the random sample comprises 267 caucasian preadolescents ( 121 girls , 146 boys ) and 330 caucasian adolescents ( 164 girls , 166 boys ) of a population belonging to the swedish part of the european youth heart study ( eyhs ) .
the eyhs is a cross - sectional school - based study of risk factors for future cardiovascular disease among preadolescents ( 910 years old ) and adolescents ( 1516 years old ) .
the mean ages in the swedish sample for preadolescents and adolescents were 9.6 years and 15.6 years , respectively .
height , weight , and birth weight were measured by internationally accepted standardised procedures ( 22 ) .
body mass index ( bmi ) was calculated as weight / height2 ( kg / m ) .
identification of sexual maturity was assessed according to tanner and whitehouse 1976 . a researcher of the same gender as the child recorded the pubertal stage after brief observation . on account of ethical reasons
the direction of one school preferred not to take part in the assessment of sexual maturity .
fifty subjects did therefore not participate in this assessment ( 3 preadolescent girls , 7 preadolescent boys , 25 adolescent girls , and 15 adolescent boys ) .
the consumption of milk was assessed by an interviewer - mediated 24-hour recall . in preadolescents ,
a qualitative food record completed the day before the interview with the help of parents served as a checklist for the data obtained during the recall .
dietary data were processed by stormats ( version 4.02 , rudans lttdata , sweden ) and analysed using the swedish national food database ( version 99.1 ) . as part of a broad - ranging questionnaire , parents of the participants were asked if their child had a chronic illness or adhered to a special diet . since under - nourishment is practically non - existent in a nutritionally replete population such as sweden and milk intake accounts in part for daily energy intake , the bmi was taken as proxy for energy intake .
furthermore , bmi is traditionally used to validate energy intake data ( 23 ) . for the genetic analysis of lp and lnp ,
genomic dna was isolated from edta whole blood samples from the individuals with the qiaamp dna blood mini kit spin procedure .
the dna fragment spanning the -13910-c / t polymorphic site was amplified using a biotinylated forward - primer ( 5 gggctggcaatacagataagata-3 ) and an unbiotinylated reverse - primer ( 5 agcagggctcaaagaacaatcta-3 ) .
the applied sequencing primer was : 5-ctttgaggccaggg-3. sequencing was performed using a psq96 snp reagent kit and a psq 96ma system ( pyrosequencing ab ) psq 96ma 2.0.1 software .
the procedure has been previously described in detail ( 24 , 25 ) . for the determination of socio - economic status ( ses ) we used the dichotomous variable , below versus above the mean income level in the catchment areas of the sample ( below or above the mean in their municipality ) on account of the relatively equal distribution of income in sweden ( 26 ) . for four subjects , data for ses
student 's t - test was used to determine differences in milk intake ( g / d ) between lp and lnp subjects .
milk intake ( g / d ) was somewhat skewed to the right and therefore split in quintiles of milk intake .
stepwise backward multiple linear regression analysis was performed in order to study the relationship between milk intake in quintiles and body height ( cm ) after adjustments for sex , birth weight ( g ) , father 's and mother 's height ( cm ) , bmi , tanner stage ( 15 ) , and ses .
the study was approved by the research ethics committees of rebro county council and huddinge university hospital .
parents and 15-year - olds gave specific written informed consent to participate in the study .
the variables included in the backward multivariate regression model were : milk intake , lct c > t-13910 polymorphism ( lp = ct , tt vs. lnp = cc ) , sex , birth weight , father 's and mother 's height , bmi , tanner stage , and ses .
characteristics of preadolescents and adolescents by quintile of milk intake are given in table 2 .
basic characteristics of covariates to body height in the study population of swedish preadolescents and adolescents variables associated to body height by quintiles of milk intake fifty - six ( 9% ) of the subjects of the whole sample were lnp . where lnp may restrict the intake of milk in these subjects and
can thereby have an effect on the dependent variable , body height as well as the exposure variable , i.e. milk intake .
milk intake tended to be lower in lnp compared to lp but the difference did not reach statistical significance ( p=0.067 ) .
the studied exposure variables in this model were milk intake in quintiles and the lct c > t-13910 polymorphism , and the outcome variable was body height .
the model explained 90% of the observed variance of body height in preadolescents and adolescents ( adjusted r 
 = 0.89 ) .
non - significant variables were removed in a stepwise manner by elimination when p(f )  0.10 .
model 1 included the variables : milk intake in quintiles , lct-13910 c > t polymorphism , sex , birth weight , parents height , tanner stage , bmi , and ses . in a first step
the variable bmi was eliminated and in a second step ses ( table 3 ) .
milk intake ( g / d ) in quintiles remained significantly different ( =0.46 ; 95% ci : 0.040 , 0.87 and p=0.032 ) in the final model .
in addition , the lct c > t-13910 polymorphism ( lp vs. lnp ) , remained significant in the final model ( =2.05 ; 95% ci : 0.18 , 3.92 and p=0.032 ) , showing a positive association of lp with height .
