Age and gender specific lung function predictive equations provide similar predictions for both a twin population and a general population from age 6 through adolescence

The impact of different spirometric definitions on the prevalence of airway obstruction and their association with respiratory symptoms

The fixed ratio criterion of forced expiratory volume in 1 s/forced vital capacity <0.70 for diagnosing airway obstruction may overdiagnose the condition, particularly in the elderly, so the lower limit of normal (LLN) is recommended as the most appropriate criterion. Our aim was to compare LLN versus fixed ratio on the prevalence of chronic obstructive pulmonary disease (COPD) and examine the association between respiratory symptoms and airway obstruction defined by LLN and fixed ratio. 12 449 twins aged 40-80 years participated in a nationwide survey using the Danish Twin Registry. They completed a questionnaire, underwent clinical examination and recorded prebronchodilator spirometry. Individuals with self-reported asthma were excluded. Clinical COPD was defined by respiratory symptoms together with airway obstruction. 10 329 individuals were included, with a mean±sd age of 58.4±9.6 years and mean body mass index of 26.6±4.4 kg·m-2; 20% were current smokers, 37% former smokers and 43% never-smokers; and 48% were male. The prevalence of LLN airway obstruction (LLN-AO) and fixed ratio airway obstruction (FR-AO) was 5.6% and 18.0%, respectively (p<0.001). Overall, 26% reported current respiratory symptoms, but 50% of those with LLN-AO had respiratory symptoms compared to 39% with FR-AO, p<0.001. The prevalence of clinical LLN-COPD and fixed ratio COPD was 2.6% and 6.3%, respectively (p<0.001). Individuals with LLN-AO had a significantly higher probability of reporting respiratory symptoms compared with both healthy individuals and FR-AO when adjusted for sex, age and ever-smoking. The use of fixed ratio more than doubled the prevalence of clinical COPD compared with LLN, this being more pronounced with increased age, and identified subjects with a lower prevalence of respiratory symptoms than LLN-AO.

Body mass index and waist circumference rather than body adiposity index are better surrogates for body adiposity in a Chinese population

BACKGROUND

Several studies have found that body adiposity index (BAI) is a better index of body adiposity than body mass index (BMI) in African and Mexican American adults. This study aims to evaluate the ability of BAI to predict body adiposity in Chinese children and adults.

MATERIALS AND METHODS

In total, 2425 children and 5726 adults were recruited from rural China. All participants completed whole-body dual-energy X-ray absorptiometry (DXA) and anthropometric measures. The correlation of BMI, BAI, and waist circumference (WC) to DXA adiposity indexes was performed across sex-specific adult and age- and sex-specific child cohorts, using Spearman correlation and linear regression models, respectively.

RESULTS

Both BMI and WC had a higher correlation with all adiposity indexes (whole body fat, percent body fat [Bfat%], trunk fat, and percent trunk fat [Tfat%]) measured by DXA than did BAI in both adults and children. Meanwhile, most of the linear regression model associations for BMI with Bfat% and Tfat% had a greater adjusted R(2) than those for BAI among both children and adults.

CONCLUSION

This study indicates that BMI and WC are better tools than BAI for estimating whole body fat and central body fat in a Chinese population.

Exploring the origins of asthma: Lessons from twin studies

This thesis explores the contribution of twin studies, particularly those studies originating from the Danish Twin Registry, to the understanding of the aetiology of asthma. First, it is explored how twin studies have established the contribution of genetic and environmental factors to the variation in the susceptibility to asthma, and to the variation in several aspects of the clinical expression of the disease such as its age at onset, its symptomatology, its intermediate phenotypes, and its relationship with other atopic diseases. Next, it is explored how twin studies have corroborated theories explaining asthma's recent increase in prevalence, and last, how these fit with the explanations of the epidemiological trends in other common chronic diseases of modernity.

Association of adipokines with blood pressure in rural Chinese adolescents

Previous research has demonstrated that adipokines influence blood pressure (BP). Limited data exist in healthy adolescents, who are in a critical period for preventing the development of high BP. This study investigated the association of leptin, adiponectin and the leptin-to-adiponectin ratio (LAR) with BP in rural Chinese adolescents. This report included 1245 adolescents (average body mass index: 19.3 kg m(-2)) aged 13-21 years from an established twin cohort. We examined gender-specific associations between plasma adipokines and BP, with adjustment for measures of adiposity and insulin resistance (IR). We estimated the genetic contribution to adipokines using the twin design and Cholesky decomposition models. There was no correlation between leptin and adiponectin levels. Leptin was positively associated with systolic blood pressure (SBP) in males and diastolic blood pressure in females, but the association disappeared after adjusting for adiposity and IR. LAR was positively associated with SBP (β(s.e.): 1.94(0.45)), P<0.01), adiponectin was negatively associated with SBP (β(s.e.): -2.18(0.63)), P<0.001) only in males, and such associations were independent of adiposity and IR. A test of gender × adiponectin interaction was significant (P=0.01). Heritability estimation showed that both environmental and genetic factors contribute to variance in adipokines. In these relatively lean Chinese adolescents, leptin was positively associated with BP in both genders, but was adiposity/IR dependent. Adiponectin was negatively associated with SBP in males, independent of adiposity/IR.

