Biomedical subjects

M S Faith

Publications and source records attributed to M S Faith.

At least 19 recordsLinked to original sources

PubMed1999

Meta-analysis of the effect of excluding early deaths on the estimated relationship between body mass index and mortality.

OBJECTIVES: Prospective cohort studies typically observe U- or J-shaped relationships between body mass index (BMI) (kg/m2) and mortality. However, some studies suggest that the elevated mortality at lower BMIs is due to confounding by pre-existing occult disease and recommend eliminating subjects who die during the first several (k) years of follow-up. This meta-analysis tests the effects of such early death exclusion on the BMI-mortality association. RESEARCH METHODS AND PROCEDURES: Studies identified from MEDLINE, review articles, ancestry analyses, and the "invisible college." INCLUDED STUDIES: 1) measured relative body weight at baseline; 2) included at least 1000 subjects; 3) reported results with and without early-death exclusion, or relevant data; and 4) did not study exclusively diseased populations. Blank tables were mailed to 131 investigators covering 59 databases. Completed tables (n = 16 databases), electronic raw data (n = 7 databases), and original articles (n = 6 databases) provided final data. Meta-analytic regressions compared the BMI-mortality association with and without early death exclusion. The sample included 29 studies and 1,954,345 subjects. RESULTS: The effect of eliminating early deaths was statistically significant but minuscule in magnitude. Implementation of early death exclusion was estimated to shift the BMI associated with minimum mortality only 0.4 units for men and 0.6 units for women at age 50. Even at a BMI 16, the estimated relative risk (compared to BMI 25) decreased only 0.008 units for men and 0.076 units for women at age 50. DISCUSSION: Results indicate that either pre-existing disease does not confound the BMI-mortality association or eliminating early deaths is inefficient for reducing that confounding.

Adolescent↗

PubMed1999

Is the prevalence of successful weight loss and maintenance higher in the general community than the research clinic?

OBJECTIVE: The prevalence of successful weight loss remains unclear. In 1982, Schachter concluded that in the general population, the rate of "self-cured" obesity approached 63%-much higher than the rate from clinical trials. Several subsequent studies have addressed this issue. RESEARCH METHODS AND PROCEDURES: Our initial goal was to meta-analyze these studies to evaluate the validity of the original hypotheses and the extent to which additional investigations supported the findings. We began by restating Schachter's hypotheses in precise, testable terms. RESULTS: A systematic review of these studies found many methodological limitations and much heterogeneity among the samples studied, hypotheses addressed, and operational definitions. Some of these limitations appear to stem from the lack of clear, precise statements of the exact hypotheses tested. Differences among studies are delineated, and we outline why meta-analytic pooling of these data appears inappropriate. CONCLUSIONS: The current data are inadequate to draw any definite conclusions regarding the cure rate of obesity. Criteria for the adequate study of success rates with "self-cure" in the general population are proposed.

Female↗

PubMed1999

Simulation study of the effects of excluding early deaths on risk factor-mortality analyses in the presence of confounding due to occult disease: the example of body mass index.

PURPOSE: Estimating the effects of continuous chronic disease risk factors on mortality is an area that generates confusion and controversy. The frequently observed U-shaped or J-shaped relationships between the risk factors and mortality are often in contrast with presumed monotone relationships. Therefore, some investigators suggest that subjects dying during the first k years of follow-up (where k is some positive number less than the total length of follow-up) be excluded from statistical analyses. The rationale for this approach is that subjects dying during the first k years of follow-up are likely to have some pre-existing occult disease that confounds the relationship between the risk factors and mortality. Excluding such subjects purportedly reduces bias due to this confounding. The purpose of this study was to test the effects of excluding subjects who die during the first k years of follow-up on the reduction of bias under a variety of situations. METHODS: Using body mass index (BMI; kg/m2) as an example, we conducted Monte Carlo simulations to investigate such effects. RESULTS: Results suggest that under the conditions investigated, the method of excluding early deaths does not reliably or substantially reduce bias due to confounding introduced by occult disease. CONCLUSION: Excluding subjects dying during the first k years of follow-up may not be a judicious strategy for handling confounding due to occult disease. Investigators are encouraged to develop alternative methods.

Bias↗

PubMed1999

Evidence for genetic influences on human energy intake: results from a twin study using measured observations.

