicated by QT interval duration.

DESIGN AND SUBJECTS

The electrocardiogram QT, RR, and QTc (QT/RR(0.5)) intervals were assessed in 30 healthy obese (BMI > 27 kg/m2) men and women ingesting a balanced 1120 kcal/d formula diet (23% kcal as fat and 52 g protein/d).

RESULTS

Weight loss (3.9 +/- 1.7 kg) over seven days had no measurable effect on the RR interval and there was a significant (P = 0.02) shortening of the QT interval (mean +/- s.d., 0.395 +/- 0.028-0.386 +/- 0.027 s) and borderline significant (P = 0.11) shortening of the QTc (0.411 +/- 0.028-0.404 +/- 0.025 s) interval. The QTc interval normalized in three subjects with prolonged baseline QTc (> 0.44 s). A statistically significant (P = 0.007) reduction in systolic blood pressure early in the course of weight loss (day 2) accompanied the electrocardiographic changes.

CONCLUSION

These results fail to support the development of cardiac repolarization abnormalities in obese patients who ingest a nutritionally balanced low calorie diet over seven days. A significant reduction in QT interval duration and amelioration of abnormally prolonged baseline QTc intervals suggest improved cardiac repolarization with early weight loss.

Human body composition: advances in models and methods

The field of human body composition research is reaching a mature stage in its development: The three interconnected areas that define body composition research--models and their rules, methodology, and biological effects--are well-defined and are actively investigated by scientists in diverse disciplines from many different nations; and methods are available for measuring all major atomic, molecular, cellular, and tissue-system level body composition components in research, clinical, and epidemiological settings. This review summarizes main body composition research concepts, examines new component-measurement methodologies, and identifies potential areas of future research.

Dual-energy X-ray absorptiometry body composition model: review of physical concepts

Although dual-energy X-ray absorptiometry (DXA) is widely used in clinical research as a means of quantifying body composition, there remains at present little published information that reviews the method's underlying physical basis. Because a clear understanding of DXA physical concepts is integral to appropriate use and interpretation, we present here a three-section review that includes both relevant in vitro and in vivo experimental demonstrations. In the first section we describe the main physical principles on which DXA is based. The section that follows presents a step-by-step analysis of the DXA two-component soft tissue model. In the final section we demonstrate how knowledge of physical concepts can lead to resolution of important methodological concerns, such as the influence of hydration changes on DXA fat estimates. A thorough understanding of DXA physical concepts provides a basis for appropriate interpretation of measurement results and stimulates many new and important research questions.

Nutrition Societies Presidents' Forum: future challenges and opportunities for nutrition societies in the 21st century

The Chair introduced "Pasteur's Quadrant" as a potentially useful paradigm for modern science. Developed by Princeton's Donald E Stokes, the quadrant is two-by-two matrix that classifies knowledge as fundamental and/or applied. The Chair also noted the effect of competitive pressures, and the necessity for cooperation among nutrition societies. The Presidents of The American Society for Nutritional Sciences (ASNS), The American Society for Clinical Nutrition (ASCN), The American Society for Parenteral and Enteral Nutrition (ASPEN), and the Chair of the Institute of Food Technologist's (IFT) Nutrition Division presented their views on how societies can prepare to meet their members' upcoming needs. The Director of the Center for Food and Nutrition Policy discussed the future role of nutrition societies and how they might interact with various interest groups. The Forum, which included an opportunity for audience participation, took place soon after the February 1996 release of "Meeting the Challenge: A Research Agenda for America's Health, Safety, and Food." Published by the Executive Office of the President's Office of Science and Technology Policy, the report highlights the importance of nutrition to our nation's health.