Association between short sleep duration and the risk of sensitization to food and aero allergens in rural Chinese adolescents

BACKGROUND

Both long and short sleep duration have been associated with obesity, cardiovascular disease, and diabetes. However, there have been no previous studies investigating the potential relationship between altered sleep duration and allergen sensitization.

OBJECTIVE

To explore the association between sleep duration and sensitization to food and aeroallergens.

METHODS

This study includes 1534 rural Chinese adolescent twins aged 12-21 years who completed standard sleep questionnaires and skin prick tests (SPTs) to nine food and five aeroallergens. Total sleep time was defined as the interval from bedtime to wake-up time minus sleep latency. Sensitization was defined as having at least one positive SPT.

RESULTS

Compared with individuals with the highest (third) tertile of sleep duration, those who slept less were more likely to be sensitized to any food allergen with odds ratios (ORs) of 1.9 [95% confidence interval (CI): 1.3-2.7] and 1.4 (95% CI: 1.0-1.9) for the first and second tertiles (trend test P(trend)=3×10(-4)), respectively. The corresponding ORs for sensitization to any aeroallergen were 1.5 (95% CI: 1.1-2.0) and 1.3 (95% CI: 1.0-1.7) (P(trend)=8×10(-3)). These associations were independent of percent body fat. In addition, we observed a significant dose-response association between the number of positive SPTs and percentage of shortest sleep duration (first tertile) (P(trend)=1×10(-3)).

CONCLUSIONS AND CLINICAL RELEVANCE

In this sample of relatively lean rural Chinese adolescents, we found that short sleep duration was associated with increasing risk of sensitization to food and aeroallergens, independent of percent body fat. Longitudinal studies are needed to further determine the temporal and causal relationships. If short sleep duration indeed is one of the risk factors for allergic sensitization, the global burden of allergic diseases could be dramatically reduced by providing appropriate guidance on sleep duration for youth.

Prediabetes is not all about obesity: association between plasma leptin and prediabetes in lean rural Chinese adults

OBJECTIVE

This study investigated the associations of plasma leptin levels with insulin resistance (IR) and prediabetes in relatively lean, rural Chinese men and women.

DESIGN AND METHODS

This study included 574 subjects aged 21-45 years from a community-based twin cohort. Plasma leptin concentrations were measured by sandwich immunoassays using flowmetric xMAP technology. Prediabetes was defined based on fasting plasma glucose and 75-g oral glucose tolerance test. Multivariate linear and logistic regression analyses were used to investigate gender-specific associations of leptin with IR measures and prediabetes, adjusting for intra-twin correlation, measures of adiposity, and other pertinent covariates.

RESULTS

The body mass index is 22.3+/-2.7 kg/m(2) in men and 22.5+/-2.7 kg/m(2) in women. Leptin levels were positively associated with IR. Individuals with higher tertiles of leptin also had increased risk of prediabetes with odds ratios (OR) of 2.6 (95% confidence interval (CI): 1.4-5.1) and 4.3 (95% CI: 2.1-8.7) in men; OR of 1.1 (95% CI: 0.6-2.1) and 3.1 (95% CI 1.5-6.2) in women for second and third tertile respectively. These associations were attenuated after further adjusting for adiposity measurements only in men. The leptin-prediabetes associations disappeared after adjusting for the homeostatic model assessment of IR in both genders.

CONCLUSION

In this sample of relatively lean rural Chinese adults, plasma leptin levels were associated with IR and prediabetes in a dose-response fashion, which were not totally explained by adiposity. Our data emphasize that prediabetes is not all about obesity, and leptin may be an additional biomarker for screening individuals at high risk for prediabetes in this population.

