the growing literature on the effects of obesity on children 's self - reported health - related quality of life ( hrqol ) has shown negative associations between some body mass index percentile ( bmi% ) categories and hrqol [ 19 ] . these studies , however , have mainly been clinic based , used small samples at the extreme ends of the bmi distribution , and included limited numbers of minority children , who suffer the greatest burden from obesity . although there were two community - based studies that analyzed the relationship between bmi and hrqol among ethnically diverse children , the percentages of african american and hispanic children were small and the hrqol instrument used were health status and not preference weighted [ 3 , 6 ] . preference - weighted quality of life ( qol ) measurements , also known as quality - adjusted life - years ( qalys ) , is the measurement recommended by the us panel on cost - effectiveness in health and medicine for cost - effectiveness analysis ( cea ) . qaly measures are based on economic theories ( utility and game theories ) that quantify the way in which people make choices when faced with uncertainty . health status instruments ask people to describe the level of disability in several domains ( e.g. , vision , hearing , and mobility ) . qaly measures provide additional information , asking people to determine the risk of death they are willing to take to improve that level of disability . qaly scores usually range from 0 to 1 , where 0 represents death and 1 represents perfect health . for example a score of  0.80 means that an individual is willing to give up  0.20 of their life to live in perfect health . the qaly classification system intention is to put a worth or  monetary term  to health outcomes . by measuring cost and health outcomes to date , studies examining the relation between bmi% categories and hrqol have not reported preference - weighted scores among ethnically diverse children . we report the associations between bmi% categories and preference - weighted scores among a large cohort of ethnically diverse sixth grade children who participated in the healthy school - based type 2 diabetes risk factor prevention study . health utility index 2 ( hui2 ) and health utility index 3 ( hui3 ) and the feeling thermometer ( ft ) were the preference - weighted qol instruments used to measure student 's preference scores . we hypothesized that bmi% categories are negatively associated with preference - weighted qol scores in ethnically diverse middle school children . the healthy intervention , which focused on environmental and individual changes in nutrition , physical activity , and behavior , was conducted in 42 middle schools recruited by 7 field centers across the usa . the goal of healthy was to reduce or moderate 4 risk factors for type 2 diabetes : bmi% , waist circumference , and fasting blood glucose and insulin levels . healthy was initiated among sixth grade students at the beginning of the 2006 - 2007 school year and continued through the eighth grade in the 2008 - 2009 school year . eligible schools had at least 50% minority students , defined as african american , hispanic , or american indian , or at least 50% of the students eligible for free or reduced price meals from the national school lunch program ( nslp ) . federalwide assurance to conduct federally funded research was obtained for all schools in the study . written parent measures were collected at baseline from sixth grade students during the 2006 - 2007 school year . the hui2 , hui3 , and the ft from the euroqol   were the preference - weighted qol instruments used . the hui instrument asks respondents to rate their current level of health function across a number of domains . the hui2 assesses seven health domains : sensation , mobility , emotion , cognition , self - care , pain , and fertility . the hui3 assesses 8 domains : vision , hearing , speech , ambulation , dexterity , emotion , cognition , and pain . preference scores are assigned to these ratings by use of utility scoring rules that have been developed by use of samples from the general public [ 15 , 16 ] . reliability and validity of the instrument have been shown to be acceptable in children as young as 10 years ; several studies , proxy and self - reported , have used the instrument to assess preference - weighted scores among children younger than 11 [ 1924 ] . we asked participants to rate how good or bad their current health is on a 0 to 100 scale , where 0 represented  worst imaginable health  and 100 represented ft ratings were divided by 100 in order to make them comparable to hui scores . the ft has been shown to be reliable and valid in children as young as 8 year [ 25 , 26 ] ; civita et al . have reported that the ft has been used with children as young as 7 years of age . the hui questionnaire was administered to the students under staff