Human obesity is associated with greater-than-average energy intake, although relatively few studies have tested the heritability of food intake. The present study examined the genetic architecture of measured caloric intake during laboratory test meals in 36 monozygotic and 18 dizygotic twin pairs. A series of analyses tested the hypotheses that (1) there would be a genetic influence on total caloric intake, (2) there would be genes influencing total caloric intake above and beyond those influencing body composition, (3) there would be a phenotypic association between total caloric intake and fat mass above and beyond any genetic influences, and (4) there would be genetic influences on macronutrient intake (i.e., fat, carbohydrate, and protein intake) above and beyond total caloric intake. Results suggested genetic influences on age- and sex-adjusted total caloric intake (24-33% of the variance), although 95% confidence intervals were wide and suggested that "true" heritability estimates might be considerably lower or higher. Caloric intake was influenced by both common and unique environmental factors. Greater-than-average caloric intake was associated with increased adiposity, despite probable genetic influences on both phenotypes. Finally, there was evidence for macronutrient-specific familial influences, although the extent to which they were genetic or environmental in origin could not be teased apart. Results suggest that human obesity may be influenced by behaviors that are themselves genetically regulated. However, further studies are needed to obtain more precise heritability estimates and a better understanding of the conditions under which genetic influences on energy intake emerge.

Adult↗

PubMed1999

Weight loss increases and fat loss decreases all-cause mortality rate: results from two independent cohort studies.

OBJECTIVE: In epidemiological studies, weight loss is usually associated with increased mortality rate. Contrarily, among obese people, weight loss reduces other risk factors for disease and death. We hypothesised that this paradox could exist because weight is used as an implicit adiposity index. No study has considered the independent effects of weight loss and fat loss on mortality rate. We studied mortality rate as a function of weight loss and fat loss. DESIGN: Analysis of 'time to death' in two prospective population-based cohort studies, the Tecumseh Community Health Study (1890 subjects; 321 deaths within 16y of follow-up) and the Framingham Heart Study (2731 subjects; 507 deaths within 8y of follow-up), in which weight and fat (via skinfolds) loss were assessable. RESULTS: In both studies, regardless of the statistical approach, weight loss was associated with an increased, and fat loss with a decreased, mortality rate (P < 0.05). Each standard deviation (s.d.) of weight loss (4.6 kg in Tecumseh, 6.7 kg in Framingham) was estimated to increase the hazard rate by 29% (95% confidence interval CI), (14%, 47%, respectively) and 39% (95% CI, 25%, 54% respectively), in the two samples. Contrarily, each s.d. of fat loss (10.0 mm in Tecumseh, 4.8 mm in Framingham) was estimated to reduce the hazard rate 15% (95% CI, 4%, 25%) and 17% (95% CI, 8%, 25%) in Tecumseh and Framingham, respectively. Generalisability of these results to severely (that is, body mass index BMI) > or = 34) obese individuals is unclear. CONCLUSIONS: Among individuals that are not severely obese, weight loss is associated with increased mortality rate and fat loss with decreased mortality rate.

Adipose Tissue↗

PubMed1999

Regional skeletal muscle measurement: evaluation of new dual-energy X-ray absorptiometry model.

Although there is growing interest in studying muscle distribution, regional skeletal muscle (SM) mass measurement methods remain limited. The aim of the present study was to develop a new dual-energy X-ray absorptiometry (DEXA) model for estimating regional adipose tissue-free skeletal muscle mass (AT-free SM). Relationships were derived from Reference Man data between tissue-system- level components (i.e., AT-free SM, AT, skeleton, and skin) and molecular-level components including fat-free soft tissue, fat, and bone mineral. The proposed DEXA-SM model was evaluated by multiscan computerized axial tomography (CT). Twenty-seven male subjects [age, 36 +/- 12 (SD) yr; body mass, 73.2 +/- 12.4 kg; 20 were healthy, and 7 had acquired immunodeficiency syndrome] completed DEXA and CT studies. Identical landmarks for DEXA and CT measurements were selected in three regions, including calves, thighs, and forearms. There was a strong correlation for AT-free SM estimates between the new DEXA and CT methods (e.g., sum of three regions, r = 0.86, P < 0.001). Regional AT-free SM measured in the 27 subjects by DEXA and CT, respectively, were 3.44 +/- 0.60 and 3. 47 +/- 0.55 kg (difference 0.9%, P > 0.05) for calves, 10.49 +/- 1. 77 and 10.05 +/- 1.79 kg (difference 4.4%, P < 0.05) for thighs, 1. 36 +/- 0.49 and 1.20 +/- 0.41 kg (difference 13.3%, P < 0.01) for forearms, and 15.29 +/- 2.33 and 14.72 +/- 2.33 kg (difference 3.9%, P < 0.05) for the sum all three regions. Although the suggested DEXA-SM model needs minor refinements, this is a promising in vivo approach for measurement of regional SM, because DEXA is widely available, relatively inexpensive, and radiation exposure is low.

