Genetic and Environmental Contributions to BMI in Adolescent and Young Adult Women

The objective of this study was to determine the genetic and environmental contributions to variation in BMI over time in European‐American (EA) and African‐American (AA) adolescent and young adult women. Self‐reported BMI (kg/m²) data from 2,816 EA (1,306 twin pairs, 56.5% monozygotic (MZ)) and 404 AA (178 twin pairs, 42.7% MZ) women at baseline (T1; median age 15 years) and 3,225 EA (1,511 twin pairs, 55.3% MZ) and 539 AA (252 pairs, 43.3% MZ) women at follow‐up (T2; median age 22 years) from a Midwestern US, population‐based twin registry were used to construct biometrical genetic models. For EA women, the majority of the variance in BMI was attributable to additive genetic effects at both time points (82% for each), with the remaining variance attributable to nonshared environment. Genetic and nonshared environment correlations between adolescent and young adult BMI were 0.87 and 0.23, respectively. Among AA women, nonadditive genetic effects comprised 68% of the variance at T1 and 73% at T2, and were highly correlated (r_D = 0.94). The proportions of variance attributable to nonshared environment at T1 (29%) and T2 (25%) were more modestly correlated (r_E = 0.31). The remaining variance in AA women could be attributed to additive genetic effects. Additive vs. nonadditive genetic effects contribute differentially to BMI in AA vs. EA adolescent and young adult women. Additional research is needed to better characterize the environmental and genetic factors related to BMI in persons of different races to aid understanding of the complex determinants of body weight in individuals.     

Effects of Heritability,Shared Environment,and Nonshared Intrauterine Conditions on Child and Adolescent BMI

Heritability studies of BMI, based upon twin samples, have identified genetic and shared environmental components of BMI, but have been largely silent about the nonshared environmental factors. Intrauterine factors have been identified as having significant long‐term effects on BMI and may be a critical source of nonshared environmental influence. Extant studies based on samples of either unrelated individuals or twins cannot separate the effects of genetics, shared environments, and nonshared intrauterine conditions because the one lacks variation in the degree of relatedness and the other has insufficient variation in intrauterine conditions. This study improves upon these prior studies by using a large, sibling‐based sample to examine heritability, shared environmental, and nonshared intrauterine influences on BMI during two age periods in childhood (6–8 years; 12–14 years). The primary interest was in determining the effects of the intrauterine environment on BMI as a component of the nonshared environment and in determining whether there were sex‐specific differences in heritability and/or in the intrauterine factors. These were estimated using regression‐based techniques introduced by DeFries and Fulker. Heritability of BMI was estimated to be 0.20–0.28 at 6–8 years and 0.46–0.61 at 12–14 years. Differences in heritability were found at 12–14 years between same‐sex as compared to mixed‐sex pairs. The shared environmental effect was significant at 6–8 years but insignificant at 12–14 years. Differences in birth weight were significant in all groups at 6–8 years suggesting long‐term effects of the nonshared intrauterine environment; at 12–14 years, birth weight was no longer significant for girls.     

Common genetic components of obesity traits and serum leptin

To estimate common and distinct genetic influences on a panel of obesity-related traits and serum leptin level in adults. In a cross-sectional study of 625 Danish, adult, healthy, monozygotic, and same-sex dizygotic twin pairs of both genders, we carried out detailed anthropometry (height, weight, waist and hip, and skin-fold thickness, body composition assessment by bioimpedance (fat mass and fat-free mass), and measurement of serum leptin level. Bivariate variance component analyses estimated the additive genetic correlations between these measurements. The genetic correlations between the traits for overall fatness (BMI and fat mass index, kg/m(2)) were 0.94 in men and 0.98 in women, and their correlations with the various local fatness measures ranged from 0.49 to 0.83 in men and from 0.70 to 0.87 in women. The correlations between the truncal measures (waist circumference and truncal skin folds) and between the peripheral measures (hip circumference and peripheral skin folds) were 0.57 and 0.47 in men and 0.71 and 0.70 in women, respectively. The correlations between the truncal and peripheral measures ranged between 0.49 and 0.72 in men and between 0.61 and 0.82 in women. For leptin vs. the various measures of overall and local fatness the correlations ranged from 0.54 to 0.74 in men and from 0.48 to 0.75 in women. All correlations were significantly <1.00. The study supports control of overall fat mass and peripheral and truncal fat mass by both shared and different genetic components, which suggests that it is important to distinguish between the different phenotypes in the search for genes involved in the development of obesity.     