supervision by use of  personal digital assistants . weight was measured by use of seca alpha 882 digital scales ( seca corporation , chino , ca , usa ) ; height was measured by use of pe - aim-101 stadiometers ( perspective enterprises , portage , mi , usa ) . bmi% was calculated from the centers for disease control and prevention bmi - for - age - and - sex growth charts and categorized as underweight ( < 5 ) , normal weight ( 5 to < 85 ) , overweight ( 85 to < 95 ) , obesity ( 95 to 99 ) , and severe obesity ( > 99 ) . we categorized fasting glucose as < 100  mg / dl , 100 to < 110  mg / dl , 110 to < 126  mg / dl , and 126 + mg / dl , and fasting insulin as < 30  u / ml and 30 +  u / ml . we collected self - report information on student age , gender , pubertal status ( by use of the tanner scale ) , and race / ethnicity . parents provided information about family history of diabetes and , as a measure of socioeconomic status , the highest educational grade attained in the household . age , gender , race / ethnicity , and parental education have been commonly controlled for in the literature that has studied the relationship between children 's self - reported hrqol and bmi% [ 13 , 58 ] . exclusion criteria for this analysis were the following : children who were underweight , age 13 years or older , and had missing qol scores and covariate data . children who were underweight ( < 5 bmi% ) were excluded because of the small proportion , the mean qol scores for underweight and normal weight were nearly similar , and the study aim was to evaluate the relationship between qol scores and greater bmi% ranges . because the average age of a sixth grade student is 11 years , students age 13 years or more may have been retained in the sixth grade for reasons other than health and thus might have influenced qol scores independent of bmi% categories . comparisons were performed using analysis of variance for self - ratings of preference - weighted qol scores , and a p value < 0.05 was considered significant with no adjustment for multiple comparisons . we used linear mixed model analysis that accounts for the clustering of students within schools to assess the association between bmi% categories and qol scores adjusted for covariates . covariates included were age , sex , blood glucose and insulin , tanner stage , race / ethnicity , family history of diabetes , and educational attainment . for covariates that had only 2 categories , a single difference in qol score and p value was reported compared to the reference group . for covariates that had more than 2 categories , an overall p value as well as differences and p values for the named categories versus the reference group was reported . reference groups were normal weight , fasting glucose < 100  mg / dl , fasting insulin < 30  u / ml , age of 11 years , female , tanner stage 1 , no family history of diabetes , non - hispanic white children , and college graduate . of the approximately 11,158 sixth grade students at 42 schools , 6358 ( 57.0% ) had written parent / guardian consent and student assent prior to baseline measurement . ninety - nine children were underweight ( 1.6% ) , 279 were 13 years or older ( 4.4% ) , 389 had missing preference - weighted qol data ( 6.1% ) , and 743 had missing covariate data ( 11.7% ) . after applying the exclusion criteria , 4979 students comprised the analytic sample ( 44.6% of the 11,158 sixth grade students enrolled or 78.3% of the 6,358 students with consent / assent ) . of the analytical sample , 92.2% answered the questionnaire in english and 7.8% in spanish . students who were excluded from the analysis were more likely to be male ( 22.8% versus 20.7% ; p = 0.045 ) . no differences were seen for bmi% categories ( p = 0.13 ) or for the other variables we collected . the rate of combined obesity and severe obesity was 30.5% ( 23.6% obesity and 6.9% severe obesity ) . african american and hispanic children made up 78.8% of the participants , and 27.0% were from families with low educational attainment ( as measured by no high school diploma ) . the average percent of students eligible for nslp in the schools that participated in the healthy study was 76.6% . the mean ( sd ) preference - weighted qol scores were 0.846 ( 0.160 ) for the hui2 , 0.796 ( 0.237 ) for the hui3 , and 0.806 ( 0.161 ) for the ft . bmi% categories were negatively associated with qol scores on all three instruments ( hui2 p < 0.001 , hui3 p = 0.004 , and ft p < 0.001 ) . scores for obese children ( hui2 p = 0.007 , hui3 p = 0.026 , and ft p < 0.001 ) and severely obese ( hui2 p < 0.001 , hui3 p < 0.001 , and ft p < 0.001 ) children were significantly lower than those for normal weight children . when overweight children were compared with normal weight , hui2 and hui3 scores showed no significant difference . other clinical and demographic categories that showed significance after