asthma is one of the most common chronic illnesses in children , and obesity is another important public health problem that can affect children ( 1 , 2 ) . recently the prevalence of both asthma and obesity have increased substantially in many countries , leading to speculation that obese people might be at an increased risk of developing asthma ( 3 , 4 ) . although a large number of studies have been performed , the possibility that obesity might increase the risk of asthma development remains controversial ( 5 ) . nevertheless , potential mechanisms have been suggested that obesity - related changes in lung volumes , systemic inflammation , and other adipocyte - derived factors such as leptin , adiponectin , resistin , and visfatin , as well as cytokines and chemokines that might alter smooth muscle function in such a way as to promote airway narrowing ( 2 , 6 ) . asthma is recognized as a chronic inflammatory disease in the airways ( 7 ) , and the obese state is characterized by low - grade systemic inflammation , as indicated by increased levels of the inflammatory markers c - reactive protein ( crp ) and il-6 in the circulation of obese subjects ( 6 ) . the effects of increased body mass index ( bmi ) on asthma may be mediated by upregulation of inflammatory mechanisms in the airway epithelium ( 8) . the current view of adipose tissue is that of an active secretory organ , sending out and responding to signals that modulate appetite , energy expenditure , insulin sensitivity , endocrine and reproductive systems , bone metabolism , and inflammation and immunity ( 6 ) . the serum concentrations of many adipokines could impact airway function , which might lead to asthma in both human and murine subjects with obesity ( 9 ) . a recent study showed that leptin , an adipokine , might play a role in atopic asthma in children ( 8) . retinol - binding protein ( rbp ) 4 is highly expressed in isolated mature human adipocytes and secreted by differentiating human adipocytes ( 10 ) . previous studies have reported significant associations between rbp4 levels and bmi ( 11 , 12 ) . in contrast , other studies have not found a link between rbp4 levels and bmi ( 13 , 14 ) . recent reports showed that rbp4 correlated with inflammatory factors such as crp and il-6 ( 15 ) , and rbp4 mrna had a strong positive correlation with markers of adipose inflammation ( monocyte chemoattractant protein-1 and cd68 ) ( 16 ) . several studies have been reported on the relationship between rbp4 and obesity , adipose inflammation but , there has been no reports showing association between rbp4 and childhood asthma . we evaluated the role of rbp4 in childhood asthma and its association with atopy markers , pulmonary function , and bronchial hyperresponsiveness . one hundred sixty prepubertal subjects , aged from 6 to 10 yr , were recruited from severance hospital , yonsei university college of medicine . asthma was defined as recurrent wheezing or coughing episodes in the absence of common cold in the preceding 12 months . additionally a physician 's diagnosis of bronchial hyperresponsiveness upon methacholine challenge ( pc2016 mg / ml ) and at least 12% reversibility of forced expiratory volume in 1 second ( fev1 ) after inhaling a 2 agonist , as per criteria of the american thoracic society ( ats ) , were required ( 17 ) . children with history of asthma attacks requiring systemic corticosteroids within the preceding 6 months were excluded . the control participants were age - matched healthy children who visited the hospital for a general health work - up or vaccination and had no history of wheezing , recurrent chronic disease , infection during the preceding two weeks , or hypersensitivity to methacholine . height and weight were measured in light clothing without shoes to calculate bmi in kg / m . obesity and overweight classification were defined by bmi for age and sex based on an international survey ( 18 ) . absolute bmi values were then converted to previously established bmi percentiles of korean children for data analysis ( 19 ) . the 85th and 95th percentiles of bmi for age and sex were used as cut off values to identify normal weight , overweight and obesity . children were eligible if they could perform reproducible spirometry and had an fev1 of at least 70% of the predicted value ( 20 ) . in short , after saline inhalation , doses of methacholine ( 0.075 , 0.15 , 0.31 , 0.62 , 1.25 , 2.5 , 5 , 10 , and 25 mg / ml ) were delivered for 0.6 sec through a devilbiss 646 nebulizer using a rosenthal - french dosimeter ( ferraris , hertford , england ) until either fev1 decreased by 20% or more , or the highest dose administered . the challenge test was considered positive if the pc20 was 16 mg / ml or less . the ne-8000 system ( sysmex , kobe , japan ) was used to count eosinophils automatically in peripheral blood . serum total ige and ecp were measured by the cap radioallergosorbent technique ( unicap ; pharmacia and upjohn , uppsala , sweden ) . blood samples to measure serum rbp4 levels were obtained between 6 a.m. and 9 a.m. after an overnight fast . after clotting at 4 , the serum was separated by centrifugation at 1,300 g for 10 min and stored at -70 until the assays . serum rbp4 concentrations were measured with a human rbp4 competitive elisa kit ( adipogen , seoul , korea ) according to the manufacturer 's instructions . the minimum rbp4 detection limit was 1 ng / ml and all assays were performed in duplicate for each sample , with the mean values reported here . descriptive statistics were calculated as the meansd or as median ( interquartile range ) for continuous variables . comparisons between groups were made using a student 's t - test , mann - whitney u test or chi - square test . simple linear regression analysis was performed to assess the age , bmi , fev1 , fev1/forced vital capacity ( fvc ) , forced expiratory flow between 25% and 75% of fvc ( fef25 - 75% ) , pc20 , eosinophil count , serum total ige and ecp on rbp4 levels . all analyses were performed on the statistical package for the social sciences software ( version 13.0 , spss inc , chicago , il , u.s.a . ) . one hundred sixty prepubertal subjects , aged from 6 to 10 yr , were recruited from severance hospital , yonsei university college of medicine . asthma was defined as recurrent wheezing or coughing episodes in the absence of common cold in the preceding 12 months . additionally a physician 's diagnosis of bronchial hyperresponsiveness upon methacholine challenge ( pc2016 mg / ml ) and at least 12% reversibility of forced expiratory volume in 1 second ( fev1 ) after inhaling a 2 agonist , as per criteria of the american thoracic society ( ats ) , were required ( 17 ) . children with history of asthma attacks requiring systemic corticosteroids within the preceding 6 months were excluded . the control participants were age - matched healthy children who visited the hospital for a general health work - up or vaccination and had no history of wheezing , recurrent chronic disease , infection during the preceding two weeks , or hypersensitivity to methacholine . height and weight were measured in light clothing without shoes to calculate bmi in kg / m . obesity and overweight classification were defined by bmi for age and sex based on an international survey ( 18 ) . absolute bmi values were then converted to previously established bmi percentiles of korean children for data analysis ( 19 ) . the 85th and 95th percentiles of bmi for age and sex were used as cut off values to identify normal weight , overweight and obesity . lung function was measured by spirometry ( vmax encore ; viasys healthcare inc . , conshohocken , pa , u.s.a . ) according to ats standards ( 17 ) . children were eligible if they could perform reproducible spirometry and had an fev1 of at least 70% of the predicted value ( 20 ) . in short , after saline inhalation , doses of methacholine ( 0.075 , 0.15 , 0.31 , 0.62 , 1.25 , 2.5 , 5 , 10 , and 25 mg / ml ) were delivered for 0.6 sec through a devilbiss 646 nebulizer using a rosenthal - french dosimeter ( ferraris , hertford , england ) until either fev1 decreased by 20% or more , or the highest dose administered . the challenge test was considered positive if the pc20 was 16 mg / ml or less . the ne-8000 system ( sysmex , kobe , japan ) was used to count eosinophils automatically in peripheral blood . serum total ige and ecp were measured by the cap radioallergosorbent technique ( unicap ; pharmacia and upjohn , uppsala , sweden ) . blood samples to measure serum rbp4 levels were obtained between 6 a.m. and 9 a.m. after an overnight fast . after clotting at 4 , the serum was separated by centrifugation at 1,300 g for 10 min and stored at -70 until the assays . serum rbp4 concentrations were measured with a human rbp4 competitive elisa kit ( adipogen , seoul , korea ) according to the manufacturer 's instructions . the minimum rbp4 detection limit was 1 ng / ml and all assays were performed in duplicate for each sample , with the mean values reported here . descriptive statistics were calculated as the meansd or as median ( interquartile range ) for continuous variables . comparisons between groups were made using a student 's t - test , mann - whitney u test or chi - square test . simple linear regression analysis was performed to assess the age , bmi , fev1 , fev1/forced vital capacity ( fvc ) , forced expiratory flow between 25% and 75% of fvc ( fef25 - 75% ) , pc20 , eosinophil count , serum total ige and ecp on rbp4 levels . all analyses were performed on the statistical package for the social sciences software ( version 13.0 , spss inc , chicago , il , u.s.a . ) . the participants consisted of 102 boys ( 63.8% ) and 58 girls ( 36.2% ) ( mean age , 8.61.2 yr ) . the mean values for age and bmi percentile were not different between the asthmatics and healthy children . eosinophil count , serum total ige and ecp were statistically higher in the asthmatics than the control subjects . fef25 - 75% was significantly lower in the asthmatics than the control subjects ( table 1 ) . in asthmatics , there was no gender difference in asthma severity ( p=0.256 ) ( table 2 ) . 1 , was not significantly different between the asthmatics and control subjects ( meansd , 256.9193.37 g / ml vs. 264.20151.79 g / ml , p = ns ) . simple linear regression analysis was performed with rbp4 as the dependent variable and age , bmi percentile , eosinophil count , serum total ige , ecp , fev1 , fev1/fvc , fef25 - 75% and pc20 as the independent variables for asthmatics divided by sex ( table 3 ) . the results indicate that fev1/fvc and fef25 - 75% contribute significantly to serum rbp4 level in boys with asthma . overweight , and obesity group , no variables were correlated to rbp4 in both sexes and between the asthmatics and the control subjects ( data not shown ) . according to asthma severity , serum total ige and fef25 - 75% contribute to serum rbp4 only in mild asthma group ( data not shown ) . in male asthmatics , serum total ige , fev1 , and fef25 - 75% also contribute significantly to serum rbp4 in mild group ( data not shown ) . no variables were associated with serum rbp4 in moderate and severe group both in all