Familial clustering of abdominal visceral fat and total fat mass: the Québec Family Study

Beyond correlates: the social gradient in childhood overweight

BACKGROUND

Health (in)equity has a high priority on research and policy agendas. Even though it is known that inequalities in overweight prevalence accumulate with age and are already existent among children below the age of six, research on this topic is scarce. In this young age group, parents play an important role in preventing overweight and associated adverse consequences. This study examines the magnitude of parental misclassification of child weight status and its correlates, focussing on the factors that determine social status and equity.

METHODS

Preschool children's weight and height was measured objectively. Parents gave information on their socioeconomic background. Family education was dichotomised into tertiary and non-tertiary educational level, according to CASMIN. Binary logistic regression, adjusted for parental BMI, was applied to detect odds of childhood overweight.

RESULTS

Data on family educational level and anthropometrics were available from 643 children (4.5 ± 0.82 years, 52.7% male) and their parents of which 46.5% (n = 299) had a tertiary educational background. The groups (tertiary vs. non-tertiary educational level) differ significantly in overweight prevalence (3.7% vs. 11.9%, p ≤ 0.001). Odds of overweight were two times higher in children with non-tertiary educational background (OR: 2.123, CI: 1.010-4.461, p < 0.05), adjusted for parental BMI.

CONCLUSION

Children from families with low educational background have an elevated risk of overweight, already at a very young age. Education in general (not explicitly health education) seems to play a tremendous role in the prevention of overweight and obesity and should therefore be implied in policies enhancing health equity.

TRIAL REGISTRATION

DRKS-ID: DRKS00010089.

Abdominal Adiposity and Total Body Fat as Predictors of Cardiometabolic Health in Children and Adolescents With Obesity

Objective: We aimed to assess the role of adipose tissue distribution in cardiometabolic risk (in particular insulin sensitivity) in a population of children and adolescents with obesity. Methods: In this cross-sectional study, participants were 479 children and adolescents with obesity (322 boys and 157 girls) aged 3 to 18 years attending the Children's Hospital at Zhejiang University School of Medicine (Hangzhou, China). Clinical assessments included anthropometry, body composition (DXA scans), carotid artery ultrasounds, and OGTT. Insulin sensitivity was assessed using the Matsuda index. Participants were stratified into groups by sex and pubertal stage. Key predictors were DXA-derived android-to-gynoid-fat ratio (A/G) and total body fat percentage (TBF%). Results: Irrespective of sex and pubertal stage, there was a strong association between increasing A/G (i.e., greater abdominal adiposity) and lower insulin sensitivity. In multivariable models, every 0.1 increase in A/G was associated with a reduction in insulin sensitivity in prepubertal boys [-29% (95% CI -36%, -20%); p < 0.0001], pubertal boys [-13% (95% CI -21%, -6%); p = 0.001], and pubertal girls [-16% (95% CI -24%, -6%); p = 0.002]. In contrast, TBF% was not associated with insulin sensitivity when A/G was adjusted for, irrespective of pubertal stage or sex. In addition, every 0.1 increase in A/G was associated with increased likelihood of dyslipidemia in prepubertal boys [adjusted odds ratio (aOR) 1.62 (95% CI 1.05, 2.49)], impaired glucose tolerance in pubertal boys [aOR 1.64 (95% CI 1.07, 2.51)] and pubertal girls [aOR 1.81 (95% CI 1.10, 2.98)], and odds of NAFLD in both prepubertal [aOR 2.57 (95% CI 1.56, 4.21)] and pubertal [aOR 1.69 (95% CI 1.18, 2.40)] boys. In contrast, higher TBF% was only associated with higher fasting insulin and ALT in pubertal boys, being also predictive of NAFLD in this group [aOR 1.15 per percentage point (95% CI 1.06, 1.26)], but was not associated with the likelihood of other cardiometabolic outcomes assessed in any group. Conclusions: A/G is a much stronger independent predictor of cardiometabolic risk factors in children and adolescents with obesity in China, particularly glucose metabolism.