Percent fat mass is inversely associated with bone mass and hip geometry in rural Chinese adolescents

This study was an attempt to examine the phenotypic, genetic, and environmental correlations between percent fat mass (PFM) and bone parameters, especially hip geometry, among 786 males and 618 females aged 13 to 21 years from a Chinese twin cohort. PFM, bone area (BA), bone mineral content (BMC), cross-sectional area (CSA), and section modulus (SM) were obtained by dual-energy X-ray absorptiometry. Multiple linear regression models were used to assess the PFM-bone relationships. A structural equation model for twin design was used to estimate genetic/environmental influences on individual phenotype and phenotypic correlations. After controlling for body weight and other pertinent covariates, we observed inverse associations between PFM and bone parameters: Compared with the lowest age- and gender-specific tertile of PFM, males in the highest tertile of PFM had lower measures of whole-body-less-head BA (WB-BA), lumbar spine BA (L(2)-L(4)-BA), total-hip BA (TH-BA), total-hip BMC, CSA, and SM (p < .005 for all, adjusted p < .05). Similar inverse associations were observed in females for all the preceding parameters except WB-BA and L2-L(4)-BA. These associations did not vary significantly by Tanner stages. In both genders, the estimated heritabilities were 80% to 86% for BMC, 67% to 80% for BA, 74% to 77% for CSA, and 64% for SM. Both shared genetics and environmental factors contributed to the inverse PFM-bone correlations. We conclude that in this sample of relatively lean Chinese adolescents, at a given body weight, PFM is inversely associated with BA, BMC, and hip geometry in both genders, and such associations are attributed to both shared genetic and environmental factors.

Association of central adiposity with prediabetes and decreased insulin sensitivity in rural Chinese normal-weight and overweight women

This study investigated whether high central adiposity was associated with prediabetes and decreased insulin sensitivity (IS) in both normal-weight (body mass index [BMI] <23 kg/m(2)) and overweight (BMI >or=23 kg/m(2)) rural Chinese women. Adipose variables measured by dual-energy x-ray absorptiometry (percentage body fat, percentage lower-body fat [%LF], and percentage trunk fat [%TF]) and general adipose variables (BMI and waist circumference) were used for examining the association of adiposity with prediabetes among 4071 rural Chinese women aged 20 to 60 years. Furthermore, the association of adiposity with IS was tested in both normal- and overweight women with normal glucose tolerance. BMI was highly correlated with percentage body fat and waist circumference, but was weakly correlated with %LF and %TF. Both high %TF (top quartile of %TF) and low %LF (lower 3 quartiles of %LF) were associated with higher prevalence of prediabetes in both normal- and overweight women. Compared with normal-weight women in low %TF, the odds of prediabetes were similarly increased for women with high %TF regardless of whether they were overweight (odds ratio [95% confidence interval] = 1.6 [1.3-2.0]) or not (odds ratio [95% confidence interval] = 1.5 [1.2-2.0]). Similarly, among 3280 women with normal glucose tolerance, high %TF was associated with increased fasting insulin, 2-hour oral glucose tolerance test insulin, and homeostasis model assessment of insulin resistance regardless of weight status (normal or overweight). Among relatively lean, rural Chinese women, high %TF was associated with increased odds of prediabetes and lower IS regardless of weight status (normal or overweight).

Plasma adipokines, bone mass, and hip geometry in rural Chinese adolescents

CONTEXT

Adipokines have been linked to bone phenotypes recently, but with conflicting results. Few such studies have been conducted in adolescents.

OBJECTIVE

The aim of the study was to examine the associations of adiponectin and leptin with multiple bone phenotypes in Chinese adolescents and estimate the genetic contribution to these associations.

DESIGN AND SETTING

This was a cross-sectional study conducted in rural China.

PARTICIPANTS

A total of 675 males and 575 females aged 13-21 yr were included.

OUTCOME MEASURES

Fat mass (FM), lean mass (LM), bone area (BA), bone mineral content (BMC), cross-sectional area (CSA), and section modulus (SM) were measured by dual-energy x-ray absorptiometry. Plasma adipokine concentration was determined using sandwich immunoassays.

RESULTS

Adiponectin was inversely associated with all BMCs in males (P < 0.01), but not in females, after adjusting for LM, body weight, or BMI singly, or for LM and FM simultaneously. No such relationships were observed for CSA or SM in both genders. Leptin was inversely associated with all BAs, total-hip BMC, CSA, and SM in both genders, when adjusting for body weight or BMI. These associations, except for whole-body BA and lumbar spine BA in females, disappeared when simultaneously adjusting for LM and FM. By Cholesky decomposition models using twin design, significant genetic correlations were detected between adiponectin and total-hip BMC in males and between leptin and total-hip BMC in both genders.

CONCLUSIONS

We demonstrated that adiponectin and leptin were inversely associated with adolescent bone phenotypes but showed differential associations by gender, type of bone phenotypes, and adjustment of FM. This study also suggested that adipokines and bone phenotypes may share a common set of genes.

Association of plasma leptin concentrations with adiposity measurements in rural Chinese adolescents

CONTEXT

There is evidence that leptin is involved in the etiology of obesity-related cardiovascular disease in adults. This raises the question of whether leptin levels in adolescence are indicative of adiposity-related cardiovascular risk.