Urinary creatinine-skeletal muscle mass method: a prediction equation based on computerized axial tomography

A classic body composition method is estimation of total body skeletal muscle mass (SM, kg) from 24 h urinary creatinine excretion (Cr, g). Two types of prediction equations were suggested: one assumes a constant SM/Cr ratio; and the other assumes a highly variable SM/Cr ratio. We explored these two extreme possibilities by measuring SM with whole-body computerized axial tomography and collecting Cr during meat-free dietary conditions in 12 healthy young men. Prediction equations were developed in the men that fit these two equation types, SM = 21.8 x Cr (SD and CV of SM/Cr ratio, 1.3 kg and 6.0%, respectively) and SM = 18.9 x Cr + 4.1 (r = 0.92, p = 2.55 x 10(-5), and SEE = 1.9 kg). The validity of each model is reviewed. This is the first investigation of Cr-SM method using modern techniques for quantifying total body SM mass.

Effects of gender, body composition, and menopause on plasma concentrations of leptin

Circulating concentrations of leptin ([leptin]) vary directly with body mass index and percentage body fat, and may thus constitute an afferent limb of a system regulating body fatness. We tested the hypotheses that: 1) Plasma [leptin] vary more directly with absolute fat mass than with fractional body fatness per se: and 2). The relationship between fat mass and [leptin] is significantly affected by gender and by menopausal status. [Leptin] in the post-absorptive state was examined in 67 subjects (26 male, 20 premenopausal female, 21 postmenopausal females; 43 never-obese, 24 obese) at usual body weight. Body composition was determined by hydrodensitometry, and [leptin] was determined by a double antibody ELISA assay. In male and pre-menopausal female subjects, subcutaneous adipose tissue aspirations were performed for determination of adipocyte volume by the osmium fixation method, and a 3 hour oral glucose tolerance tests was performed. At usual body weight, ([leptin]) was better correlated with absolute fat mass than with body mass index (BMI) or percentage body fat. BMI and % body fat did not account for any of the variance in [leptin] beyond that attributable to FM, per se. The regression equations relating FM to [leptin] did not differ significantly between obese and never-obese subjects. [Leptin] and fasting serum insulin concentrations were significantly correlated in males only. [Leptin] was significantly higher in pre- and post-menopausal females compared to males, even when [leptin] was corrected for differences in body composition (pre-menopausal females > post-menopausal females > males). While plasma [leptin], corrected for FM, declines significantly in women post-menopause, this decline is not sufficient to account for the striking sexual dimorphism in the relationship of leptin to fat mass. This sexual dimorphism is apparently also due, in part, to a suppressive effect of circulating androgens on [leptin].

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 field's growth.

Metabolically active component of fat-free body mass: influences of age, adiposity, and gender

Fat-free body mass (FFM) is often considered the metabolically active compartment and is widely used to adjust between-subject differences in resting energy expenditure for body composition. The use of FFM as the metabolically active portion of body weight makes the assumption that the body cell mass (BCM) component which is more difficult to measure, maintains a relatively constant relationship to FFM within and between subjects. The aim of this study was to test the hypothesis that BCM and FFM are associated independently of age, adiposity (as represented by body density), and gender in healthy white women and men. BCM and FFM were estimated by whole-body 40K-counting and dual-energy x-ray absorptiometry (DXA), respectively. Multiple regression analysis was used to model the relationships between BCM as the dependent variable and FFM, age, body density, and gender as potential independent variables. FFM alone explained 51% and 63% of between-individual BCM differences in women (n = 269) and men (n = 204) (both P = .0001), respectively. Age contributed significantly (P = .0001) to BCM prediction after adjusting first for FFM in both women and men. Body density also added significantly (P = .004 and P = .0001) to FFM and age prediction of BCM in women and men, respectively. Lastly, gender contributed significantly to the composite model, with 91% of between-individual differences in BCM explained by FFM, age, body density, and gender. Hence, BCM does not maintain a fixed relationship to FFM, as often assumed, but varies significantly and independently of FFM with age, adiposity, and gender. These findings have implications for the study of metabolic indices such as resting energy expenditure.