The genetics of middle-age spread in middle-class males.

This study provides findings to assist in identifying factors that contribute to the current clinical and public health debate of the obesity epidemic. The study examined the genetics of adult-onset weight change in middle-aged male-male twins controlling for weight in early adulthood, lifetime history of tobacco use and alcohol dependence, and aimed to estimate the proportion of genetic factors that influence weight change between early adulthood and middle age in white middle-class males. The study was a classic longitudinal twin design and used Body Mass Index (BMI) for three waves of data collection from the Vietnam Era Twin Registry--induction physicals (approximately 1968), 1987 and 1990--or periods corresponding between young adulthood and middle age. Univariate heritability estimates for BMI at all three data periods were conducted as well as a Cholesky longitudinal genetic analysis for weight change controlling for BMI at military induction, smoking and alcohol use. Frequency data indicated that the sample was on average classified as normal BMI in their 20s; but BMI gradually increased during the next twenty years. Univariate data for each data period indicated that additive genetic factors accounted for between 63% and 69% of total variance in BMI. The Cholesky longitudinal genetic analysis of BMI87 and BMI90, controlling for BMI at military induction, indicated that more than half of the change in BMI from early adulthood to middle age remains heritable. No shared environmental factors were identified, thus the remainder of the variance was accounted for by nonshared, or unique, environmental factors and error. The data analysis suggests that treatments and public health interventions need to recognize the magnitude of genetic factors if short-term and long-term interventions are to be effective.     

Sex difference in heritability of BMI in South Korean adolescent twins

Twin studies of BMI on the basis of Asian twins are extremely rare. Eight hundred eighty-eight pairs of twins [279 monozygotic (MZ) and 82 dizygotic (DZ) pairs of male twins, 319 MZ and 82 DZ pairs of female twins, and 126 opposite-sex pairs of DZ twins] completed items concerning height and weight through a mail and a telephone survey. A general sex-limitation model was applied to the data. Heritability estimate was greater among women than among men. However, there was little evidence of sex-specific genes. Under the best-fitting model, additive genetic variances were 82% [95% confidence interval (CI): 72% to 95%] for men and 87% (95% CI: 77% to 99%) for women; shared environmental variances were negligible in both men and women. These estimates of genetic and environmental factors in BMI found among South Korean adolescent twins were broadly in the range of those reported in previous studies of BMI based on Western twin samples.     

Genetic and non-genetic factors affecting birth-weight and adult Body Mass Index.

Birthweight affects neonatal mortality and morbidity and has been used as a marker of foetal undernutrition in studies of prenatal effects on adult characteristics. It is potentially influenced by genetic and environmental influences on the mother, and effects of foetal genotype, which is partially derived from the maternal genotype. Interpretations of variation in birthweight and associated characteristics as being due to prenatal environment ignore other possible modes of materno-foetal transmission. Subjects were adult twins recruited through the Australian Twin Registry, aged 17 to 87 years, and the sample comprised 1820 men and 4048 women. Twins reported their own birthweight as part of a health questionnaire. Body Mass Index (BMI) was calculated from self-reports of height and weight. Correlations between co-twins' birthweights were high for both monozygotic (r = 0.77) and dizygotic (r = 0.67) pairs, leading to substantial estimates of shared environmental effects (56% of variance) with significant additive genetic (23%) and non-shared environmental (21%) components. Adult BMI was mainly influenced by genetic factors, both additive (36% of variance) and nonadditive (35%). The correlation between birthweight and BMI was positive, in that heavier babies became on average more obese adults. A bivariate model of birthweight and adult BMI showed significant positive genetic (r(g) = 0.16, p = 0.005) and environmental (r(e) = 0.08, p = 0.000011) correlations. Intra-uterine environmental or perinatal influences shared by cotwins exercise a strong influence on birthweight, but the factors which affect both birthweight and adult BMI are partly genetic and partly non-shared environmental.     