UPDATING THE CONCEPT OF METABOLICALLY HEALTHY OBESITY

Obesity is a well-recognized risk factor for type 2 diabetes, cardiovascular disease, and several types of cancer. However, a proportion of the obese individuals display a significantly lower risk for metabolic complications than expected for their degree of body mass index, and this subtype of obesity was described as "metabolically healthy obesity" (MHO). No universally accepted criteria for the diagnosis of MHO exists and the prevalence of this subtype of obesity varies largely according to criteria used. Broadly, MHO is characterized by a lower amount of visceral fat, a more favorable inflammatory profile, and less insulin resistance as compared to the metabolically unhealthy obesity. Currently, controversies exist regarding the risk of cardiovascular events and all-cause mortality associated with MHO as compared to metabolically-healthy non-obese individuals. Further research is needed in order to identify the MHO phenotype and if MHO is truly healthy for a long period of time or if it is a transient state from normal metabolic/normal weight to abnormal metabolic/obese state. This review will discuss the MHO definition criteria; the differences between MHO and metabolically unhealthy obesity; the possible underlying mechanisms and clinical implications of MHO.

The Relationship between Dietary Patterns and Metabolic Health in a Representative Sample of Adult Australians

Studies assessing dietary intake and its relationship to metabolic phenotype are emerging, but limited. The aims of the study are to identify dietary patterns in Australian adults, and to determine whether these dietary patterns are associated with metabolic phenotype and obesity. Cross-sectional data from the Australian Bureau of Statistics 2011 Australian Health Survey was analysed. Subjects included adults aged 45 years and over (n = 2415). Metabolic phenotype was determined according to criteria used to define metabolic syndrome (0-2 abnormalities vs. 3-7 abnormalities), and additionally categorized for obesity (body mass index (BMI) ≥30 kg/m2 vs. BMI <30 kg/m2). Dietary patterns were derived using factor analysis. Multivariable models were used to assess the relationship between dietary patterns and metabolic phenotype, with adjustment for age, sex, smoking status, socio-economic indexes for areas, physical activity and daily energy intake. Twenty percent of the population was metabolically unhealthy and obese. In the fully adjusted model, for every one standard deviation increase in the Healthy dietary pattern, the odds of having a more metabolically healthy profile increased by 16% (odds ratio (OR) 1.16; 95% confidence interval (CI): 1.04, 1.29). Poor metabolic profile and obesity are prevalent in Australian adults and a healthier dietary pattern plays a role in a metabolic and BMI phenotypes. Nutritional strategies addressing metabolic syndrome criteria and targeting obesity are recommended in order to improve metabolic phenotype and potential disease burden.

Metabolic and cardiovascular implications of a metabolically healthy obesity phenotype

Metabolically healthy obesity (MHO) is a new concept in which an individual may exhibit an obese phenotype in the absence of any metabolic abnormalities. There are a number of definitions of MHO that utilize a variety of components. The findings of clinical and basic studies indicate that subjects with MHO do not exhibit an increased mortality, an increased risk of cardiovascular disease, or an increased risk of type 2 diabetes mellitus, as compared to normal-weight controls. Although these findings imply that metabolic health is a more important factor than obesity, several studies have shown that subjects with MHO have a similar risk of metabolic or cardiovascular diseases as those with metabolically unhealthy obesity. Thus, there is still debate regarding not only the implications of the MHO phenotype but its very existence. Accordingly, future studies should focus on developing a unified definition of MHO and distinguishing subjects who will be at a high risk for metabolic and cardiovascular diseases.