OBJECTIVE

This study investigated gender-specific patterns of plasma leptin during adolescence, assessed which adiposity measurements are most strongly associated with plasma leptin, and estimated to what degree leptin-adiposity associations are influenced by genetic factors.

METHODS

Plasma leptin concentrations were determined using a sandwich immunoassay. Associations between plasma leptin and several adiposity measurements were examined using generalized estimating equations. Genetic contribution to leptin-adiposity association was estimated by Cholesky decomposition models using twin design.

RESULTS

Plasma leptin levels were higher in females than males at all Tanner stages. In females, body mass index, waist circumference, fat mass index (FMI), and percentage body fat (%BF) had similar associations with leptin levels. In males, %BF and FMI were more strongly associated with leptin levels than body mass index and waist circumference. In both males and females, percentage trunk fat had the weakest association with leptin among the five adiposity measures. Shared genetic factors account for more than 80% of the phenotypic correlation between %BF and leptin.

CONCLUSIONS

We found gender differences in leptin levels and leptin-adiposity associations. In both genders, leptin levels were strongly associated with %BF and FMI, particularly in males. Shared genetic factors contributed largely to the phenotypic correlation between leptin and %BF. Our findings underscored the importance of further investigation of leptin as a biomarker of adiposity in adolescents.

Genetic and environmental contributions to phenotypic components of metabolic syndrome: a population-based twin study

The increasing prevalence of metabolic syndrome (MS) poses a serious public-health problem worldwide. Effective prevention and intervention require improved understanding of the factors that contribute to MS. We analyzed data on a large twin cohort to estimate genetic and environmental contributions to MS and to major MS components and their intercorrelations: waist circumference (WC), systolic (SBP) and diastolic blood pressure (DBP), fasting plasma glucose (FPG), triglycerides (TGs), and high-density lipoprotein-cholesterol (HDL-C). We applied structural equation modeling to determine genetic and environmental structure of MS and its major components, using 1,617 adult female twin pairs recruited from rural China. The heritability estimate for MS was 0.42 (95% confidence interval (CI): 0.00-0.83) in this sample with low MS prevalence (4.4%). For MS components, heritability estimates were statistically significant and ranged from 0.13 to 0.64 highest for WC, followed by TG, SBP, DBP, HDL-C, and FPG. HDL-C was mainly influenced by common environmental factors (0.62, 95% CI: 0.58-0.62), whereas the other five MS components were largely influenced by unique environmental factors (0.32-0.44). Bivariate Cholesky decomposition analysis indicated that the clinical clustering of MS components may be explained by shared genetic and/or environmental factors. Our study underscores the importance of examining MS components as intercorrelated traits, and to carefully consider environmental and genetic factors in studying MS etiology.

Genetic and environmental contributions to allergen sensitization in a Chinese twin study

BACKGROUND

Allergic disease is on the rise worldwide. Effective prevention of allergic disease requires comprehensive understanding of the factors that contribute to its intermediate phenotypes, such as sensitization to common allergens.

OBJECTIVE

To estimate the degree of genetic and environmental contributions to sensitization to food and aeroallergens.

METHODS

Sensitization was defined as a positive skin prick test to an allergen. We calculated the zygosity-specific concordance rates and odds ratios (ORs) for sensitization to food and aeroallergens in 826 Chinese twin pairs [472 monozygotic (MZ) and 354 dizygotic (DZ)] aged 12-28 years. We also applied structural equation modelling procedures to estimate genetic and environmental influences on sensitization.

RESULTS

The concordance rates and risk of sensitization in one twin given the presence vs. the absence of sensitization in the other twin were higher in MZ twins than those in DZ twins. However, a large number of MZ twins were discordant in sensitization to common allergens. These observations suggest both genetic and environmental factors influence sensitization. Consistently, the estimated heritability and individual environmental components of the liability to sensitization ranged from 0.51 to 0.68 and 0.32 to 0.49, respectively, based on the best-fitted structural equation model. We also observed high phenotypic correlations between sensitization to two aeroallergens (cockroach and dust mite: 0.83) and two food allergens (peanut and shellfish: 0.58), but only moderate correlations for the pairs between sensitization to a food and an aeroallergen (0.31-0.46). The shared genetic and environmental factors between paired sensitizations contribute to the observed correlations.

CONCLUSION

We demonstrated that sensitization to common food and aeroallergens were influenced by both genetic and environmental factors. Moreover, we found that paired allergen sensitizations might share some common sets of genes and environmental factors. This study underscores the need to further delineate unique and/or pleiotropic genetic and environmental factors for allergen sensitization.