Comparisons for body mass index and body fat percent among Puerto Ricans, blacks, whites and Asians living in the New York City area

The latest data (NHANES III) from the National Center for Health Statistics (NCHS) show that the black population has the highest proportion of overweight among all adult populations in the United States. The present study compared the body mass index (BMI) and body fat percent from dual-photon absorptiometry in 1,324 healthy adults aged 18 to 107 years recruited from four ethnic groups in the New York City area; 523 whites, 280 blacks, 267 Asians and 254 Puerto Ricans. Puerto Ricans had the largest BMI and the largest percent of subjects with body weight more than 120% of their ideal weight, and the largest fat percent of the four ethnic groups: 76% of Puerto Rican males had fat percent above the median value for white males (fat percent = 19.6%) and 95% of Puerto Rican females had fat percent above the median for white females (fat percent = 30.8%). Asians had the smallest BMI, but 63% of them had fat percent above the median values for whites in each gender. Puerto Ricans also had the largest waist-to-hip ratios among the four ethnic groups. In blacks, the percent of subjects with fat percent larger than the median for whites was slightly smaller than that for Puerto Ricans, 64% and 82% of males and females respectively. These results differ from the latest NCHS data and show that Puerto Ricans in this sample are heavier and fatter than blacks.

Total-body skeletal muscle mass: evaluation of 24-h urinary creatinine excretion by computerized axial tomography

A classic body-composition method is estimation of total-body skeletal muscle mass (SM, in kg) from 24-h urinary creatinine excretion (in g). Two approaches of unknown validity have been used to calculate SM from creatinine: one assumes a constant ratio of SM to creatinine, the so-called creatinine equivalence (k), and that SM = k x creatinine; the other suggests a highly variable ratio of SM to creatinine and is based on regression equations of the form SM = b + a x creatinine. We explored these two extreme possibilities by measuring SM with whole-body computerized axial tomography and collecting urinary creatinine during meat-free dietary conditions in 12 healthy adult men. Prediction equations were developed in the men that fit these two models: SM = 21.8 x creatinine (SD and CV of the ratio of SM to creatinine: 1.3 kg and 6.0%, respectively) and SM = 18.9 x creatinine + 4.1 (r = 0.92, P = 2.55 x 10(-5), SEE = 1.89 kg). The validity of each model is reviewed in the context of theoretical aspects of creatine-creatinine metabolism. This first investigation of the method of measuring urinary creatinine excretion to determine SM by using modern techniques raises important practical and basic questions related to SM prediction.

Obesity and risk of gallstone development on a 1200 kcal/d (5025 Kj/d) regular food diet

OBJECTIVE

Previous studies report a greatly increased risk of gallstone formation in obese persons during periods of caloric restriction on very low calorie formula diets. The aim of the present study was to assess the risk of gallstone development in moderately obese patients losing weight on a prescribed 1200 kcal/d (5025 Kj/d) regular food diet.

DESIGN AND SUBJECTS

A consecutive sample of 70 men and women (body mass index > 25 kg/m2) (mean +/- SD, 28.9 +/- 2.8 kg/m2) responding to an announcement of an outpatient weight loss program in a major metropolitan research and teaching hospital were enrolled in a meal replacement program which prescribed 1200 kcal/d (5025 Kj/d) consisting of regular foods with approximately 20 g/d fat (15% of kcal) for 16 weeks.

RESULTS

Participants who completed the study (n = 34) lost a mean of 5.1 +/- 3.6 kg (p < 0.001) (range, +2.7 kg to -12.5 kg; 6.7 +/- 5.0% of body weight, range +/-3.1% to -17.0%; 0.36 +/- 0.25 kg/week) with no clinically significant adverse effects. There were no discernible new gallstones as measured by ultrasonography during the study period. Liver enzyme blood concentrations did not change significantly after 16 weeks except for alkaline phosphatase, which decreased by 4.5 mu/l from a mean initial level of 72.7 mu/l (p < 0.05).

CONCLUSION

Weight loss over 16 weeks on a 1200 kcal/d (5025 Kj/d) regular food/approx 20 g/d fat (15% of kcal) diet was not accompanied by a high rate of gallstone formation in moderately obese persons.