Findings from the Quebec Family Study on the Etiology of Obesity: Genetics and Environmental Highlights

The Quebec Family Study (QFS) was an observational study with three cycles of data collection between 1979 and 2002 in Quebec City, Canada. The cohort is a mixture of random sampling and ascertainment through obese individuals. The study has significantly contributed to our understanding of the determinants of obesity and associated disease risk over the past 35 years. In particular, the QFS cohort was used to investigate the contribution of familial resemblance and genetic effects on body fatness and behaviors related to energy balance. Significant familial aggregation and genetic heritability were reported for total adiposity, fat-free mass, subcutaneous fat distribution, abdominal and visceral fat, resting metabolic rate, physical activity level and other behavioral traits. The resources of QFS were also used to study the contribution of several nontraditional (non-caloric) risk factors as predictors of excess body weight and gains in weight and adiposity over time, including low calcium and micronutrient intake, high disinhibition eating behavior trait, and short sleep duration. An important finding relates to the interactions between dietary macronutrient intake and exercise intensity on body mass and adiposity.

Comparison of father-offspring and mother-offspring associations of cardiovascular risk factors: family linkage within the population-based HUNT Study, Norway

BACKGROUND

Cardiovascular risk factors are transmitted from parents to offspring; however, the relative contributions of fathers and mothers remain unclear. If maternal exposures during pregnancy influence offspring through the intrauterine environment, associations between mothers and offspring are expected to be stronger than between fathers and offspring. In this family linkage study we compared father-offspring and mother-offspring associations of several cardiovascular risk factors.

METHODS

The study population consisted of 36,528 father-mother-offspring trios who participated at one or more surveys of the HUNT Study, Norway in 1984-86, 1995-97 and 2006-08. Parent-offspring associations were assessed using unstandardized and standardized residuals from linear regression analysis, and possible non-paternity was accounted for in sensitivity analyses.

RESULTS

Age- and sex-adjusted parent-offspring associations for anthropometric factors, blood pressure, blood lipids, blood glucose and resting heart rate were largely similar between fathers and mothers. Use of standardized values and analyses adjusted for non-paternity further emphasized this similarity.

CONCLUSIONS

This study found largely similar father-offspring and mother-offspring associations across all cardiovascular risk factors under study, arguing against strong maternal effects transmitted through intrauterine mechanisms.

Pathophysiology of human visceral obesity: an update

Excess intra-abdominal adipose tissue accumulation, often termed visceral obesity, is part of a phenotype including dysfunctional subcutaneous adipose tissue expansion and ectopic triglyceride storage closely related to clustering cardiometabolic risk factors. Hypertriglyceridemia; increased free fatty acid availability; adipose tissue release of proinflammatory cytokines; liver insulin resistance and inflammation; increased liver VLDL synthesis and secretion; reduced clearance of triglyceride-rich lipoproteins; presence of small, dense LDL particles; and reduced HDL cholesterol levels are among the many metabolic alterations closely related to this condition. Age, gender, genetics, and ethnicity are broad etiological factors contributing to variation in visceral adipose tissue accumulation. Specific mechanisms responsible for proportionally increased visceral fat storage when facing positive energy balance and weight gain may involve sex hormones, local cortisol production in abdominal adipose tissues, endocannabinoids, growth hormone, and dietary fructose. Physiological characteristics of abdominal adipose tissues such as adipocyte size and number, lipolytic responsiveness, lipid storage capacity, and inflammatory cytokine production are significant correlates and even possible determinants of the increased cardiometabolic risk associated with visceral obesity. Thiazolidinediones, estrogen replacement in postmenopausal women, and testosterone replacement in androgen-deficient men have been shown to favorably modulate body fat distribution and cardiometabolic risk to various degrees. However, some of these therapies must now be considered in the context of their serious side effects. Lifestyle interventions leading to weight loss generally induce preferential mobilization of visceral fat. In clinical practice, measuring waist circumference in addition to the body mass index could be helpful for the identification and management of a subgroup of overweight or obese patients at high cardiometabolic risk.

Mapping body fat distribution: a key step towards the identification of the vulnerable patient?