BMI may overestimate the prevalence of obesity among women of lower socioeconomic status

Objective: Our objective was to examine gender differences in height and weight associated with socioeconomic status (SES) and the consequent effect on body mass index in a multiethnic society. Research Methods and Procedures: A cross‐sectional study, the First Israeli National Health and Nutrition Survey, was performed on a representative population sample of 3246 adults 25 to 64 years of age, between the years 1999 to 2001. Height and weight were measured, and BMI and other weight‐height indices were calculated. SES was assessed by income and education. Results: Age‐adjusted height was significantly lower at lower levels of SES among both women and men (p < 0.001). As opposed to men, women of lower SES were heavier than those of higher SES, and the mean age‐adjusted weight was 4.6 kg higher among those of lower SES (p < 0.001). Thus, using the standard index of BMI, the prevalence of obesity was significantly higher among shorter women. Discussion: In this group of Israeli adults, the unfavorable effect of low SES on BMI was evident among women, partly due to their decreased height combined with increased weight common in this socioeconomic sector. Since BMI is only partly independent of height, it may overestimate the prevalence of obesity among women of lower SES. Alternative measures for classifying obesity in the lower SES groups that put less emphasis on height may be considered and studied.     

Sex differences in heritability of BMI: a comparative study of results from twin studies in eight countries.

Body mass index (BMI), a simple anthropometric measure, is the most frequently used measure of adiposity and has been instrumental in documenting the worldwide increase in the prevalence of obesity witnessed during the last decades. Although this increase in overweight and obesity is thought to be mainly due to environmental changes, i.e., sedentary lifestyles and high caloric diets, consistent evidence from twin studies demonstrates high heritability and the importance of genetic differences for normal variation in BMI. We analysed self-reported data on BMI from approximately 37,000 complete twin pairs (including opposite sex pairs) aged 20-29 and 30-39 from eight different twin registries participating in the GenomEUtwin project. Quantitative genetic analyses were conducted and sex differences were explored. Variation in BMI was greater for women than for men, and in both sexes was primarily explained by additive genetic variance in all countries. Sex differences in the variance components were consistently significant. Results from analyses of opposite sex pairs also showed evidence of sex-specific genetic effects suggesting there may be some differences between men and women in the genetic factors that influence variation in BMI. These results encourage the continued search for genes of importance to the body composition and the development of obesity. Furthermore, they suggest that strategies to identify predisposing genes may benefit from taking into account potential sex specific effects.     

Genetic and Environmental Influences on Body‐Fat Measures among African‐American Twins

Objective: To investigate the genetic and environmental influences on body‐fat measures including waist circumference (WC), waist‐to‐hip ratio (WHR), and body mass index (BMI) among African‐American men and women. Research Methods and Procedures: Measurements were taken as part of the Carolina African American Twin Study of Aging. This sample currently comprises 146 same‐sex African‐American twins with an average age of 50 years (range, 22 to 88 years). This analysis included 26 monozygotic and 29 dizygotic men and 45 monozygotic and 46 dizygotic women. Maximum likelihood quantitative genetic analysis was used. Results: In men, additive genetic effects accounted for 77% of the variance in WC, 59% in WHR, and 89% in BMI. In women, additive genetic effects accounted for 76% of the variance in WC, 56% in WHR, and 73% in BMI. The remaining variance in both men and women was attributed to unique environmental effects (WC, 21%; WHR, 36%; BMI, 11% in men and WC, 22%; WHR, 38%; BMI, 27% in women) and age (WC, 2%; WHR, 5% in men and WC, 2%; WHR, 6% in women). When BMI was controlled in the analysis of WC and WHR, it accounted for a portion of the genetic and environmental variance in WHR and over one‐half of the genetic and environmental variance in WC. Discussion: There are both genetic and environmental influences on WC, WHR, and BMI, and independent of BMI, there are genetic and environmental effects on WC and WHR among both genders. The results from this African‐American twin sample are similar to findings among white twin samples.     

The genetic and environmental effects on depressive symptoms among older female twins.