Skeletal muscle mass: evaluation of neutron activation and dual-energy X-ray absorptiometry methods

Although skeletal muscle (SM) is a major body component, whole body measurement methods remain limited and inadequately investigated. The aim of the present study was to evaluate the Burkinshaw in vivo neutron activation analysis (IVNA)-whole body 40K-counting and dual-energy X-ray absorptiometry (DXA) methods of estimating SM by comparison to adipose tissue-free SM measured using multiscan computerized axial tomography (CT). In the Burkinshaw method the potassium-to-nitrogen ratios of SM and non-SM lean tissue are assumed constant; in the DXA method the ratio of appendicular SM to total SM is assumed constant at 0.75. Seventeen healthy men [77.5 +/- 13.8 (SD) kg body wt] and eight men with acquired immunodeficiency syndrome (AIDS; 65.5 +/- 7.6 kg) completed CT, IVNA, and DXA studies. SM measured by CT was 34.4 +/- 6.2 kg for the healthy subjects and 27.2 +/- 4.0 kg for the AIDS patients. Compared with CT, the Burkinshaw method underestimated SM by an average of 6.9 kg (20.1%, P = 0.0001) and 6.3 kg (23.2%, P = 0.01) in the healthy men and the men with AIDS, respectively. The DXA method minimally overestimated SM in both groups (2.0 kg and 5.8% in healthy men, P = 0.001; 1.4 kg and 5.1% in men with AIDS, P = 0.16). This overestimate could be explained by a higher actual than assumed ratio of DXA-measured appendicular SM to total body SM (actual = 0.79 +/- 0.05, assumed = 0.75). The current study results reveal that large errors are present in the Burkinshaw SM method and that substantial refinements in the models that form the basis of this IVNA approach are needed. The model on which the DXA-SM method is based also needs further minor refinements, but this is a promising in vivo approach because of less radiation exposure and lower cost than the IVNA and CT methods.

A comparative study of different means of assessing long-term energy expenditure in humans

We compared three independent techniques for measurement of total energy expenditure (TEE) in human subjects: 1) weight-maintaining energy intake (dietTEE), 2) 24-h chamber calorimetry (chamberTEE), and 3) differential elimination rates 2H2O and H(2)18O (isotopeTEE). Twenty-three healthy adult in-patients [19 never obese (NO), 2 obese (OB), and 2 formerly-obese (RO); 9 female, 14 malel] ingested a liquid formula diet (40% of calories as fat, 45% carbohydrate, 15% protein), the volume of which was adjusted until body weight was stable for at least 14 days. Body composition was then determined by hydrodensitometry, isotope dilution, and dual photon beam absorptiometry (DXA). The thermic effect of feeding (TEF) and resting energy expenditure [REE; measured before arising (dietREE) and after arising (chamberREE)] were determined by indirect calorimetry. Non-resting energy expenditure (NREE) was calculated as NREE = TEE - (REE + TEF). Subjects then gained or lost 10% of their body weight and were restudied as described above. All measures of TEE were significantly correlated (dietTEE vs. chamberTEE r2 = 0.75; dietTEE vs. isotopeTEE r2 = 0.88; isotopeTEE vs. chamberTEE r2 = 0.73; P < 0.0001). ChamberTEE (mean +/- SE = 2,107 +/- 64 kcal/day) was approximately 20% lower than either dietTEE (2,536 +/- 94 kcal/day, P < 0.0001) or isotopeTEE (2,564 +/- 83 kcal/day, P < 0.0001). When data were normalized to metabolic mass, weight gain of 10% was associated with significant increases in dietTEE (P < 0.005) and isotopeTEE (P < 0.05) but not chamberTEE; weight loss of 10% was associated with significant reductions in dietTEE (P < 0.005) and isotopeTEE (P < 0.05) but not chamberTEE. We conclude that measures of energy expenditure obtained in a highly controlled environment by caloric titration (dietTEE) or differential excretion rates of 2H2O and H(2)18O (isotopeTEE) are not significantly different and that measurements of TEE obtained in a respiratory chamber (chamberTEE) are significantly lower than dietTEE or isotopeTEE, probably largely due to limitations on physical activity in the chamber.