Although excess body fat is a significant health hazard, estimation of body fat content with the body mass index may not adequately reflect the amount of atherogenic adipose tissue (AT), i.e. visceral and ectopic fat. As opposed to subcutaneous AT that supposedly acts as a metabolic sink buffering excess dietary energy, visceral or intra-abdominal AT depots respond to several external stimuli that trigger lipolysis and secretion of free fatty acids (FFAs). Reaching the liver, FFAs accumulate in the liver and, over time, promote a chronic condition known as non-alcoholic fatty liver disease (NAFLD). The liver of the typical NAFLD patient secretes large amounts of very-low-density lipoproteins, the lipid content of which may accumulate in additional organs (skeletal muscle, heart, and pancreas). Here, we review the evidence emerging from functional and population studies that point towards an important role of ectopic fat accumulation in the pathophysiology of type 2 diabetes and cardiovascular disease. We conclude that although patients with impaired glycemic control or type 2 diabetes are at increased cardiovascular disease (CVD) risk, estimating cardiovascular risk goes wellbeyond the assessment of glycemic control and traditional CVD risk factors, and the estimation of visceral/ectopic fat deposition via readily available imaging techniquesshould be considered.

Corticosteroid Binding Globulin Gene Polymorphism Influences Cortisol Driven Fat Distribution in Obese Women

Hypothalamo-pituitary-adrenal axis has been reported to influence fat mass distribution in obesity. We investigated the hypothesis that corticosteroid-binding globulin (CBG) polymorphism could influence obesity, metabolic, or hypothalamo-pituitary adrenal (HPA) axis activity parameters. In 44 obese pre-menopausal women, a microsatellite located within the CBG gene was analyzed, providing three genotypes: 86/86 (n = 29), 86/90 (n = 14), and 90/90 (n = 1). No significant difference was found for obesity, metabolic, and HPA axis activity parameters between the genotypes 86/86 and 86/90. Looking for differences in correlations between HPA axis activity parameters and obesity or metabolic parameters between the two genotypes, genotype 86/90 showed a strong correlation between salivary cortisol after dexamethasone (0.25 mg) suppression test and waist-to-hip ratio (r = -0.84, p = 0.0007), whereas this correlation was weaker for genotype 86/86 (r = -0.34, p = 0.09). These data were completed with an analysis of the BclI polymorphism of the glucocorticoid receptor (GR) gene. There was an association between this GR polymorphism and both awakening salivary cortisol and postdexamethasone salivary cortisol but no association for obesity or metabolic parameters. We concluded that CBG gene polymorphisms might modulate the influence of the HPA axis on the fat mass distribution in this population.

Use of MRI and CT for fat imaging in children and youth: what have we learned about obesity, fat distribution and metabolic disease risk?

Childhood obesity is a matter of great concern for public health. Efforts have been made to understand its impact on health through advanced imaging techniques. An increasing number of studies focus on fat distribution and its associations with metabolic risk, in interaction with genetics, environment and ethnicity, in children. The present review is a qualitative synthesis of the existing literature on visceral and subcutaneous abdominal, intrahepatic and intramuscular fat. Our search revealed 80 original articles. Abdominal as well as ectopic fat depots are prevalent already in childhood and contribute to abnormal metabolic parameters, starting early in life. Visceral, hepatic and intramuscular fat seem to be interrelated but their patterns as well as their independent contribution on metabolic risk are not clear. Some ethnic-specific characteristics are also prevalent. These results encourage further research in childhood obesity by using imaging techniques such as magnetic resonance imaging and computed tomography. These imaging methods can provide a better understanding of fat distribution and its relationships with metabolic risk, compared to less detailed fat and obesity assessment. However, studies on bigger samples and with a prospective character are warranted.

Assortative mating for obesity

BACKGROUND

Assortative mating is the nonrandom mating of individuals with respect to phenotype and cultural factors. Previous studies of assortative mating for obesity have indicated that it may have contributed to the obesity epidemic. However, those studies all used body mass index or skinfold thicknesses to measure obesity and did not always account for potential confounding factors.

OBJECTIVE

We aimed to assess the level of assortative mating for obesity by using dual-energy X-ray absorptiometry to characterize body composition.

DESIGN

This was a cross-sectional study of 42 couples.