The aim of the present study was to examine the contribution of genetic and environmental factors to depressive symptoms among older women. The participants were 102 monozygotic and 115 dizygotic female twin pairs aged 64 to 76 years. Depressive symptoms were assessed by the Center for the Epidemiologic Studies Depression Scale. The contribution of genetic and environmental effects was estimated for the constructed depressiveness factor and for the subscales which were depressed mood, psychomotor retardation, lack of wellbeing and interpersonal difficulties. Of the variance in depressiveness, shared environmental influences accounted for 39% and nonshared environmental influences 61%. For the subscales, 24% to 62% of the variance was explained by individual, and 13% to 23% by shared, environmental factors. Lack of wellbeing had its own moderate additive genetic effect explaining 30% of the variance. This study showed that in older women predominantly environmental factors underlay individual differences in depressiveness; however, the factors varied to some extent between dimensions measured by the subscales.     

Relationships Between the Body Mass Index and Body Composition

The aims of this study were to evaluate the Body Mass Index (BMI) (weight/stature²) as a proxy for percent body fat (%BF) and to determine its association with fat-free mass (FFM). Multivariate analysis of variance and partial correlations were used to examine relationships between BMI and %BF and FFM from densitometry for 504 men and 511 women, aged 20 to 45 years. Sensitivity/specificity analyses used cut offs of 28 kg/m² in men and 26 kg/m² in women for BMI, and 25% in men and 33% in women for %BF. Significantly higher associations existed in each gender between BMI and %BF in the upper BMI tertile than in the lower BMI tertiles. In the lower BMI tertiles, correlations between BMI and FFM were approximately twice as large as those between BMI and %BF. The BMI correctly identified about 44% of obese men, and 52% of obese women when obesity was determined from %BF. BMI is an uncertain diagnostic index of obesity. Results of Receiver Operator Characteristic (ROC) analyses using %BF and total body fat, both provided a BMI of 25 kg/m² in men and 23 kg/m² in women as diagnostic screening cut offs for obesity.     

Heritability of adult body height: a comparative study of twin cohorts in eight countries.

A major component of variation in body height is due to genetic differences, but environmental factors have a substantial contributory effect. In this study we aimed to analyse whether the genetic architecture of body height varies between affluent western societies. We analysed twin data from eight countries comprising 30,111 complete twin pairs by using the univariate genetic model of the Mx statistical package. Body height and zygosity were self-reported in seven populations and measured directly in one population. We found that there was substantial variation in mean body height between countries; body height was least in Italy (177 cm in men and 163 cm in women) and greatest in the Netherlands (184 cm and 171 cm, respectively). In men there was no corresponding variation in heritability of body height, heritability estimates ranging from 0.87 to 0.93 in populations under an additive genes/unique environment (AE) model. Among women the heritability estimates were generally lower than among men with greater variation between countries, ranging from 0.68 to 0.84 when an additive genes/shared environment/unique environment (ACE) model was used. In four populations where an AE model fit equally well or better, heritability ranged from 0.89 to 0.93. This difference between the sexes was mainly due to the effect of the shared environmental component of variance, which appears to be more important among women than among men in our study populations. Our results indicate that, in general, there are only minor differences in the genetic architecture of height between affluent Caucasian populations, especially among men.     

Associations of Fat Distribution and Obesity with Hypertension in a Bi‐ethnic Population: The ARIC Study

Objective: To examine associations of hypertension with obesity and fat distribution among African American and white men and women. Research Methods and Procedures: The analysis sample included 15,063 African American and white men and women between the ages of 45 and 64 years who were participants in the 1987 through 1989 examination of the Atherosclerosis Risk in Communities Study (ARIC). Odds ratios and adjusted prevalences of hypertension were calculated across sexspecific quintiles of body mass index (BMI), waist‐to‐hip ratio (WHR), waist circumference, and waist‐to‐height ratio (waist/height) and adjusted for age, research center, smoking, education, physical activity, alcohol consumption, hormone replacement therapy, and menopausal status. Results: The prevalence of hypertension was higher among African Americans than whites. In the lowest quintile of BMI, 41% of African American women and 43% of African American men had hypertension compared with 14% of white women and 19% of white men. Elevated BMI, WHR, waist circumference, and waist/height were associated with increased odds of hypertension in African American and white men and women. In women, but not in men, there were significant interactions between ethnicity and the anthropometric variables studied here. The direction of the interaction indicated larger odds ratios for hypertension with increasing levels of anthropometric indices in white compared with African American women. Discussion: Obesity and abdominal fat preponderance were associated with increased prevalence of hypertension in African American and white men and women. Associations were similar among African American and white men, but obesity and fat patterning were less strongly associated with hypertension in African American than in white women.     

