中国儿童青少年腰围胸围比与肺功能的关系

ORIGINAL ARTICLE Ratio of waist circumference to chest circumference is inversely associated with lung function in Chinese children and adolescents KUI FENG,1 LI CHEN,1 SHAO-MEI HAN2 AND GUANG-JIN ZHU1 Departments of 1Physiology and Pathophysiology and 2Epidemiology and Statistics, Institute of Basic Medical Sciences, Chinese Academy of Medical Sciences, Peking Union Medical College, Beijing, China ABSTRACT Background and objective: In White children, waist circumference (WC) is positively correlated with forced vital capacity (FVC) and forced expiratory volume in 1 s (FEV1). Because fat distribution differs among different races, the relationship between WC and lung function in Asian children may differ from that in White children. The present study aimed to examine the effect of WC on ventilatory function in Chinese children. Methods: A cross-sectional study was performed on 1572 healthy subjects aged 9–18 years. Height, weight, chest circumference (CC), WC and lung function (FVC, FEV1, peak expiratory flow (PEF) and maximal mid- expiratory flow (MMEF)) were measured. To avoid the problem of colinearity, a model that combined CC and WC as the waist-to-chest ratio (WCR) was used. The relative contributions of WCR and body mass index (BMI) to spirometric parameters were determined by linear regression analysis. Results: WCR was inversely associated with all spiro- metric parameters. On average, each 0.01 increase in WCR was associated with decreases of 8.14 mL for FVC, 9.36 mL for FEV1, 6.54% for FEV1/FVC, 19.81 mL/s for PEF and 17.25 mL/s for MMEF. BMI was positively asso- ciated with all spirometric parameters except FEV1/ FVC. These results suggest that WC was inversely associated with lung function parameters. Conclusions: Inverse associations were identified between WCR, as well as WC, and lung function in a population of Chinese children. The underlying mechanisms need to be further explored. Key words: chest circumference, child, Chinese, lung function, waist circumference. INTRODUCTION Numerous epidemiological studies of adults have shown that abdominal adiposity is a risk factor for deterioration of pulmonary function.1–4 Waist circum- ference (WC), one of the indicators of abdominal adi- posity, has a similar relationship with pulmonary function.3,4 However, there have been few studies on WC and its relationship with pulmonary function in children. Recently, Chen et al. reported that WC was positively correlated with forced vital capacity (FVC) and forced expiratory volume in 1 s (FEV1) in children. However, this study was performed in a White popu- lation, and it is not known whether the results can be extrapolated to other racial groups. Besides serving as measures of overweight and obesity, body mass index (BMI) and WC are also indicators of body size, espe- cially in young people.5 Compared with WC, chest cir- cumference (CC) may be a better indicator of body size and is positively correlated with lung function in both children and adults.6,7 The relationship between WC and lung function, after adjustment for CC and other confounders, has not been investigated. The Enlarged Population Investigation of Human Physiological Constant Database of China provided a population that was suitable for examining the rela- tionship between WC and lung function, as the par- ticipants were a well-characterized group of generally healthy individuals who had undergone high-quality anthropometric and spirometric measurements. The purpose of the present study was to examine the effect of WC on ventilatory function, after adjustment for confounders, including CC, in a population of Chinese children and adolescents. Correspondence: Guang-Jin Zhu, Department of Physiology and Pathophysiology, Institute of Basic Medical Sciences, Chinese Academy of Medical Sciences, Peking Union Medical College, Road Dongdansantiao No. 5, Beijing 100005, China. Email: zhugj@pumc.edu.cn Received 23 October 2011; invited to revise 22 November 2011, 6 March 2012, 9 March 2012; revised 19 February 2012, 8 March 2012, 31 March 2012; accepted 31 March 2012 (Associate Editor: Chi Chiu Leung). SUMMARY AT A GLANCE This study showed that there was an inverse asso- ciation between the ratio of waist circumference to chest circumference and forced spirometric parameters in a large population of Chinese chil- dren. The mechanisms underlying this association need to be further explored. bs_bs_banner © 2012 The Authors Respirology © 2012 Asian Pacific Society of Respirology Respirology (2012) 17, 1114–1118 doi: 10.1111/j.1440-1843.2012.02219.x METHODS Subjects The study was approved by the Ethics Committee of the Institute of Basic Medical Sciences, Chinese Academy of Medical Sciences (IRB no. 005–2008). Written informed consent was obtained from all subjects and their parents. As part of the Enlarged Population Investigation of Human Physiological Constant Database of China, a total of 1814 students aged 9–18 