RESULTS

Raw spousal correlations showed assortative mating for age, weight, body mass index, lean mass, and fat mass. Removing the effect of age on fat mass strengthened the spousal correlation (r = 0.405). Social homogamy did not appear to be important, because in this sample there was no significant effect of area of origin on age-corrected fat and lean tissue masses for either sex. Regional body-composition analysis showed that subjects with disproportionately large arms (both fat and lean) assortatively mated with partners with the same trait. However, both men and women with high lean tissue in their arms assortatively mated with partners that had a disproportionately low fat content in their legs.

CONCLUSIONS

These data confirm that assortative mating for obesity exists when dual-energy X-ray absorptiometry is used to evaluate adiposity. We hypothesize that assortative mating may have contributed to the obesity epidemic because the time course of obesity development has shifted progressively earlier, allowing singles in their late teens and early twenties to more easily distinguish partners with obese and lean phenotypes.

Genetic determinants of upper airway structures that predispose to obstructive sleep apnea

Genetic factors are thought to play an important role in human development. Recent data indicate that obstructive sleep apnea may have a genetic basis. Sleep apnea is a very common disorder with significant cardiovascular and neurophysiologic morbidity. The pathogenesis of sleep apnea is related to a reduction in the size of the upper airway. The reduction in airway size is secondary to increased adipose tissue (enlargement of the parapharyngeal fat pads), alterations in craniofacial structure (reduction in mandibular size) and enlargement of the surrounding soft tissue structures (tongue, lateral pharyngeal walls). Genetic factors are one of the factors that have been proposed to mediate the size of each of these anatomic risk factors for sleep apnea. Recent evidence is accumulating about the genetic loci for these structural risk factors that predispose to the development of obstructive sleep apnea.

Twin study of genetic and environmental influences on adult body size, shape, and composition

OBJECTIVE

To investigate the genetic and environmental influences on adult body size, shape, and composition in women and men, and to assess the impact of age.

MATERIALS AND METHODS

In this cross-sectional study of 325 female and 299 male like-sex healthy twin pairs, on average 38 y old (18-67 y), we determined zygosity by DNA similarity, and performed anthropometry and bioelectrical impedance analysis of body composition. The contribution to the total phenotypic variance of genetic, common environment, and individual environment was estimated in multivariate analysis using the FISHER program. Further, these variance components were analysed as linear functions of age.

RESULTS

In both women and men genetic contributions were significant for all phenotypes. Heritability for body mass index was 0.58 and 0.63; for body fat%, 0.59 and 0.63; for total skinfolds, 0.61 and 0.65; for extremity skinfolds 0.65 and 0.62; for truncal skinfolds, 0.50 and 0.69; for suprailiac skinfolds, 0.49 and 0.48; for waist circumference, 0.48 and 0.61; for hip, 0.52 and 0.58; for lean body mass/height2, 0.61 and 0.56; and for height, 0.81 and 0.69, respectively. There was no strong evidence of common environmental effects under the assumptions of no nonadditive effect. The pattern of age trends was inconsistent. However, when significant there was a decrease in heritability with advancing age.

DISCUSSION

These findings suggest that adult body size, shape, and composition are highly heritable in both women and men, although a decreasing tendency is seen with advancing age.

Genetics of abdominal visceral fat levels

The purpose of this review is to explore the evidence accumulated thus far that suggests a genetic component to the observed variation in abdominal visceral fat (AVF) levels. The precise determination of AVF levels in humans is limited to methods such as computerized tomography and magnetic resonance imaging; thus, few studies have examined the role of genetic factors on this phenotype. Evidence from the Québec Family Study (QFS) and the HERITAGE Family Study indicates that between 50-55% of the variance in AVF levels, adjusted for total fatness, is attributable to genetic factors. Additionally, a major gene hypothesis for AVF was supported in the both the QFS and HERITAGE Family Study. However, after adjustment for total fat mass the support for a major gene was reduced, suggesting that a major gene which affects fat mass may also affect AVF either directly (pleiotropy), or indirectly. The search for candidate genes that may impact AVF levels is in its infancy, and few candidate genes have been identified. However, the glucocorticoid receptor (GRL), ss3 adrenergic receptor (ADRB3), and fatty acid binding protein 2 (FABP2) genes have been significantly associated with AVF or intra-abdominal fat levels in humans. In addition, three quantitative trait loci obtained from crosses of mice, the Do2, Mob4, and Qbw1 loci have been linked with mesenteric or abdominal fat and are thus considered positional candidate genes for AVF levels. The search for candidate genes or random genetic markers associated with AVF levels is a challenging prospect. However, given the significant heritability of this phenotype, the quest remains promising. Am. J. Hum. Biol. 11:225-235, 1999. Copyright 1999 Wiley-Liss, Inc.

