it seems that complications of vitamin d deficiency have been completely removed by having enriched food by vitamin d , but rachitism is the top of iceberg of vitamin d deficiency . vitamin d deficiency can also causes osteopenia , osteoporosis , and muscle weakness and increases risk of hip fracture in adults . the most important effect of vitamin d is on parathyroid and bone and also plays a key role on the calcium absorption from the gut . the discovery of vitamin d receptor in most tissues and organs has developed a new perception from effects of vitamin d. for instance , vitamin d can decrease risk of many chronic diseases , cancers , autoimmune disease , infectious , and cardiovascular disease . it has been estimated that one billion people are vitamin d deficient in the world . this deficiency is common in all different cities with the same level of the sea or cities with lower geographic latitude . 20 ng / ml is the level of vitamin d deficiency and 20 < 25 ( oh ) d < 30 ng / ml is recognized as a vitamin d insufficiency . there are different ways to detect cut - off points for vitamin d deficiency . for instance , based on inverse relation between 25 ( oh ) d and pth , a direct relation between 25 ( oh ) d and 1 , 25 ( oh ) d and effect of vitamin d on fracture risk . the level of 25 ( oh ) d that would be enough for pth suppression , probably , is a suitable criterion for bone health and cut - off point for vitamin d deficiency . the aim of this study was detection of prevalence of vitamin d deficiency in secondary students in arak , a centrally located city in iran , and also detection of cut - off points for vitamin d deficiency . in a cross - sectional study in november 2010 , 420 students including 220 girls and 200 boys from secondary students from 10 to 16 years old were selected by a multistage sampling . first , all of the secondary schools were divided into two areas and then 21 schools including 11 girls schools and 10 boys , schools were selected by a cluster sampling . finally , the samples were picked up by a systematic random procedure based on the student 's initials . all the students and their parents were informed about the study and signed off the consent form . inclusion criteria were lack of hepatic , renal , bone disease and cancer , no history of intake of anticonvulsant drugs , calcium , vitamin d , and antineoplastic drugs . laboratory tests including hepatic , renal , calcium , phosphoros , alp ( alkaline phosphatase ) , albumin , pth , 25 ( oh ) d. 25 ( oh ) d levels by ria and pth by irma were measured ( made by bio source europe sa , belgium ) and its intensity categorized into three levels . mild deficiency : 15  25 ( oh ) d < 20 ng / ml.moderate deficiency : 8  25 ( oh ) < 15 ng / ml.severe deficiency : 25 ( oh ) < 8 ng / ml . 
  mild deficiency : 15  25 ( oh ) d < 20 ng / ml . moderate deficiency : 8  25 ( oh ) < 15 ng / ml . severe deficiency : 25 ( oh ) < 8 ng / ml . the relation between vitamin d and pth was assessed by the scatter chart . the level of vitamin d that pth was increasing , defined as cut - off points for vitamin d deficiency . the prevalence of vitamin d deficiency in different intensities was assessed by spss 15 and variables compared between girls and boys , and also between deficient and sufficient vitamin d groups by student 's test or mann -- whitney u based on normal distribution based on k -- s ( kolmogrov -- smirnov ) test . the pearson test was used to identify the relation between 25 ( oh ) d and age , weight , and bmi . a total of 420 students including 46.9% boys and 53.1% girls participated in this study . mean age for boys and girls was 12.5  1 and 12.7  1 years old respectively p = 0.1 [ table 1 ] . mean characteristic of students ( mean  sd ) mean age of vitamin d - deficient and nondeficient groups based on cut - off 25 ( oh ) d < 20 ng / ml were 12.6  1 and 12.6  2 years old respectively ( p = 0.5 ) . but mean bmi in two groups were 20  3.5 and 19  2.1 respectively ( p = 0.02 ) . there was no meaningful relation between 25 ( oh ) d and age ( p = 0.5 ) but there was an inverse relation between 25 ( oh ) d level and bmi ( r = -0.17 ) ( p = 0.01 ) , weight ( r = -0.14 ) ( p = 0.05 ) , pth ( r = -0.262 ) ( p = 0.001 ) . there was also a meaningful relation between vitamin d deficiency and sex ( p = 0.001 ) . based on the logistic regression analysis , the girls have 54.4 times more chance than boys for vitamin d deficiency ( or 54.4 ) ( p < 0.001 ) . totally 84% of the students had vitamin d deficiency [ table 2 ] . three cut - off points 33 , 21 , and 13 ng / ml were defined for mild , moderate , and severe vitamin d deficiency [ figure 1 ] using the scatter chart . the prevalence of vitamin d deficiency based on different intensities was evaluated in both genders [ table 3 ] . prevalence of vitamin d deficiency in students ( number/% ) mean of pth of different cut - off points for vitamin d deficiency prevalence of vitamin d deficiency based on local cut - off points ( % ) our findings in arak , a centrally located city of iran , showed a high prevalence of vitamin d deficiency of 84% among the students . the results showed that females were more likely to be vitamin d - deficient than males . the present study suggests three local cut - off points for vitamin d deficiency based on 25 ( oh ) d < 13 , 21 , and 33 ng / ml . vitamin d deficiency is a common medical problem throughout the world . it has been predicted that one billion people throughout the world are vitamin d deficient . prevalence of vitamin d deficiency in the countries where food is enriched by vitamin d ( scandinavian countries ) is 1.614.8% . in other european countries , among middle - aged and elderly people , 14 - -59.6 in tunessia 47.6% and finally among teenagers in boston was 24.1% . a total of 30 - -50% of people in india , lebanon , and turkey and also 45.2% of females in china were vitamin d deficient . several studies in different parts of iran and among different age groups have shown a high prevalence of vitamin d deficiency.[1114 ] prevalence of vitamin d deficiency in tehran ( the capital city of iran ) was 60% in males and 91% in females . the prevalence of vitamin d deficiency in isfahan , another central city of iran , was 46.2% among students including 72.1% girls and 18.3% boys and is similar to our findings in tabriz , another city in the north west of iran , which was 64.2% among females ( mild 33.7% , moderate 15.5% , severe 15.1% ) that in comparison with the present study is lower . another study in different cities of iran , with different climate , showed that 27.2% of females and 37.25% of males had a mild vitamin d deficiency , and also 47.85% of females and 34.75% of males had moderate to severe vitamin d deficiency that confirms the present study . different etiologies , such as insufficient sun exposure , clothing habits , and insufficient intake of vitamin d , may explain high prevalence of vitamin d deficiency . it has been shown that 2-years intake of calcium and vitamin d , increased serum 25 ( oh ) d level up to 28% . serum 25 ( oh ) d levels were lowest in blacks and highest in white people . although some studies found that sun expose and clothing habits were not different between sufficient and deficient groups . decreased calcium intake lead to increased serum pth level and increased catabolism of 25 ( oh ) d. awumey et al . showed higher activity of 24-hydroxylase in fibroblasts that lead to increased catabolism of 25 ( oh ) d of indian americans . the darker skin of the asian immigrants to european countries is another risk for vitamin d deficiency . found that 61% of the turkish group ( in turkey ) and 55% of the turkish immigrant group in germany had bone and muscle pain , while it was 15% within the german group with higher serum 25 ( oh ) d levels . our findings showed that the prevalence of vitamin d deficiency is much more in girls than boys ( girls 99.1% , 66.5% boys ) ( p < 0.001 ) . another study in iran , isfahan , also found that the prevalence of vitamin d deficiency was four times more in girls than boys and severe vitamin d deficiency in girls and boys were 14.5% and 0.6% respectively . another study in iran also found that moderate - to - severe vitamin d deficiency was much more in females than males . differences according to gender and age group maybe because of biological differences but might also due to behavioral differences . there might be gender - specific clothing differences ( necessarily not based on a culture religious ) . there also might be that females spend less time outside , resulting in less exposure to the sun . another study in the united states among female soldiers found that vitamin d levels decrease during military training in the summer due to the type of clothing worn during training , coupled with inadequate vitamin d intake . our study found that vitamin d deficiency was not associated with the age , while most studies found higher prevalence of vitamin d deficiency among elderly , probably due to lower capacity of the skin to produce vitamin d after sun exposure or less vitamin d intake in elderly , although a few studies reported higher prevalence of vitamin d deficiency in young people . our study shows a significant inverse association between bmi ( body mass index ) and weight in one hand with vitamin d deficiency in another hand , same as the tabriz study and against the isfahan study . considering the role of fat tissue as a depot of vitamin d , obese have increased storage capacity of 25 ( oh ) d , that leads to lower serum vitamin d levels . we determined three cut - off points for vitamin d deficiency based on 25 ( oh ) d < 33 , 21 , and 13 ng / ml for mild , moderate , and severe vitamin d deficiency , using scatter chart , versus 12.5 , 25 , and 40 ng / ml in another study in iran and 32 ng / ml in isfahan . high prevalence of vitamin d deficiency probably could be the result of above - mentioned combination . we recommend that some efforts could be made to encourage people to intake vitamin d and calcium supplements and also have more exposure to the sunlight .