Absorptiometry, Photon↗

PubMed1999

Evidence for independent genetic influences on fat mass and body mass index in a pediatric twin sample.

OBJECTIVE: Insight into genetic and environmental influences on fat mass, independent of body mass index (BMI; kg/m2), is expected to enhance methods for treating pediatric obesity. However, few studies have estimated the heritability of fat mass in pediatric samples, and those conducted have relied primarily on BMI measurements. PRESENT STUDY: Using bioimpedance analysis, the present study tested a series of hypotheses predicting significant genetic and environmental influences on percent body fat (PBF) above and beyond BMI. Subjects were 66 pairs of twins, including 41 monozygotic and 25 dizygotic pairs, from 3 to 17 years of age. Structural equation modeling tested hypotheses, adjusting for demographic variables. RESULTS: Analyses indicated significant genetic influences on PBF, with genes estimated to account for 75% to 80% of the phenotypic variation. The remaining variation was attributable to nonshared environmental influences. Multivariate analyses revealed sizable genetic correlations and environmental correlations between BMI and PBF (rg =.74 and re =.67, respectively), suggesting that some genes and environmental experiences influence both phenotypes. However, analyses confirmed genetic and environmental influences on PBF above and beyond BMI. For example, 62.5% of the total genetic variation in PBF was attributable to genes that influenced PBF but not BMI. CONCLUSION: There seems to be a substantial genetic contribution to fat mass distinct from BMI in a sample of children and adolescents. Studies testing putative genetic or environmental determinants of pediatric obesity might be strengthened further by including research-based body composition methods.

Adipose Tissue↗

PubMed1999

Childhood obesity: results of a multicenter study of obesity treatment in Italy.

The prevalence of pediatric obesity is increasing and many patients are followed by specialized centers or private doctors. The aim of this study was to verify short- and medium term results of a therapeutic approach based on nutritional intervention in a large pediatric population: 1383 subjects (695 females, 688 males) aged 10.1 +/- 2.7 yr, followed in 11 pediatric departments in Italy. No difference was found between centers in age, height, weight, BMI and IBW. The drop-out rate after the first visit was 30.2% (58.1% IBW > 140%) in females and 34.2% (70.7% IBW > 140%) in males. After two years of follow-up only 9.7% of females and 6.4% of males remained on treatment. Of these patients only 7.3% of females and 6.4% of males had IBW < 120%. These data show that an approach based on nutritional intervention alone is not sufficient for long-term treatment of pediatric obesity. Only an approach started early and involving the family can produce permanent results.

Adolescent↗

PubMed1998

Power and sample size for survival analysis under the Weibull distribution when the whole lifespan is of interest.

Accessible and readily utilized software, tables and approximation formulae have been developed to estimate power and sample size for studies of time to event (survival times) when the survival times are assumed to be exponential. These methods can markedly misestimate power when the distribution is Weibull and not exponential. The Weibull distribution with increasing hazard is common in aging research, especially when the whole life span of the subjects is of interest. This note considers an extension of power and sample size calculations, previously developed under the exponential distributional assumption, to the more general case of the Weibull distribution for a prospective comparative follow-up study. The hypotheses are defined in terms of the ratio of the median survival times between two groups. It is shown that the power and sample sizes are heavily dependent on the shape parameter of the Weibull distribution. Using the extensions developed, investigators can use existing software and tables to calculate power and sample size under the assumption of a Weibull distribution.

Algorithms↗

PubMed1998

Demand characteristics of the research setting can influence indexes of negative affect-induced eating in obese individuals.

Measures of negative affect-induced eating (NAIE) are common in clinical research with obese individuals. However, previous studies suggest that measures of NAIE can be induced by social desirability tendencies or demand characteristics of the research setting. Using an experimental design, the present study tested the effects of demand characteristics of the research setting on self-report and behavioral indexes of NAIE. Obese and nonobese subjects (n=107) were randomly assigned to an experimental or control group. Experimental subjects received a lecture indicating an association between emotional eating and obesity; control subjects received no information. Outcome measures were indexes of NAIE from a questionnaire, food diaries, and food intake during a laboratory "taste test." We hypothesized that the association between relative body mass and NAIE would be stronger among subjects receiving the lecture manipulation than control subjects. Results indicated that NAIE indexes, particularly from food diaries, were influenced modestly by research demand characteristics. We conclude that demand characteristics of the research setting can affect indexes of NAIE among obese persons.

Adolescent↗

PubMed1998

Prader-Willi syndrome: relationship of adiposity to plasma leptin levels.