and male asthmatics ( data not shown ) . the aim of this study was to investigate serum rbp4 levels and its relationship between the parameters of atopy , pulmonary function , and bronchial hyperresponsiveness . there were no differences in mean serum rbp4 concentrations between the asthmatics and the control subjects . there was also no significant correlation between serum rbp4 and bmi for both the asthmatics and control group . fev1/fvc and fef25 - 75% contributed to serum rbp4 levels in male asthmatics . there are several proposed mechanisms by which obesity can affect airway function ( 21 ) . a recent review touched on the mechanical , immunological , hormonal and inflammatory effects of obesity that may play a role in the development and persistence of asthma ( 21 , 22 ) . although a large number of cross - sectional , case - control , prospective and weight loss studies have been performed , the possibility that obesity might increase the risk of asthma development still remains controversial ( 3 , 21 ) . although a small study sample and a relatively small obese group may have affected the results , the proportion of obese people in our study is similar to the proportion of obese people in the general korean population . both the low proportion of obesity and the rarity of severe obesity in korea may also have affected the results . the fact that the subjects were prepubertal ( younger than 10 yr ) may also have influenced the results by excluding the effects of sex hormones . although the possibility that obesity might increase the risk of asthma remains controversial , their correlation is still a matter of concern . it is well documented that obesity is an inflammatory state that leads to increased levels of hormones such as leptin , cytokines such as interleukin-6 and tumor necrosis factor- , chemokines and other inflammatory mediators that could potentially influence airway smooth muscle ( 6 ) . rbp4 is a newly discovered fat - derived peptide that modulates glucose metabolism and consequently induces insulin resistance ( 23 ) . a recent report suggests that rbp4 is a central mediator of obesity - induced insulin resistance in mice and humans ( 24 ) . mean serum rbp4 levels were elevated in both nondiabetic and diabetic obese subjects , and serum rbp4 levels were positively correlated with bmi ( 25 ) . previous studies have shown that rbp4 levels are elevated in obese patients ( 12 , 24 , 25 ) . on the other hand , rbp4 level was not different between the asthmatics and normal control groups , and there were no differences in bmi between the asthmatics and controls in our study . the relatively narrow bmi distribution of our study subjects might explain the lack of a relationship . simple linear regression analysis revealed that both fev1/fvc and fef25 - 75% are predictive factors of rbp4 for male asthmatics in our study . the fev1 to fvc ratio is a useful assessment of airflow limitation ( 26 ) . recently , fev1/fvc was reported to be a predictive factor for basement membrane thickness , which is a known early airway remodeling marker ( 27 ) . among the respiratory indices derived from the forced vital capacity maneuver , maximal midexpiratory flow rate ( mmef ) namely fef25 - 75% , which measures the average flow of gas through the middle lung volumes , was originally considered a more sensitive and earlier marker of obstruction in the small airways than fev1 ( 28 , 29 ) . the relationship between serum rbp4 and pulmonary function might be indirectly associated with lung inflammation , changes in small airways , and airflow obstruction , all part of asthma pathogenesis . it is known that asthma is more prevalent in boys younger than 10 yr or the mid - teens , at which point asthma is more prevalent in girls until sexual maturity ( 30 ) . a current study shows that high serum leptin levels in asthmatic boys may partly explain the higher prevalence of childhood asthma in males ( 8) . the often hypothesized role of sexual hormones still remains to be demonstrated ( 30 ) . the small number of participants and the relatively small obese group are limitations of this study . relatively small number of female asthmatics may also have affected the results . in conclusion ,
recently the prevalence of both asthma and obesity have increased substantially in many countries . 
 the aim of this study was to evaluate the role of retinol - binding protein ( rbp ) 4 in childhood asthma and its association with atopy markers , pulmonary function , and bronchial hyperresponsiveness in relation to obesity . 
 we studied 160 children between the ages 6 to 10 yr , including 122 asthmatics and 38 controls . 
 the body mass index , pulmonary function tests , and methacholine challenge tests were measured on the same day . 
 total eosinophil count , serum total ige , serum eosinophil cationic protein , and serum rbp4 were measured in all subjects . 
 there was no difference in serum rbp4 levels between the asthmatics and the control group . in all subjects or subgroups , 
 serum rbp4 was not associated with total eosinophil count , serum total ige , serum eosinophil cationic protein , or pc20 . 
 there was no relationship between serum rbp4 and pulmonary function in female asthmatics . 
 forced expiratory volume in 1 second / forced vital capacity ( fvc ) and forced expiratory flow between 25% and 75% of fvc contributed to serum rbp4 in male asthmatics . 
 our findings show an association between rbp4 and pulmonary function in prepubertal male asthmatics 
 . this relationship may indirectly affect the high prevalence of childhood asthma in males .