vitamin d deficiency , a pandemic health problem , is a major cause of rickets in infants and toddlers and of osteopenia in adolescents [ 16 ] .
the production of vitamin d in the skin depends on sunshine exposure , latitude , skin - covering clothes , the use of sun block , and skin pigmentation .
although the mediterranean region generally has a sunny climate , higher rates of hypovitaminosis d are seen in european and mediterranean countries [ 710 ] .
vitamin d deficiency is especially common in the middle east because of the prevalence of wearing skin - covering clothes and because of staying out of the sun . besides acting as a regulatory hormone in calcium metabolism , noncalciotropic effects of vitamin d such as cellular differentiation and replication in many organs
vitamin d is also critical in glucose homeostasis and insulin secretion via its endocrine mechanisms [ 1214 ] , besides its autocrine and paracrine role in adipocytes [ 15 , 16 ] .
insulin resistance plays a major role in obesity , and as a population gets heavier at younger ages , the age of onset of non - insulin - dependent diabetes mellitus also decreases .
regrettably , obesity and adiposity is an emerging trend in the industrialized world it is a result of alimited exercise , a sedentary lifestyle , and replete diets with high - calorie , low - nutrient foods .  
the literature provides conflicting data as to whether vitamin d deficiency and insufficiency is a risk factor for the development of impaired glucose tolerance in childhood obesity .
low 25-hydroxyvitamin d level is found to be associated with insulin resistance in adults , but no relationship was found with parameters of glucose homeostasis and insulin sensitivity in healthy youth .
this study was conducted to study the relationship of vitamin d deficiency and insufficiency with insulin resistance in obese children and adolescents and the prevalence of vitamin d deficiency among children living in the metropolitan area of istanbul , turkey .
enrolled in the study were 118 obese children ( 50% males , 50% females ; mean age 12  2.2 years ) and a control group of 68 healthy nonobese children ( 48.5% males , 51.5% females ; mean age 12.6  1.7 years ) . they had been admitted to the bezmialem vakif university general pediatrics clinic in the marmara region between january 2011 and january 2012 .
each participant underwent a detailed physical examination ( including evaluation for syndromes and endocrine diseases ) , as well as a laboratory evaluation .
standing height was measured to the nearest 0.1  cm with a harpenden fixed stadiometer .
body weight ( kg ) was measured on a seca balance scale to the nearest 0.1  kg , with each subject dressed in a light t - shirt and shorts .
body mass index ( bmi ) was calculated by dividing weight by height ( kg / m ) .
obesity was defined as the bmi > 97th percentile , the definition of the international task force of obesity in childhood and population - specific data [ 21 , 22 ] .
children whose obesity was the result of a syndromal problem ( prader willi , laurence - moon biedl syndrome , etc . ) were excluded , as were those whose obesity had an endocrinal cause such as cushing 's syndrome or hypothyroidism .
none of the participants was using meneedication or had a history or evidence of current metabolic , cardiovascular , respiratory , or hepatic disease .
all blood analyses were performed on fasting samples in both the study and control groups .
cholesterol , high - density lipoprotein ( hdl ) , low - density lipoprotein ( ldl ) , and triglycerides were measured by the homogeneus colorimetric enzyme technique ( roche , cobas 8000 ) .
serum 25-ohd levels were determined by the electrochemiluminescence enzyme immunoassay method ( eclia ) ( advia centaur , usadpc co. , usa ) .
glucose was measured by the glucose oxidase technique ( siemens advia 1800 ) and insulin levels were analyzed with direct chemiluminescence technique ( siemens centaur , usa ) .
insulin resistance was estimated from fasting plasma measurements using homa - ir ( insulin ( mu / l )  glucose ( mmol / l)/22.5 ) .
insulin resistance criteria were homa - ir > 2.5 for prepubertal children and homa - ir > 4.0 for adolescents
. vitamin d status was classified as either deficient , insufficient , or sufficient ( serum 25-ohd : < 10  ng / ml , 1020  ng / ml , and > 20  ng / ml , resp . ) . statistical analysis was performed with pasw statistics , v.13.0 . a paired t - test was used to calculate the difference of two parameters in groups ; one - way anova was used in the calculation of difference of two parameters in groups with more than two in the same group and between different groups .
categorical data were evaluated using the chi - square test ; p < 0.05 was accepted as statistically significant .
the study was approved by the local ethical committee . written informed consent was obtained from parents .
age and gender distribution were not statistically different between the two groups ( p = 0.79 and 0.839 , resp . ) .
subjects of the obese group had significantly higher waist circumference sds ( 0.027 ) , bmi ( p < 0.001 ) , bmi sds ( p > 0.001 ) , and bmi percentage ( p > 0.001 ) compared with the control group ( table 1 ) . 
 table 2 presents the biochemical characteristics of all subjects . compared with the nonobese subjects , obese subjects had higher fasting insulin , homa - ir , serum alt , triglyceride , and total and ldl - cholesterol levels .
the 25-ohd levels of obese children were significantly lower than those of the non - obese ( p = 0.002 ) .
hdl - cholesterol levels of the obese subjects were significantly lower than the non - obese ones .
the obese group was classified according to 25-ohd levels as vitamin d deficient ( 10  ng / ml ) , insufficient ( 1020  ng / ml ) , and sufficient ( 25-ohd > 20  ng / dl ) .
the correlation of vitamin d levels with insulin resistance in the obese group was evaluated with homa - ir .
the homa - ir levels of the obese subjects who were vitamin d deficient and insufficient did not statistically differ from those with vitamin - d - sufficient ones ( p = 0.72 ) ( table 3 ) . 