Subcutaneous and visceral adipose tissue: their relation to the metabolic syndrome

Methods for assessment, e.g., anthropometric indicators and imaging techniques, of several phenotypes of human obesity, with special reference to abdominal fat content, have been evaluated. The correlation of fat distribution with age, gender, total body fat, energy balance, adipose tissue lipoprotein lipase and lipolytic activity, adipose tissue receptors, and genetic characteristics are discussed. Several secreted or expressed factors in the adipocyte are evaluated in the context of fat tissue localization. The body fat distribution and the metabolic profile in nonobese and obese individuals is discussed relative to lipolysis, antilypolysis and lipogenesis, insulin sensitivity, and glucose, lipid, and protein metabolism. Finally, the endocrine regulation of abdominal visceral fat in comparison with the adipose tissue localized in other areas is presented.

Familial aggregation of central obesity in Southern Indians

BACKGROUND

High prevalence of diabetes in South Asians is associated with a pronounced tendency to abdominal obesity. This intermediate quantitative trait may be more amenable than type 2 diabetes to genetic linkage studies.

OBJECTIVES

To derive a measure of central obesity independent of total adiposity and adjusted for factors under environmental influence, and to estimate the genetic contribution to familial aggregation of this trait.

SUBJECTS AND METHODS

A total of 1,295 individuals from 300 families were studied in a community-based cross-sectional study in Chennai, India. Central fat was measured using sagittal abdominal diameter, and adjusted for age, BMI and body fat percentage measured by impedance. Intra-sibship correlations were calculated for adjusted sagittal abdominal diameter and a comparison variable, adjusted body fat percentage.

RESULTS

Among individuals free of diabetes, intra-sibship correlations were 0.48 for adjusted sagittal abdominal diameter and 0.14 for adjusted body fat percentage.

CONCLUSIONS

Even after adjustment for possible assortative mating, these results are consistent with a heritability exceeding 90% for a trait defined as abdominal fat accumulation adjusted for total adiposity, sex and age. Linkage studies of abdominal obesity that map one or more of the genes underlying this high heritability are therefore a possible route to identifying genes for type 2 diabetes in South Asians.

Physical findings and the risk for obstructive sleep apnea. The importance of oropharyngeal structures

In this study, we hypothesized that anatomic abnormalities of the oropharynx, particularly narrowing of the airway by the lateral pharyngeal walls, tonsils, and tongue, would be associated with an increased likelihood for obstructive apnea among patients presenting to a sleep disorders center. To test this hypothesis, we used data from a cohort of 420 patients presenting to the Penn Center for Sleep Disorders. Associations between individual variables in the clinical evaluation model and sleep apnea as defined by a respiratory disturbance index greater than or equal to 15 events per hour were characterized by odds ratios (ORs) with 95% confidence intervals (CIs). Multivariable logistic regression was used to simultaneously estimate ORs for multiple variables and to control for other relevant patient characteristics. Results showed that narrowing of the airway by the lateral pharyngeal walls (OR = 2.5; 95% CI, 1.6-3.9) had the highest association with obstructive sleep apnea (OSA) followed by tonsillar enlargement (OR = 2.0; 95% CI, 1.0-3.8), enlargement of the uvula (OR = 1.9; 95% CI, 1.2-2.9), and tongue enlargement (OR = 1.8; 95% CI, 1.0-3.1). Low-lying palate, retrognathia, and overjet were not found to be significantly associated with OSA. Controlling for BMI and neck circumference, only lateral narrowing and enlargement of the tonsils maintained their significant (OR = 2.0 and 2.6, respectively). A subgroup analysis examining differences between male and female subjects showed that no oropharyngeal risk factor achieved significance in women while lateral narrowing was the sole independent risk factor in men. These findings suggest that enlargement of the oropharyngeal soft tissue structures, particularly the lateral pharyngeal walls, is associated with an increased likelihood of OSA among patients presenting to sleep disorders centers.

