Biomedical subjects

M S Faith

Publications and source records attributed to M S Faith.

18 recordsLinked to original sources

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↗

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↗

PubMed1997

Issues in mapping genes for eating disorders.

Recent twin studies show that both genetic and environmental factors contribute to the development of eating disorders. As in many other fields, there is much enthusiasm regarding the possibility of locating the specific genes that influence the risk of eating disorders. Advances in molecular and statistical technology have made this task more feasible than it was in the past, and continued enhancements in new technology are expected in the future. Despite these advances, the resources required to map a gene for traits as complex as eating disorders are likely to be enormous. Researchers considering such an undertaking may wish to look for ways to reduce this demand, such as (1) using multivariate analyses, (2) studying intermediate quantitative phenotypes, (3) using large sibships, (4) analytic enhancements (e.g., multipoint analyses), (5) reconceptualizing power, (6) data pooling, and (7) disequilibrium mapping.

Chromosome Mapping↗

PubMed1996

Hypnosis as an adjunct to cognitive-behavioral psychotherapy for obesity: a meta-analytic reappraisal.

I. Kirsch, G. Montgomery, and G. Sapirstein (1995) meta-analyzed 6 weight-loss studies comparing the efficacy of cognitive-behavior therapy (CBT) alone to CBT plus hypnotherapy and concluded that "the addition of hypnosis substantially enhanced treatment outcome" (p.214). Kirsch reported a mean effect size (expressed as d) of 1.96. After correcting several transcription and computational inaccuracies in the original meta-analysis, these 6 studies yield a smaller mean effect size (.26). Moreover, if 1 questionable study is removed from the analysis, the effect sizes become more homogeneous and the mean (.21) is no longer statistically significant. It is concluded that the addition of hypnosis to CBT for weight loss results in, at most, a small enhancement of treatment outcome.

Cognitive Behavioral Therapy↗

PubMed1996

On estimating the minima of BMI-mortality curves.

OBJECTIVE: To identify the 'optimal' BMI, that is, the BMI associated with minimal mortality, researchers frequently fit a quadratic function to data and identify the nadir of the BMI-mortality curve. However, Waaler (1984)8 has argued that this approach systematically overestimates the optimal BMI because the true curve might not be quadratic. The purpose of this study was to test this proposition. DESIGN: We simulated 25 decidedly nonquadratic curves in which the true minimum corresponded to a BMI of 24. RESULTS: When fitting a quadratic model to this data and solving for the nadir of the curve, the estimated optimal BMI was 21.14 (SD = 0.586). CONCLUSIONS: It is concluded that there is no way of knowing a priori whether the BMI associated with minimum mortality will systematically overestimate, underestimate, or estimate in an unbiased manner the true optimal BMI when the true model underlying the data is not known.

Body Mass Index↗

PubMed1996

Publication bias in obesity treatment trials?

OBJECTIVE: The present investigation examined the extent of publication bias (namely the tendency to publish significant findings and file away non-significant findings) within the obesity treatment literature. DESIGN: Quantitative literature synthesis of four published meta-analyses from the obesity treatment literature. Interventions in these studies included pharmacological, educational, child, and couples treatments. METHODS: To assess publication bias, several regression procedures (for example weighted least-squares, random-effects multi-level modeling, and robust regression methods) were used to regress effect sizes onto their standard errors, or proxies thereof, within each of the four meta-analysis. A significant positive beta weight in these analyses signified publication bias. RESULTS: There was evidence for publication bias within two of the four published meta-analyses, such that reviews of published studies were likely to overestimate clinical efficacy. The lack of evidence for publication bias within the two other meta-analyses might have been due to insufficient statistical power rather than the absence of selection bias. CONCLUSIONS: As in other disciplines, publication bias appears to exist in the obesity treatment literature. Suggestions are offered for managing publication bias once identified or reducing its likelihood in the first place.

Bias↗

PubMed1996

Risch's lambda values for human obesity.

OBJECTIVE: Risch's lambda statistic (lambda R) is related to the heritability of traits and can be useful in several contexts, including the conduct of power analyses to determine sample size for gene mapping studies. However, values of lambda R have not been presented for human obesity. DESIGN AND RESULTS: Using both analytic and empirical approaches, the present study calculates estimates of lambda R. Examples are provided to illustrate the use of these estimates for determining sample size for genetic mapping studies.

Adolescent↗

PubMed1993

The role of body image in sexually avoidant behavior.

"Spectatoring" refers to a cognitive self-absorption, wherein individuals fixate on and carefully monitor personal body parts and/or the adequacy of personal sexual functioning. To examine this process within a university population, undergraduate and graduate students (108 male and 140 female) filled out questionnaires that assessed body image, sexual knowledge, global sexual attitudes (i.e., liberal-conservative), general psychological adjustment, and frequency of sexual behaviors. Multiple regression analyses were used to determine if spectatoring, operationalized by measures of body image, would significantly predict sexually avoidant behavior. Results indicated that body image scores significantly predicted frequency of sexual behaviors for both genders, while general sexual knowledge and psychological adjustment did not predict sexual behavior. Overall, sexual attitude scores were the best predictors of sexual approach/avoidance behaviors for both genders. Implications are drawn for future research using the assessment of more global sex attitudes in the study of spectatoring.

Adaptation, Psychological↗

PubMedDate not supplied

Body weight and health care among women in the general population.

OBJECTIVE: To examine the relation between body mass index ([BMI] calculated as weight in kilograms divided by the square of height in meters) and the use of medical care services among a nationally representative sample of women. DESIGN AND SETTING: Multistage cluster-area probability sampling survey. Data are from the Cancer Control and Health Insurance supplements of the 1992 National Health Interview Survey conducted by the National Center for Health Statistics. Respondents were 6981 women aged 18 years or older residing in the United States who self-reported sociodemographic information and the use of health care services. MAIN OUTCOME MEASURES: Interval (< or = 3 years vs > 3 years) since most recent mammography, clinical breast examination, gynecologic examination, and Papanicolaou smear and the number of physician visits in the year before the survey. RESULTS: When age, race, income, education, smoking, and health insurance status were adjusted for, the BMI was directly related to delaying clinical breast examinations, gynecologic examinations, and Papanicolaou smears. Obese women (BMI of 35) were more likely than nonobese women (BMI of 25) to delay clinical breast examinations (odds ratio, 1.26; 95% confidence interval, 1.00-1.58), gynecologic examinations (odds ratio, 1.39; 95% confidence interval, 1.15-1.69), and Papanicolaou smears (odds ratio, 1.29; 95% confidence interval, 1.04-1.58). The BMI was not significantly related to delays in mammography. It was also related to increased physician visits (P = .001). CONCLUSION: Among women, an increased BMI is associated with decreased preventive health care services, which may exacerbate or even account for some of the increased health risks of obesity.

Adult↗