years were randomly recruited through schools located in the Inner Mongolia Autonomous Region of China. A total of 1572 (87%) of these stu- dents were enrolled in the study after completing a questionnaire and physical examination. The exclu- sion criteria were: a personal history of smoking; a common cold within the last 4 weeks; a history of chest injury; respiratory disease (self-reported or physician-diagnosed asthma, pulmonary tuberculo- sis, pneumonia, frequent bronchitis or chronic bron- chitis); hypertension; respiratory symptoms during the last 12 months (chronic cough, wheeze or phlegm) and clinically relevant changes on physical examination of the heart, lungs and chest wall. Measurements Age was calculated as the number of completed years from birth to the date of the study. Standing height was measured to the nearest 0.1 cm without shoes. Weight was measured to the nearest 0.1 kg with the subjects wearing light indoor clothing. BMI was cal- culated as weight divided by height squared. WC was measured horizontally between the lowest rib and the iliac crest, through the narrowest part of the torso. CC was measured at the level of the nipples with the subjects in a resting state. Spirometry was performed using a portable spirometer (MasterLab 5.10; Jaeger, Wurzburg, Germany). The system was calibrated every morning using a 1-L syringe and recalibrated at least every 2 h. Integrated volumes were corrected for body temperature, ambient pressure and saturated with water vapour. Spirometry was performed in accor- dance with the American Thoracic Society guide- lines.8 All measurements were performed by an experienced research fellow and a technician. Nose clips were not used, but nose breathing during testing was avoided by manual occlusion. Children were instructed in the technique for spirometry manoeuvres in small groups followed by reinforce- ment of the instructions to each child individually before the test. At least three acceptable tests were required for each child, with the two highest FVC values, as well as the two highest FEV1 values being reproducible to within 5% of each other. The curve with the highest FVC and FEV1 values was selected. Peak expiratory flow (PEF) and maximal mid- expiratory flow (MMEF) were automatically recorded by the spirometer. All tests were performed between 8:00 and 13:00 h. Statistical analyses Statistical analyses were performed using SPSS for Windows version 15.0 software (IBM, Somers, NY, USA). The data for anthropometric and spirometric parameters are expressed as means � standard deviation. The relative contributions of WC, CC and BMI to spirometric parameters were determined by linear regression analyses. Stepwise multiple regres- sion models were constructed using FVC, FEV1, FEV1/ FVC, PEF and MMEF as dependent variables. The independent variables included gender (male = 1, female = 2), height, BMI and waist-to-chest ratio (WCR). Because there were strong correlations among BMI, CC and WC, inclusion of these correlated variables may have substantially inflated the variance and affected the robustness of the multivariate model; therefore, WCR was introduced into the model as an independent variable. Results were considered sig- nificant when the P value was <0.05. The variance inflation factor was used to evaluate whether colin- earity was a problem, which would be indicated by a variance inflation factor > 2. RESULTS The subjects (n = 1572) comprised 782 boys (50.0%) and 790 girls (50%). The characteristics of these sub- jects are presented in Table 1. Table 2 shows the correlations among the different anthropometric measurements. There were strong correlations between BMI and WC, CC and waist- to-height ratio, with correlation coefficients of 0.778, Table 1 Anthropometric and lung function characteris- tics of the subjects Males (n = 782) Females (n = 790) Age, years 13.65 � 2.1 13.95 � 2.23 Height, cm 157.6 � 11.85 154.75 � 8.08 Weight, kg 48.25 � 12.17 47.04 � 9.42 BMI, kg/m2 19.16 � 3.03 19.49 � 2.82 CC, cm 75.39 � 8.27 76.0 � 7.13 WC, cm 67.43 � 8.62 66.36 � 7.48 WHtR 0.43 � 0.05 0.43 � 0.04 WHR 0.8 � 0.07 0.77 � 0.06 WCR 0.9 � 0.07 0.87 � 0.06 FVC, L 3.42 � 0.91 2.98 � 0.58 FEV1, L 3.17 � 0.84 2.85 � 0.54 FEV1/FVC, % 93.29 � 5.29 95.84 � 4.2 PEF, L/s 6.97 � 1.75 6.45 � 1.27 MMEF, L/s 3.85 � 1.19 3.88 � 0.95 All values are means � standard deviation. BMI, body mass index; CC, chest circumference; FEV1, forced expiratory volume in 1 s; FVC, forced vital capac- ity; MMEF, maximal mid-expiratory flow; PEF, peak expiratory flow; WC, waist circumference; WCR, waist- to-chest ratio; WHR, waist-to-hip ratio; WHtR, waist-to- height ratio. WCR and lung function in Chinese youth 1115 © 2012 The Authors Respirology © 2012 Asian Pacific Society of Respirology Respirology (2012) 17, 1114–1118 0.823 and 0.644, respectively, whereas the correlations between BMI and WCR, and waist-to-hip ratio were weak, with correlation coefficients of 0.11 and 