Inheritance of the waist-to-hip ratio in the National Heart, Lung, and Blood Institute Family Heart Study

OBJECTIVE

Considering that waist-to-hip ratio (WHR) is a simple anthropometric measure of obesity and is a better predictor of coronary heart disease than body mass index (BMI), the genetic underpinnings of WHR are of interest. The inheritance pattern of WHR, before and after adjustment for BMI (WHR-BMI), was investigated in 2713 individuals from 1038 nuclear families in the National Heart, Lung, and Blood Institute Family Heart Study (NHLBI-FHS).

RESEARCH METHODS AND PROCEDURES

Waist and hip measurements were taken twice, and the means of the measurements were used to calculate the WHR. Adjustments for age were carried out separately by sex, using stepwise multiple regression procedures for WHR and WHR-BMI phenotypes. Segregation analysis was applied using the unified model as implemented in the computer program POINTER.

RESULTS

For age-adjusted WHR, the segregation results suggested an additive major gene that accounts for 35% of the phenotypic variance, and approximately 30% of the sample are homozygous for the "high" genotype. The results for age- and BMI-adjusted WHR were also compatible with a major gene; however, the multifactorial model provided the most parsimonious fit to the data.

DISCUSSION

Although the genetic mechanisms for several obesity traits have been studied, tests of Mendelian segregation on this simple anthropometric measure (WHR) have not been reported previously. This study provides evidence for the presence of a major gene for age-adjusted WHR, suggesting that it is an appropriate trait for further genetic analysis, especially because it has strong predictive value and probably relates biologically to cardiovascular risk.

Childhood obesity, adipose tissue distribution, and the pediatric practitioner

The prevalence of pediatric obesity is increasing in the United States. Sequelae from pediatric obesity are increasingly being seen, and long-term complications can be anticipated. Obesity is the most common cause of abnormal growth acceleration in childhood. Obesity in females is associated with an early onset of puberty and early menarche. Puberty is now occurring earlier in females than in the past, and this is probably related either directly or indirectly to the population increase in body weight. The effect of obesity on male pubertal maturation is more variable, and obesity can lead to both early and delayed puberty. Pubertal gynecomastia is a common problem in the obese male. Many of the complications of obesity seen in adults appear to be related to increased accumulation of visceral fat. It has been proposed that subcutaneous fat may be protective against the adverse effects of visceral fat. Males typically accumulate fat in the upper segment of the body, both subcutaneously and intraabdominally. In females, adiposity is usually subcutaneous and is found particularly over the thighs, although visceral fat deposition also occurs. Gender-related patterns of fat deposition become established during puberty and show significant familial associations. There are no reliable means for assessing childhood and adolescent visceral fat other than radiologically. Noninsulin-dependent diabetes is being seen more commonly in the pediatric population. Diabetes and impaired glucose tolerance are noted particularly in obese children with a family history of diabetes. In this situation, a glucose tolerance test may be indicated, even in the presence of fasting normoglycemia. Hypertriglyceridemia and low high-density lipoprotein-cholesterol levels are the primary lipid abnormalities of obesity and are related primarily to the amount of visceral fat. Low-density lipoprotein-cholesterol levels are not typically elevated in simple obesity. The offspring of parents with early coronary disease tend to be obese. Very low-density lipoprotein and intermediate-density lipoprotein particles, which are small in size, may be important in atherogenesis but they cannot be identified in a fasting lipid panel. The propensity to atherogenesis cannot be interpreted readily from a fasting lipid panel, which therefore should be interpreted in conjunction with a family history for coronary risk factors. Hypertriglyceridemia may be indicative of increased visceral fat, familial combined hyperlipidemia, familial dyslipidemic hypertension, impaired glucose tolerance, or diabetes. Almost half of adult females with polycystic ovary syndrome are obese and many have a central distribution of body fat. This condition frequently has its origins in adolescence. It is associated with increased androgen secretion, hirsutism, menstrual abnormalities, and infertility, although these may not be present in every case. Adults with polycystic ovary syndrome adults are hyperlipidemic, have a high incidence of impaired glucose tolerance and noninsulin-dependent diabetes, and are at increased risk for coronary artery disease. Weight reduction and lipid lowering therefore are an important part of therapy. Obstructive sleep apnea with daytime somnolence is a common problem in obese adults. Pediatric studies suggest that obstructive sleep apnea occurs in approximately 17% of obese children and adolescents. Sleep disorders in the obese may be a major cause of learning disability and school failure, although this remains to be confirmed. Symptoms suggestive of a sleep disorder include snoring, restlessness at night with difficulty breathing, arousals and sweating, nocturnal enuresis, and daytime somnolence. Questions to exclude obstructive sleep apnea should be part of the history of all obese children, particularly for the morbidly obese. For many children and adolescents with mild obesity, and particularly for females, one can speculate that obesity may not be a great health risk