OBJECTIVE: Prader-Willi syndrome (PWS) is an autosomal dominant disorder involving the proximal long arm of chromosome 15, in which obesity is common. However, there is limited information on the underlying physiological mechanisms promoting obesity in this population. We tested whether there was a significant positive association between leptin and total body fat (TBF) in subjects with PWS, and whether this association was stronger among subjects with than without PWS. RESEARCH METHODS AND PROCEDURES: We studied 21 PWS patients and 64 non-PWS controls on whom we measured serum leptin, total body fat, glucose, insulin, and resting energy expenditure. We tested whether the slope of the regression line between leptin and TBF (in kg), measured by dual energy X-ray absorptiometry, was the same for PWS patients and non-PWS controls. RESULTS: Regression analyses indicated that the leptin-TBF association was significantly stronger among PWS patients. In contrast, the slope of the leptin-body mass index association did not significantly differ between PWS patients and non-PWS controls. None of the other outcome variables showed associations with leptin. DISCUSSION: Results suggest that the role of leptin in promoting obesity may be greater among subjects with PWS than among non-PWS controls.

Absorptiometry, Photon↗

PubMed1998

Relative body weight and self-esteem among African Americans in four nationally representative samples.

OBJECTIVE: Obesity is an increasingly common health problem among African Americans, especially women, in the United States. However, limited data are available on the psychological correlates of obesity in this population. This study examined the association between self-esteem and relative body weight (RBW) in four large nationally representative samples of African American individuals. RESEARCH METHODS AND PROCEDURES: Data from The Adolescent Health Care Evaluation Study, The National Longitudinal Survey of Youth, The High School and Beyond, and The National Survey of Black Americans were analyzed. Within each database, regression analyses tested the association between RBW and self-esteem while adjusting for age and sex. RESULTS: In three of the four databases, there was no significant association between RBW and self-esteem. In the only database detecting a statistically significant effect, the magnitude of the effect was small. The combined effects of RBW and its interaction with age and sex accounted for <2% of the variance in self-esteem across databases. DISCUSSION: Results suggest that elevated RBW is not associated with a poorer general self-concept, on average, among African American individuals.

Adolescent↗

PubMed1998

Body mass index as a measure of adiposity among children and adolescents: a validation study.

OBJECTIVES: To test the hypothesis that in a healthy pediatric population body mass index (BMI) (kilograms per meter square) is a valid measure of fatness that is independent of age for both sexes. METHODS: Total body fat (TBF) (in kilograms) and percent of body weight as fat (PBF) were estimated by dual energy x-ray absorptiometry (DXA) in 198 healthy Italian children and adolescents between 5 and 19 years of age. We developed multiple regression analysis models with TBF and percent body fat as dependent variables and BMI, age, and interaction terms as independent variables. Separate analyses were conducted for boys and girls. RESULTS: BMI was strongly associated with TBF (R2 = 0.85 and 0.89 for boys and girls, respectively) and PBF (R2 =0.63 and 0.69 for boys and girls, respectively). Confidence limits on BMI-fatness association were wide, with individuals of similar BMI showing large differences in TBF and in PBF. Age was a significant covariate in all regression models. Addition of nonlinear terms for BMI did not substantially increase the R2 for TBF and PBF models in boys and girls. CONCLUSION: Our results support the use of BMI as a fatness measure in groups of children and adolescents, although interpretation should be cautious when comparing BMI across groups that differ in age or when predicting a specific individual's TBF or PBF.

Absorptiometry, Photon↗

PubMed1998

Meta-analysis of the association of the Trp64Arg polymorphism in the beta3 adrenergic receptor with body mass index.

OBJECTIVE: As a result of efforts to isolate obesity-promoting genes, the Trp64Arg polymorphism in the beta3 adrenergic receptor locus, has been studied by many investigators. Results of the studies have varied in statistical significance and magnitude of the association of the polymorphism with body mass index (BMI: kg/m2). This has led to controversy about whether this polymorphism is associated with meaningful changes in BMI. To clarify the possible association, we conducted a meta-analysis. DESIGN: Meta-analytic study. MEASUREMENTS: For each genotype of the beta3 adrenergic receptor (Trp/Trp; Trp/Arg; Arg/Arg), we extracted the number of subjects, mean and standard deviation of BMI from 23 studies, including 36 different subgroups with a total of 7399 subjects. Other indices and obesity-related variables were not considered. RESULTS: No significant association of the Trp64Arg polymorphism with BMI was found. The weighted mean BMI difference between Trp/Trp homozygotes and Trp/Arg heterozygotes was 0.19 (s.e. = 0.11; P = 0.07). In addition, the distribution of effect sizes was not significantly heterogeneous (chi2=38.68; df 35; P = 0.31) suggesting that the variation of the effect sizes across the subgroups is not significant. A further weighted regression analysis, utilizing all three genotypes and adjusting for the random subgroup effect, also showed the effect of the polymorphism on BMI is not significant (F = 1.72, df = (2,54), P = 0.19). CONCLUSION: Based on existing data, the Trp64Arg polymorphism does not appear to be significantly associated with BMI. Moreover, we found no evidence for effect heterogeneity, suggesting that the effect of the polymorphism is not moderated by ethnicity or diabetic status.