0.229, respectively. Table 3 shows the correlations between lung function parameters and the anthropometric measurements. WCR was inversely correlated with all lung function parameters, and the associations between WCR and most of the lung function param- eters were stronger than those between waist-to-hip ratio and these parameters. Multiple regression analysis demonstrated that WCR and BMI were significant predictors of lung function (Tables 4,5). WCR was inversely associated with FVC, FEV1, FEV1/FVC, PEF and MMEF, after adjustment for gender, standing height and BMI. On average, each 0.01 increase in WCR was associated with a decrease of 8.14 mL for FVC, 9.36 mL for FEV1, 6.54% for FEV1/FVC, 19.81 mL/s for PEF and 17.25 mL/s for MMEF. BMI was positively associated with FVC, FEV1, FEV1/FVC, PEF and MMEF when gender, standing height and WCR were included in the models (Tables 4,5). On average, an increase in BMI of 1 kg/m2 was associated with an increase of 60 mL for FVC, 48 mL for FEV1, 4.2% for FEV1/FVC, 91 mL/s for PEF and 38 mL/s for MMEF. All variance inflation factors for the independent variables were <2 (Tables 4,5), indicating that there was only a small probability of colinearity in these models. Table 2 Correlations among anthropometric parameters Height BMI WC CC WCR WHR WHtR Height 1.00 0.328 0.413 0.600 -0.161 -0.137 -0.142 BMI — 1.00 0.778 0.823 0.110 0.229 0.644 WC — — 1.00 0.775 0.532 0.623 0.841 CC — — — 1.00 -0.116 0.163 0.483 WCR — — — — 1.00 0.775 0.679 WHR — — — — — 1.00 0.766 WHtR — — — — — — 1.00 All correlations were significant (P < 0.001). BMI, body mass index; CC, chest circumference; WC, waist circumference; WCR, waist-to-chest ratio; WHR, waist-to-hip ratio; WHtR, waist-to-height ratio. Table 3 Correlations between lung function and anthropometric parameters FVC, L FEV1, L FEV1/FVC, % PEF, L/s MMEF, L/s Height 0.822 0.835 NS 0.713 0.615 BMI 0.436 0.419 -0.135 0.364 0.287 WCR -0.127 -0.161 -0.147 -0.151 -0.201 WHR NS -0.073 -0.179 -0.088 -0.157 WHtR NS NS -0.169 NS -0.088 All correlations were significant (P < 0.01) unless indicated otherwise. BMI, body mass index; FEV1, forced expiratory volume in 1 s; FVC, forced vital capacity; MMEF, maximal mid-expiratory flow; NS, not significant; PEF, peak expiratory flow; WCR, waist-to-chest ratio; WHR, waist-to-hip ratio; WHtR, waist-to-height ratio. Table 4 Regression coefficients for WCR and other anthropometric parameters in models predicting FVC, FEV1 and FEV1/FVC of the subjects† FVC, L FEV1, L FEV1/FVC, % B SE P value VIF B SE P value VIF B SE P value VIF WCR -0.814 0.164 <0.001 1.109 -0.936 0.15 <0.001 1.109 -6.541 1.842 <0.001 1.043 Gender -0.309 0.022 <0.001 1.079 -0.205 0.02 <0.001 1.079 2.592 0.246 <0.001 1.034 Height, cm 0.055 0.001 <0.001 1.224 0.052 0.001 <0.001 1.224 — — — — BMI, kg/m2 0.06 0.004 <0.001 1.181 0.048 0.004 <0.001 1.181 -0.235 0.042 <0.001 1.018 †Multiple regression analysis, n = 1572. B, regression coefficient for each predictor; BMI, body mass index; FEV1, forced expiratory volume in 1 s; FVC, forced vital capacity; SE, standard error; VIF, variance inflation factor; WCR, waist-to-chest ratio. K Feng et al.1116 © 2012 The Authors Respirology © 2012 Asian Pacific Society of Respirology Respirology (2012) 17, 1114–1118 DISCUSSION The aim of the present study was to examine the asso- ciation between WC and ventilatory function, after adjustment for confounders, including CC, in Chinese children. To achieve this aim, gender and height (key determinants of lung function) were first introduced into the model, followed by BMI, which is related to obesity, then WC and CC. There were strong correlations between BMI, CC and WC. However, inclusion of these correlated variables in multiple regression models may result in colinearity; therefore, CC and WC were combined as the WCR, which showed weak correlations with height and BMI and stronger correlations with lung function parameters than either waist-to-hip ratio or waist-to-height ratio. When WCR was introduced into the regression models for predicting lung function, WCR was shown to be inversely associated with FVC, FEV1, FEV1/FVC, PEF and MMEF in Chinese children. Spirometry is used to measure dynamic lung volumes and the rate of airflow. In restrictive lung disorders, FEV1 and FVC are both decreased, but FVC is decreased more than FEV1, resulting in a normal or elevated FEV1/FVC ratio. In obstructive lung disor- ders, FEV1 is usually decreased more than FVC, and consequently, the FEV1/FVC ratio is decreased. In children, WCR appears to have an inverse association with FEV1 and to a lesser extent than FVC; therefore, WCR becomes an important predictor of the FEV1/ FVC ratio. Because the present study showed that WCR was inversely associated with both FVC and FEV1, a reduction in the FEV1/FVC ratio in children with a large WCR may indicate airway obstruction. WCR may be a better index of obesity than BMI in Chinese children. Studies on the relationship