main effect by quintiles of milk intake and lct-13910 c > t ( exposure variables ) on body height in swedish preadolescents and adolescents ( n=542 ) ; stepwise backward multiple linear regression models were used included variables in model 1 : birth weight , sex , mother 's and father 's height .
bmi , tanner stage , ses excluded variable : bmi ( p[f ]  0.10 ) . excluded variables : ses ( p[f ]  0.10 ) .
we found that milk intake and lct c > t-13910 polymorphism , known to modulate milk consumption , were significant contributors to the observed variance of body height in the studied population .
we primarily did not expect to find our hypothesis confirmed in a nutritionally replete country with already one of the highest intakes of milk and dairy products per capita in the world .
the lct c > t-13910 polymorphism might contribute with a small effect to phenotypic variation in height .
as far as we know no genome wide association studies or meta - analysis have been performed previously addressing the question whether lp or lnp affects body height .
increasing evidence suggests that cow 's milk consumption exerts a positive effect on longitudinal growth in preadolescents and adolescents , for instance , by affecting the igf - i - axis ( 8 , 11 , 2730 ) .
but not all studies show a positive effect of milk on height , especially in nordic countries , where average milk intake per capita traditionally is high ( 31 ) .
the lct c > t-13910 polymorphism , known to modulate milk intake ( 32 , 33 ) , remained significant in the final model . by including the lct c >
t-13910 polymorphism in our model , we subtract for hidden heritability in an already predominantly lp population ( 18 ) . in a recent prospective study , evidence of an association between increased infant size at birth and cow milk consumption during pregnancy
it is known that lp subjects consume on average more milk than lnp subjects . where lp might thus lead to an increased susceptibility towards already prenatally programmed increased body sizes , mediated by the higher capacity of lp mothers to consume milk and dairy products .
our study accounts for parental height as regards milk intake and its potential effects on body height and growth in preadolescents and adolescents .
despite the circumstance that body height is a highly heritable complex trait ; corrections for parental height continue to be an exception , also shown in recent studies ( 31 ) .
even twin growth studies have been performed without consideration of parental height ( 35 ) .
it has been demonstrated that at the current stage a simple prediction based on phenotype of relatives obviously outperforms sophisticated genomic predictions as regards body height ( 36 )
. limitations of this study are the sample size , and the limits inherent to cross - sectional studies as regards causal inference .
cow 's milk is an evolutionary food constructed to promote growth and development in calves and appears to affect growth in human beings also ( 13 , 14 , 28 ) .
milk has become a normative food for preadolescents and adolescents beyond weaning age , even in asian populations and developing countries , which traditionally did not have home fare based on domesticated dairy animals ( 8 , 37 ) .
the long - term effects of this changed nutritional normative are yet poorly understood , even in western countries .
we conclude that actual milk intake and the genetic lp trait is positively associated with body height in preadolescents and adolescents .
nevertheless , higher milk consumption in childhood might exert both negative and positive effects , since greater height has been associated with higher risk of some cancers ( 29 , 3841 ) .
this study was supported by grants from rebro lns landstings forskningskommitt and by nyckelfonden , rebro , sweden .
 backgroundbody height is a classic polygenic trait . about 80%90% of height 
 is inherited and 10%20% owed to environmental factors , of which the most important ones are nutrition and diseases in preadolescents and adolescents.objectivethe aim of this study was to explore potential relations between the lct ( lactase ) c > t-13910 polymorphism , milk consumption , and body height in a sample of swedish preadolescents and adolescents.designin a cross - sectional study , using a random sample of preadolescents and adolescents ( n = 597 ) , dietary intakes were determined . 
 anthropometric measurements including sexual maturity ( tanner stage ) and birth weight were assessed . parental body height and socio - economic status ( ses ) were obtained by questionnaires . 
 genotyping for the lct c > t-13910 polymorphism that renders individuals lactase persistent ( lp ) or lactase non - persistent ( lnp ) was performed by dna sequencing . 
 stepwise backward multivariate linear regression was used.resultsmilk consumption was significantly and positively associated with body height (  = 0.45 ; 95% ci : 0.040 , 0.87 , p = 0.032 ) . 
 adjustments were performed for sex , parental height , birth weight , body mass index ( bmi ) , ses , and tanner stage . 
 this model explains 90% of the observed variance of body height ( adjusted r2 = 0.89 ) . 
 the presence of the -13910 t allele was positively associated with body height (  = 2.05 ; 95% ci : 0.18 , 3.92 , p = 0.032).conclusionsmilk consumption is positively associated with body height in preadolescents and adolescents . we show for the first time that a nutrigenetic variant might be able to explain in part phenotypic variation of body height in preadolescents and adolescents . due to the small sample size 
 further studies are needed . 
Methods
Results
Discussion
Conflict of interest and funding