The validity of obesity based on self-reported weight and height: Implications for population studies

Objective: To validate self‐reported information on weight and height in an adult population and to find a useful algorithm to assess the prevalence of obesity based on self‐reported information. Research Methods and Procedures: This was a cross‐sectional survey consisting of 1703 participants (860 men and 843 women, 30 to 75 years old) conducted in the community of Vara, Sweden, from 2001 to 2003. Self‐reported weight, height, and corresponding BMI were compared with measured data. Obesity was defined as measured BMI ≥ 30 kg/m². Information on education, self‐rated health, smoking habits, and physical activity during leisure time was collected by a self‐administered questionnaire. Results: Mean differences between measured and self‐reported weight were 1.6 kg (95% confidence interval, 1.4; 1.8) in men and 1.8 kg (1.6; 2.0) in women (measured higher), whereas corresponding differences in height were ?0.3 cm (?0.5; ?0.2) in men and ?0.4 cm (?0.5; ?0.2) in women (measured lower). Age and body size were important factors for misreporting height, weight, and BMI in both men and women. Obesity (measured) was found in 156 men (19%) and 184 women (25%) and with self‐reported data in 114 men (14%) and 153 women (20%). For self‐reported data, the sensitivity of obesity was 70% in men and 82% in women, and when adjusted for corrected self‐reported data and age, it increased to 81% and 90%, whereas the specificity decreased from 99% in both sexes to 97% in men and 98% in women. Discussion: The prevalence of obesity based on self‐reported BMI can be estimated more accurately when using an algorithm adjusted for variables that are predictive for misreporting.     

Childhood obesity: genetic and environmental overlap with normal-range BMI

Objective: To understand the overlap between the etiology of obesity and normal variation in BMI in children. Methods and Procedures: Height and weight data were available from a large UK representative sample of twins: 2,342 same‐sex pairs at 7 years and 3,526 same‐sex pairs at 10 years. The twin method and model‐fitting techniques were used to estimate genetic and environmental contributions to BMI. DeFries‐Fulker (DF) extremes analysis was used to investigate genetic and environmental influences on the mean difference between obese and normal‐weight children. Obesity was classified using the International Obesity Task Force (IOTF) criteria. Results: At both ages, BMI and obesity were highly heritable (0.60–0.74) and only modestly influenced by shared environmental factors (0.12–0.22). Extremes analyses indicated that genetic and environmental influences on obesity are quantitatively and qualitatively similar to those operating across the range of BMI. Discussion: Obesity is the extreme of the same genetic and environmental factors responsible for variation throughout the distribution of BMI. This finding implies that genes that influence obesity will also be associated with BMI in the normal range, and similar environmental influences will affect BMI in the clinical and normal range. Knowing that obesity is influenced by the same genetic and environmental factors that affect weight at all levels has implications for investigating the mechanisms for weight gain and developing interventions for weight control.     

First nationwide survey of prevalence of overweight, underweight, and abdominal obesity in Iranian adults

Objective: The goal was to estimate the prevalence of overweight, obesity, underweight, and abdominal obesity among the adult population of Iran. Research Methods and Procedures: A nationwide cross‐sectional survey was conducted from December 2004 to February 2005. The selection was conducted by stratified probability cluster sampling through household family members in Iran. Weight, height, and waist circumference (WC) of 89,404 men and women 15 to 65 years of age (mean, 39.2 years) were measured. The criteria for underweight, normal‐weight, overweight, and Class I, II, and III obesity were BMI <18.5, 18.5 to 24.9, 25 to 29.9, 30 to 34.9, 35 to 39.9, and ≥40 (kg/m²), respectively. Abdominal obesity was defined as WC ≥102 cm in men and ≥88 cm in women. Results: The age‐adjusted means for BMI and WC were 24.6 kg/m² in men and 26.5 kg/m² in women and 86.6 cm in men and 89.6 cm in women, respectively. The age‐adjusted prevalence of overweight or obesity (BMI ≥25) was 42.8% in men and 57.0% in women; 11.1% of men and 25.2% of women were obese (BMI ≥30), while 6.3% of men and 5.2% of women were underweight. Age, low physical activity, low educational attainment, marriage, and residence in urban areas were strongly associated with obesity. Abdominal obesity was more common among women than men (54.5% vs. 12.9%) and greater with older age. Discussion: Excess body weight appears to be common in Iran. More women than men present with overweight and abdominal obesity. Prevention and treatment strategies are urgently needed to address the health burden of obesity.     