Segregation analysis of abdominal visceral fat: the HERITAGE Family Study

A major gene hypothesis for abdominal visceral fat (AVF) level, both before and after adjustment for total body fat mass, was investigated in 86 white families who participated in the HERITAGE Family Study. In this study, sedentary families were tested for a battery of measures (baseline), endurance exercise trained for 20 weeks, and then remeasured again. The baseline measures reported here are unique in that the variance due to a potentially important environmental factor (activity level) was limited. AVF area was assessed at L4 to L5 by the use of computerized tomography scan, and total body fat mass was assessed with underwater weighing. For fat mass, a putative locus accounted for 64% of the variance, but there was no evidence of a multifactorial component (i.e., no polygenic and/or common familial environmental effects). For AVF area, both a major gene effect accounting for 54% of the variance and a multifactorial component accounting for 17% of the variance were significant. However, after AVF area was adjusted for the effects of total level of body fat, the support for a major gene was reduced. In particular, there was a major effect for fat mass-adjusted AVF area, but it was not transmitted from parents to offspring (i.e., the three transmission probabilities were equal). The importance of this study is twofold. First, these results confirm a previous study that suggested that there is a putative major locus for AVF and for total body fat mass. Second, the findings from the HERITAGE Family Study suggest that the factors underlying AVF area in sedentary families may be similar to those in the population at large, which includes both sedentary and active families. Whether the gene(s) responsible for the high levels of AVF area is the same as that which influences total body fat content remains to be further investigated.

Obesity QTLs on mouse chromosomes 2 and 17

The inheritance of obesity has been analyzed in an intercross between the mouse strains AKR/J and C57L/J. Two novel obesity quantitative trait loci (QTLs) have been identified using the strategy of selective DNA pooling. One QTL affecting adiposity, Obq3, was mapped to a 39-cM segment near the middle of Chromosome 2, with a peak lod score (5.1) just distal to the D2Mit15 locus. The AKR/J Obq3 allele confers increased adiposity in a nearly additive manner, and males are more affected than females. A second obesity QTL (Obq4) maps to the centromeric end of Chromosome 17, with a lod score peak of 4.6 at D17Mit143. The obesity-conferring allele is contributed by C57L/J and acts in a recessive or an additive manner. Obq4 also has more influence in males and affects the inguinal fat depot differentially. Obq3 and Obq4 account for 7.0 and 6.1% of the phenotypic variance in adiposity (gender-merged data), respectively. The possible relationships between these QTLs and previously described obesity QTLs and candidate genes are discussed. The large number of different obesity QTLs that have been described in mice and the relatively small effects contributed by individual loci suggest considerable genetic complexity.