Assortative mating for relative weight: genetic implications

Most work on the genetics of relative weight has not considered the role of assortative mating, i.e., mate selection based on similarity between mates. We investigated the extent to which engaged men and women in an archival longitudinal database were similar to each other in relative body weight prior to marriage and cohabitation. After controlling for age, a small but statistically significant mate correlation was found for relative weight (r=.13, p=.023), indicating some assortative mating. Furthermore, we examined whether mate similarity in relative weight prior to marriage predicts survival of the marriage. No significant effects were found. In sum, these results are consistent with those of other studies in suggesting that there is a small but significant intermate correlation for relative weight. However, they are unique in showing that these results cannot be explained on the basis of (a) cohabitation, (b) age similarity, or (c) selective survival of marriages between couples more similar in relative weight. The implications of these findings for heritability studies, linkage studies, and the estimation of shared environmental effects are discussed.

How useful is body mass index for comparison of body fatness across age, sex, and ethnic groups?

This study tested the hypothesis that body mass index (BMI) is representative of body fatness independent of age, sex, and ethnicity. Between 1986 and 1992, the authors studied a total of 202 black and 504 white men and women who resided in or near New York City, were ages 20-94 years, and had BMIs of 18-35 kg/m2. Total body fat, expressed as a percentage of body weight (BF%), was assessed using a four-compartment body composition model that does not rely on assumptions known to be age, sex, or ethnicity dependent. Statistically significant age dependencies were observed in the BF%-BMI relations in all four sex and ethnic groups (p values < 0.05-0.001) with older persons showing a higher BF% compared with younger persons with comparable BMIs. Statistically significant sex effects were also observed in BF%-BMI relations within each ethnic group (p values < 0.001) after controlling first for age. For an equivalent BMI, women have significantly greater amounts of total body fat than do men throughout the entire adult life span. Ethnicity did not significantly influence the BF%-BMI relation after controlling first for age and sex even though both black women and men had longer appendicular bone lengths relative to stature (p values < 0.001 and 0.02, respectively) compared with white women and men. Body mass index alone accounted for 25% of between-individual differences in body fat percentage for the 706 total subjects; adding age and sex as independent variables to the regression model increased the variance (r2) to 67%. These results suggest that BMI is age and sex dependent when used as an indicator of body fatness, but that it is ethnicity independent in black and white adults.

A multicenter comparison of dual-energy X-ray absorptiometers: in vivo and in vitro measurements of bone mineral content and density

As part of a multicenter research project, we compared dual-energy X-ray absorptiometry (DXA) instruments at three research centers (sites 1, 2 and 3) to determine both intra- and intersite variability of bone mineral content (BMC), bone mineral density (BMD), and bone area (BA). Scans of the total body and lumbar spine were performed in duplicate on five humans (in vivo), and scans of the total body were performed on two whole body phantoms with artificial skeletons and thickness overlays (in vitro) at all sites over 15 days. The average intrasite variability in two consecutive total body BMD measurements, expressed as a percent difference, was significantly higher in vitro, 1.74 +/- 1.97%, than in vivo, 0.71 +/- 0.38% (p < 0.05). Average intrasite variability of the in vivo lumbar spine BMD was 1.08 +/- 1.12%. The intersite coefficients of variation for all BMD, BMC, and BA measurements were < 2.0%. The total body BMD from site 2 was systematically lower than at sites 1 and 3 both in vivo and in vitro (p < 0.05) with no differences in BMC and BA. Although significant, the total body BMD differences between sites were small (< 1.2%) in vivo compared with in vitro (< 2.6%) and are encouraging for the comparison and pooling of human data from multicenter trials, provided that appropriate standardized cross-calibration and analysis procedures are applied.