Socioeconomic status in relation to obesity and abdominal obesity in Korean adults: a focus on sex differences

Objective: We examined the relationship between income and education level with BMI and waist circumference to provide further understanding of the relationship between socioeconomic status and obesity and to identify the presence of sex differences. Research Methods and Procedures: A total of 7962 people ≥20 years of age (3597 men; 4365 women) who participated in the 1998 Korean National Health and Nutrition Examination Survey provided data including height, weight, waist circumference, education, and income level. We examined adjusted BMI and waist circumference according to level of income and education and the association between income and education with obesity and abdominal obesity by multiple logistic regression analysis. Results: In men, significant dose‐response relationships were noted between income and obesity (trend, p < 0.05) and abdominal obesity (trend, p < 0.05). Compared with the lowest income group, the adjusted odds ratios (ORs) (95% confidence interval) of the highest income group for obesity and abdominal obesity were 1.65 (1.18 to 2.32) and 1.37 (0.94 to 1.98), respectively. However, income was not associated with obesity or abdominal obesity in the fully adjusted models in women. With regard to education, women showed significantly decreased ORs, with inverse trends for obesity and abdominal obesity across all education levels. Compared with the lowest education group, the adjusted ORs (95% confidence interval) for obesity and abdominal obesity were 0.66 (0.57 to 0.76) and 0.40 (0.35 to 0.45), respectively, among women with 7 to 12 years of schooling and 0.27 (0.21 to 0.34) and 0.15 (0.12 to 0.18), respectively, among women with 13 or more years of schooling. Discussion: Socioeconomic difference has a considerable impact on the prevalence of obesity among the Korean population, and the patterns differ substantially across sex.     

Underreporting of BMI in adults and its effect on obesity prevalence estimations in the period 1998 to 2001

Objective: To identify the determinants of underreporting BMI and to evaluate the possibilities of using self‐reported data for valid obesity prevalence rate estimations. Research Methods and Procedures: A cross‐sectional monitoring health survey was carried out between 1998 and 2002, and a review of published studies was performed. A total of 1809 men and 1882 women ages 20 to 59 years from The Netherlands were included. Body weight and height were reported and measured. Equations were calculated to estimate individuals’ BMI from reported data. These equations and equations from published studies were applied to the present data to evaluate whether using these equations led to valid estimations of the obesity prevalence rate. Also, size of underestimation of obesity prevalence rate was compared between studies. Results: The prevalence of obesity was underestimated by 26.1% and 30.0% among men and women, respectively, when based on reported data. The most important determinant of underreporting BMI was a high BMI. When equations to calculate individuals’ BMI from reported data were used, the obesity prevalence rate was still underestimated by 12.9% and 8.1% of the “true” obesity prevalence rate among men and women, respectively. The degree of underestimating the obesity prevalence was inconsistent across studies. Applying equations from published studies to the present data led to estimations of the obesity prevalence varying from a 7% overestimation to a 74% underestimation. Discussion: Valuable efforts for monitoring and evaluating prevention and treatment studies require direct measurements of body weight and height.     

Resemblance for Body Mass Index in Families of Obese African American and European American Women

Objective: We determined the levels of resemblance in body mass index (BMI) in large samples of families selected through obese African American and European American women. Research Methods and Procedures: We examined correlations among relatives in 1185 European American and African American families ascertained through age-matched obese women (BMI ≥ 30 kg/m²). A subset of 801 families were ascertained through extremely obese women (BMI ≥ 40 kg/m²). Results: Parent-offspring and sibling correlations ranged from 0.19 to 0.15, suggesting a moderate level of heritability in both groups. Mean BMI values for female relatives were lower for European Americans than for African Americans even though probands were matched, perhaps because the European American relatives regress to a lower population mean. We found significantly higher family correlations for height in European Americans, suggesting greater environmental variability among African Americans for factors affecting growth and physical development. Discussion: Our results suggest a similar level of heritability of BMI in families of obese African American and European American women. Other genetic studies will be needed to determine the extent to which the same or different genes and environmental conditions contribute to an overall similar heritability in the two racial groups.