being stratified by qol scores are shown in table 2 . table 3 shows the adjusted associations between clinical and demographic categories and the 3 qol score differences derived from the mixed model analyses . only children with severe obesity remained with significantly lower qol scores , compared to normal weight , on all three instruments ( hui2 p = 0.013 ; hui3 p = 0.025 ; and obese and overweight children did not have significantly lower scores than normal weight children on the hui2 and hui3 . hispanic and black children had significantly lower qol scores than non - hispanic white children on the hui2 and hui3 instruments but not on the ft . other characteristics that were significantly associated with one or some of the qol scores were age , gender , and tanner stage . bmi% categories were significantly associated with lower hui2 functional ratings in the mobility domain and with lower hui3 functional ratings in the speech domain ( see figure 1 ) . obese and severely obese children were 1.5 and 2.9 times more likely , respectively , to present lower levels of hui2 mobility . both obese and severely obese children were 1.3 times more likely to show lower levels of hui3 speech , although findings were not statistically significant in severely obese children . this is the first school - based study to measure preference - weighted qol scores in a large , ethnically diverse population of sixth grade students . the purpose was to determine the association between preference - weighted scores using three instruments ( hui2 , hui3 , and ft ) and bmi% among mostly minority children . students who were severely obese rated their preference - weighted qol in all three instruments significantly lower than those who were normal weight , before and after the adjustment for demographic factors and glucose and insulin levels . scores based on the ft instrument were significantly lower among overweight , obese , and severely obese students than their normal weight counterparts . a number of studies have found that children with combined obesity and severe obesity report significantly lower qol scores than do normal weight children [ 18 ] , but none to our knowledge have studied a range of bmi categories and preference - weighted qol instruments among a large population of minority children . the current paper is the first to suggest that preference - weighted qol function ratings decrease clinical at severe obesity level ( > 99% ) . although there are no studies of children to determine the clinical significance of differences in qol scores , reports from adult populations have identified differences of 0.03 as being clinically significant and differences of as little as 0.01 as being meaningful [ 3033 ] . by this measure , severely obese children in the current study had clinically and meaningful differences in all from the three instruments ( range 0.03 to 0.09 ) . there is only one other study in the usa that used the hui3 to compare scores between normal weight and overweight / obese children . this study used a convenience sample of 76 predominantly african american and hispanic children , age of 518 , drawn from hospital clinics . the overall hui3 score for the entire sample was 0.79 ( 0.17 ) which is close to the hui3 score in our population ( 0.80 ( 0.24 ) ) . also similar to the healthy study , their study did not show significant differences in hui3 scores between the normal weight and overweight / obese groups ( 0.81 versus 0.78 , resp . ) . the healthy study extends these findings into a larger group of minority children and a wider range of bmi% . the significantly lower qol scores in the healthy study were due , in part , to lower levels of functioning reported by children in the mobility domain ( bend , lift , jump , walk , and run ) for the hui2 and the speech domain ( being able to be understood when speaking and being able to speak at all ) for the hui3 . the low mobility score in obese and severely obese children is well documented in the literature [ 5 , 7 , 35 ] .   the second domain affected among obese , but not severely obese children , was speech . an extensive review of the literature was conducted , and no other study was found showing this relationship . hui3 was also analyzed in our study by english and spanish responders , and there was no difference in the speech domain between groups . because we have no explanation for this finding , further studies are needed to fully understand this association . after adjusting for covariates , being older , male , hispanic , african american , and advanced tanner stage were associated with lower qol scores . the rate of severe obesity for children 10 or younger , 11 , and 12 years of age was 5.5% , 6.2% , and 8.9% , respectively ; for males and females , it was 7.7% and 6.2% , respectively ; and for hispanics , african americans , non - hispanic white , it was 7.3% , 8.0% , and 