Arginine↗

PubMed1997

Examination of "early mortality exclusion" as an approach to control for confounding by occult disease in epidemiologic studies of mortality risk factors.

Methods for the estimation of the effects of chronic disease risk factors on mortality continue to be an area that generates confusion and controversy. In response to the frequently observed U- or J-shaped relations between risk factors and mortality, some authors suggest that subjects dying during the first k years of follow-up (where k is some positive number less than the total length of follow-up) be excluded from statistical analyses. By excluded, the authors mean completely removed from the data set. The rationale is that persons dying during the first k years are likely to have a preexisting occult disease that confounds the relation between the risk factor under study and mortality. Excluding persons dying during the first k years of follow-up purportedly reduces this confounding. However, the authors are aware of no demonstration that this procedure effectively accomplishes its goal. They show that excluding subjects who die during the first k years of follow-up does not necessarily lead to a reduction in bias in the estimated effect of a risk factor on mortality when this relation is confounded by the presence of occult disease. Moreover, it is possible for such exclusion to exacerbate the confounding due to preexisting disease. Thus, excluding subjects dying during the first k years of follow-up is not necessarily an effective strategy for dealing with confounding due to occult disease. Investigators are encouraged to pursue alternative methods.

Confounding Factors, Epidemiologic↗

PubMed1997

Hypothesis concerning the U-shaped relation between body mass index and mortality.

Numerous studies have documented a U- or J-shaped association between body mass index (BMI) (kg/m2) and mortality, such that increased mortality rate is associated with relatively low and high BMI values. It has been argued elsewhere that the elevated mortality rate observed at lower BMI values actually results from the effects of unmeasured confounding variables, in particular smoking status and preexisting disease. In this paper, the authors present an additional explanation for the phenomenon, i.e., nonspecific measurement. They propose that differential health consequences of fat mass and fat-free mass can be masked by the use of BMI when studied in relation to mortality. To illustrate this point, they use body composition data from 1,137 healthy adults and specify a hypothetical underlying BMI-mortality model in which the logit of death increased linearly with fat mass and decreased linearly with fat-free mass, and % fat increased monotonically with BMI. The results indicate that, even under these specifications, the authors can recover a U-shaped association between BMI and mortality. Consistent with previous suggestions in the literature, future epidemiologic studies that examine the association between adiposity and mortality should prioritize the use of body composition measures.

Adolescent↗

PubMed1997

Putting the behavior into the behavior genetics of obesity.

Tremendous advances in the genetic underpinnings of obesity have emerged in recent years. Curiously, behavioral genetic methods have provided relatively less information on the environmental influences and intermediary behaviors which promote human obesity. This situation in unfortunate since human obesity is, in part, environmentally determined and the result of behaviors such as eating and physical (in)activity. This article has several goals. First, it outlines reasons why behavior qua behavior should be a specific focus of obesity-oriented behavioral genetic designs. Second, possible explanations for why behavior has been underinvestigated are explored. Third, data regarding the genetic/ environmental architecture of various obesity-related phenotypes (e.g., food intake, physical activity, etc.) are reviewed. Fourth, a commentary on the importance of gene-environment interactions is offered. Finally, suggestions for future research, including a list of possible "candidate environments" and "candidate intermediary behaviors," are offered.

Adolescent↗

PubMed1997

A proposed heuristic for communicating heritability estimates to the general public, with obesity as an example.

It is well established that many continuously distributed traits have a heritable component. However, it is often difficult to communicate to the general public the meaning of quantitative estimates of heritability. To address this problem, the present paper introduces a heuristic for communicating heritability to nonscientific audiences. This heuristic involves adopting an extremely simplified model of inheritance and artificially (and somewhat arbitrarily) defining a cutoffs of "low environmental risk" and "affectation status." Using body weight and obesity as an example, we present a table which gives estimates of the proportion of obese persons who are "genetically obese" assuming varying levels of "environmental risk" for obesity and relative body weight scores for defining obesity. The resulting statistic may prove useful for lay audiences in understanding a heritability estimate.

Body Mass Index↗