table 4 presents the correlations among homa - ir with other biochemical parameters in the obese group .
homa - ir was not correlated significantly with age and serum 25-ohd levels but significantly correlated with triglyceride and ldl levels .
homa - ir correlated significantly with bmi ( p < 0.001 ) , bmi sds ( p < 0.01 ) , and bmi percentage ( p < 0.001 ) .
obese children in turn have higher risk of hypovitaminosis d. turkey 's vitamin d prophylaxis augmentation program ( started in 2005 ) has resulted in a marked decrease in vitamin d deficiency in healthy children under 1 year of age ; yet despite these improvements , most turkish adolescents are still vitamin d deficient . in our study ,
this finding revealed that obesity could be a risk factor of hypovitaminosis d in turkish children and adolescents .
recent studies from different countries have also demonstrated that vitamin d deficiency is common in obese children , possibly due to the low quality of diet . here in turkey , rates of hypovitaminosis d in healthy adolescents were 59.4% and 65% in two different studies [ 26 , 29 ] .  
vitamin d plays an important role in glucose homeostasis in the mechanism of insulin release .
most of the studies suggested vitamin d deficiency as a risk factor of disturbed glucose homeostasis in adults [ 3033 ] , but this hypothesis is still controversial in relation to children .
it remains unclear if vitamin d deficiency and insufficiency are associated with insulin resistance in obese children and adolescents .  
insulin resistance was estimated from fasting plasma measurements using homa - ir . in our study ,
insulin resistance values were higher in obese subjects compared with the healthy , vitamin - d - sufficient , age- and gender - matched children and adolescents . in order to determine the role of 25-hydroxyvitamin d levels in glucose intolerance and insulin resistance in obese children
, we compare the homa - ir levels of vitamin - d - sufficient obese children and adolescents with the vitamin - d - deficient and -insufficient obese ones .
there were no significant differences in homa - ir levels in obese children and adolescents due to 25-hydroxyvitamin d levels . in correlation analyses
, we found that homa - ir depended on the degree of obesity and correlated with serum lipid profile .
there was a significant trend towards higher insulin concentrations and insulin resistance in subjects with higher body mass index independent from 25-hydroxyvitamin d levels .  
recent studies researching the relationship between vitamin d deficiency and insulin resistance in obese children revealed controversial results . in two studies
, no relationship was found between low vitamin d status and insulin resistance [ 34 , 35 ] . on the other hand ,
the literature mostly supported the hypothesis that low vitamin d status is associated with worse glucose tolerance .
.   showed that vitamin d deficiency is associated with increased insulin resistance in children .
the study by reis et al .   showed that low serum vitamin d in adolescents was strongly associated with increased risk for fasting hyperglycemia , hypertension , and metobolic syndrome . in the study conducted by garanty - bogacka et al .
, fasting insulin levels and homa - ir were found correlated with low vitamin d levels .  
a limitation of this study is that vitamin d deficiency and insufficiency was defined only by 25-ohd levels .
it is 1,25-dihydroxyvitamin d which is active on the vitamin d receptor in the insulin producing cells .
it was also , not possible to evaluate other parameters modifying bone health such as subjects ' dietary calcium intake or the responsiveness of the vitamin d receptor .
in addition , with the lack of consensus regarding the definition of optimal vitamin d status in children , it is not possible to determine the level of vitamin d which disturbs glucose homeostasis and causes metabolic effects .   in conclusion , in our study ,
insulin resistance correlated mostly with bmi but not with 25-hydroxyvitamin d levels in the obese children and adolescents .
we found significant association between the degree of obesity and some biochemical parameters with insulin resistance , but different levels of 25 hydroxyvitamin d status among obese children were not an independent predictor of insulin resistance .
 
 objectives . we aimed to determine the relationship between insulin resistance and serum 25-hydroxyvitamin d ( 25-ohd ) levels in 
 obese children and their nonobese peers . materials and methods . included in the study group were 188 obese children ( aged 915 years ) , and 68 age- and gender - matched healthy children of normal weight as control group . 
 anthropomorphic data were collected on patients and fasting serum glucose , insulin , serum lipids , alanine aminotransaminase ( alt ) and 25-ohd were measured . 
 the homeostatic model assessment of insulin resistance ( homa - ir ) was calculated in both groups . 
 results . 
 the levels of 25-ohd in the obese group were significantly lower than those of the nonobese ( p = 0.002 ) . 
 homa - ir , triglycerides , low - density lipoprotein , and alt levels in the obese group were significantly higher than values of control group ( p < 0.001 and p = 0.002 , resp . ) . in the obese group , vitamin d deficiency , insufficiency , and sufficiency ( 25-ohd < 10  ng / dl , 
 < 20 , > 10  ng / dl ; > 20  ng / dl , resp . ) were not correlated with homa - ir ( r : 0.008 , p = 0.935 ) . 
 homa - ir was negatively correlated with bmi , bmi sds , and bmi% , and triglycerides , low - density lipoprotein , and alt levels ( p < 0.001 ) . 
 conclusion . 
 the insulin resistance of the obese subjects who were vitamin d deficient and insufficient did not statistically differ from those with vitamin d sufficiency . 
 low 25-hydroxyvitamin d levels were not related with higher insulin resistance in obese children and adolescents . in obese subjects , 
 insulin resistance was affected more from bmi , bmi sds , and bmi% than from 25-hydroxyvitamin d levels . 