between WC and lung function have mostly been performed in Asian and White adult populations, but there have been few studies in children.2–4 Recently, Chen et al. reported on the relationship between WC and lung function in White children.5 To the best of our knowledge, the present study is the first that has explored the rela- tionship between WC and lung function in a popula- tion of Asian children. The present results suggested that after adjustment for other variables, WC in children was inversely asso- ciated with dynamic lung volumes, including FVC, FEV1, FEV1/FVC and expiratory flow rates that reflect airway size, such as PEF and MMEF. These findings are consistent with the results of previous studies on the relationship between WC and pulmonary func- tion in adults. The adverse effect of abdominal obesity on lung function is likely due to at least two factors:1–4,9 (i) fat deposited in the abdominal cavity directly impedes the descent of the diaphragm and decreases thoracic compliance; and (ii) visceral fat may con- strain both large and small airways. However, there are some major differences between the present study and that of Chen et al., which was conducted in White children.5 In White children, WC was positively associated with both FVC and FEV1, which contrasts with the results from the present study. The discrepancy between these studies is pos- sibly due to several factors: (i) the independent vari- ables were different; CC was introduced into the model in the present study but was not used in the study of Chen et al.; (ii) air pollution; during at least the last three decades, economic development in China has been very rapid, resulting in serious air pollution, which has possibly caused a decrease in lung function, as suggested by previous studies;10,11 (iii) undiagnosed asthma; some subjects with undiag- nosed asthma may be obese or overweight, which may contribute to decreased lung function;12–14 (iv) differences in fat distribution among different races; regardless of whether the population studied is chil- dren or adults and given the condition of roughly equal BMI, Asians tend to have a higher percentage of body fat and a tendency to more central obesity than White people15–20; and (v) puberty; the development of puberty may be relatively delayed in Chinese com- pared with White children. The reasons for this may differ from those mentioned previously and may include socioeconomic factors, which need be further explored. In the present study, the relationship between BMI and lung function was also investigated, and BMI was shown to be positively associated with FVC, FEV1, PEF and MMEF, but inversely associated with FEV1/FVC, which is similar to the findings of Chen et al.5 The present results also suggested that when other vari- ables were fixed, CC was positively associated with all the measured spirometric parameters, which is Table 5 Regression coefficients for WCR and other anthropometric parameters in models predicting PEF and MMEF of the subjects† PEF, L/s MMEF, L/s B SE P value VIF B SE P value VIF WCR -1.981 0.427 <0.001 1.109 -1.725 0.335 <0.001 1.109 Gender -0.323 0.057 <0.001 1.079 0.163 0.044 <0.001 1.079 Height, cm 0.095 0.003 <0.001 1.223 0.06 0.002 <0.001 1.223 BMI, kg/m2 0.091 0.01 <0.001 1.18 0.038 0.008 <0.001 1.18 †Multiple regression analysis, n = 1572. B, regression coefficient for each predictor; BMI, body mass index; MMEF, maximal mid-expiratory flow; PEF, peak expiratory flow; SE, standard error; VIF, variance inflation factor; WCR, waist-to-chest ratio. WCR and lung function in Chinese youth 1117 © 2012 The Authors Respirology © 2012 Asian Pacific Society of Respirology Respirology (2012) 17, 1114–1118 similar to the results from previous studies.6,7 There- fore, in Chinese children, BMI and CC are more indicative of body size than fat mass. A strength of this study was that only non-smokers were included. Previous or current smoking is one of the most important confounding factors affecting lung function. To eliminate any influence of smoking, subjects with even a minimal history of smoking were excluded. However, as this was a cross-sectional study, a cause-and-effect relationship between WCR and decreased lung function could not be inferred. To confirm causality between WCR and pulmonary func- tion, an individualized follow-up study is required. In conclusion, the inverse association between WCR, as well as WC, and lung function in Chinese children may be an effect of abdominal fat deposition. Although the magnitude of the inverse association between WCR and lung function was relatively small in clinical terms, from a public health viewpoint, these findings indicate yet another adverse conse- quence of childhood obesity in the Asian population. ACKNOWLEDGEMENT This study was funded by the Key Basic Research Program of the Ministry of Science and Technology of China as a part of the Enlarged Population Investiga- tion of Human Physiolo