4.9% , respectively . qol scores were lower in older , male , and minority children because of their higher severe obesity rates . for tanner stage , longitudinal studies have shown that obese children have more advanced tanner stage than their lean counterparts [ 3638 ] . the strength of this study is in the use of preference - weighted qol instruments in a large school - based cohort of ethnically diverse children . this study is unique because it involves minority children who have the highest rates of obesity , and it is important to understand the role that bmi% categories may have on these children 's physical and mental function . there are only two community - based studies involving small number of minority children and none used preference - weighted qol instruments ; and the only study to use a preference - weighted qol instrument included a small number of minority children who were enrolled in hospital clinics .   despite these strengths , when we analyzed the bmi , age , ethnicity , and sex between consented and nonconsented children , however , we found no significant differences . drawing three tubes of blood to measure lipids , insulin , and glucose may have dampened response rates , but in return we collected valuable biochemistries to include as covariates . the present study had 73% african american and hispanic children , whereas nationally 39% of children enrolled in public schools are african american and hispanic . nonetheless , minority and disadvantaged children were oversampled because of their higher risk for obesity and type 2 diabetes . third , the algorithms for estimating hui preference - weighted scores were not derived from children or u.s . they were derived from white middle - class canadian adults [ 13 , 15 , 16 ] . health care cost and preference - weighted scores used for cea are usually considered from a societal perspective . it is the society that usually pays health care bills , and as the budget holder , it insists on economic evaluations to inform decisions of resource allocations . to develop hui preference scores for children , adults were asked to take risk on their children 's health outcomes given several fictitious health states . they were asked , for example , if their child had a physical or mental disability , would they prefer a treatment that would decrease the child 's lifespan to give him / her a better quality of life or leave the disability unchanged to preserve the longer lifespan . it is likely that parents anywhere would make decisions on what is best for their child given a medical condition similar to those made by the middle - class canadian adults who were involved in developing the hui preference - weighted scores . nonetheless , preference - weighted qol measure in children is still an incomplete science , and more research is needed to determine their discriminative and evaluative roles . in conclusion , we found that severely obese children of ethnically diverse backgrounds had significantly lower preference - weighted qol scores than did normal weight children in all three instruments . being overweight and obese was related to lower preference scores in one of the three instruments . lastly , although this is the first study to evaluate the relationship between preference - weighted scores and bmi% categories in a large cohort of mostly minority children , more research is needed to validate preference - weighted qol instruments in children .
to date , studies examining the relation between body mass index percentile ( bmi% ) categories and health - related quality of life ( qol ) measurements have not reported preference - weighted scores among ethnically diverse children . 
 we report the associations between bmi% categories and preference - weighted scores among a large cohort of ethnically diverse sixth grade children who participated in the healthy school - based type 2 diabetes risk factor prevention study . 
 health utility index 2 ( hui2 ) and health utility index 3 ( hui3 ) and the feeling thermometer ( ft ) were the preference - weighted qol instruments used to measure student 's preference scores . 
 of 6358 consented students , 4979 ( 78.3% ) had complete qol , height , weight , and covariate data . 
 mean ( sd ) preference scores were 0.846 ( 0.160 ) , 0.796 ( 0.237 ) , and 0.806 ( 0.161 ) for the hui2 , hui3 , and ft , respectively . 
 after adjusting for age , sex , blood glucose and insulin , tanner stage , race / ethnicity , family history of diabetes , and educational attainment , children with severe obesity ( > 99% ) had significantly lower preference scores compared to normal weight on all three instruments ( hui2 p = 0.013 ; hui3 p = 0.025 ; and 
 ft p < 0.001 ) . 
 obese and severe obese categories were significantly associated with lower hui2 functional ratings in the mobility domain and with lower hui3 functional ratings in the speech domain .