vitamin d deficiency is reemerging as a major public health problem throughout the world . it is acknowledged that the prevalence of vitamin d deficiency and its associated morbidities are higher than previously thought worldwide . in addition to metabolic bone disease , recent studies reported that vitamin d deficiency could increase the risk of certain cancers , heart disease , and autoimmune diseases including rheumatoid arthritis and multiple sclerosis in adults and diabetes mellitus [ 18 ] . chronic vitamin d deficiency may have serious adverse consequences in patients with kidney disease [ 913 ] . to test the hypothesis that low serum 25-hydroxyvitamin d ( 25(oh)d ) levels are a risk factor for kidney disease progression , melamed et al . analyzed data from 13,328 participants in the national health and nutrition examination survey ( nhanes ) iii follow - up study , in which 25(oh)d levels were measured from 1988 through 1994 , and then participants were followed for up to 12 years . the incidence of end - stage renal disease was 2.6 times greater in people whose serum 25(oh)d was less than 15  ng / ml than in those with higher levels .   however , it is still not common practice among nephrologists to monitor and correct vitamin d deficiency of patients with kidney disease , because it is widely believed that the capacity of the 1 -hydroxylase to synthesize 1,25(oh)2d3 decreases progressively because of decreased renal mass and that any vitamin d deficiency associated with calcium - phosphate disturbances is better treated with activated vitamin d.   although hypovitaminosis d has been reported frequently in patients with kidney disease , the prevalence of vitamin d deficiency among patients hospitalized in nephrology services is unknown . the aims of the study were to describe and compare the prevalence of vitamin d deficiency inpatients and outpatients in the nephrology department during early winter and to evaluate effect of assessing serum 25(oh)d levels and previous supplementation of cholecalciferol on vitamin d status . we undertook a retrospective audit of the serum 25(oh)d levels of inpatients and outpatients in the nephrology departments from october 15 , 2010 to january 1 , 2011 , and previous serum 25(oh)d analysis , and previous vitamin d supplementation history of the patients . the following information was determined from the patients ' medical records : age , gender , primary underlying renal diagnosis , blood urea nitrogen ( bun ) , serum creatinine , corrected total serum calcium , serum phosphorus , intact parathormone ( ipth ) , serum albumin levels , recent and preceding results of serum 25(oh)d analysis , and previous documentary evidence of vitamin d supplementation . estimated gfr ( egfr ) , using the abbreviated modification in diet in renal disease ( mdrd ) equation , was determined . vitamin d deficiency was defined as 25(oh)d levels below 15  ng / ml ( 37.4  nm / l ) . for subanalysis the patients were divided into four diagnostic categories according to their serum 25(oh)d levels : severe vitamin d deficiency ( serum 25(oh)d3 level , less than 5  ng / ml , ( 12,5  nm / l ) ) , moderate vitamin d deficiency ( serum 25(oh)d3 level , 5 to 15  ng / ml ( 12.537.4  nm / l ) ) , vitamin d insufficiency ( serum 25(oh)d3 level , 15 to 30  ng / ml ( 12.537.4  nm / l ) ) , and adequate vitamin d stores ( serum 25(oh)d3 level , more than 30  ng / ml ( 37.4  nm / l ) ) . the significance of difference between continuous variables was tested using unpaired t - test or mann - whitney rank sum test and for categorical variables using 2 or fisher exact test as appropriate . p values less than 0.05 were regarded as statistically significant . logistic regression analysis , binary logistic ( hosmer - lemshow goodness of fit ) by enter method , was studied . in the local biochemistry database we identified 25(oh)d requests in total 280 inpatients and outpatients in the nephrology department between october 15 , 2010 and january 1 , 2011 . a total of 49 ( 17.5% ) patients were receiving dialysis ( 17 ( 34,7% ) due to acute renal failure , and all these were inpatients . the prevalence of vitamin d deficiency among these 280 patients was 62,1% ( 174 patients ) . fifty - three patients ( 18.9% ) had severe vitamin d deficiency , 121 patients ( 43.2% ) moderate vitamin d deficiency , and 66 patients ( 23.6% ) vitamin d insufficiency . only 40 ( 14.3% ) patients had adequate vitamin d stores . whereas in patients with vitamin d deficiency , age , bun , and ipth were higher , serum albumin , calcium levels were lower . there was no significant difference according to serum phosphorus , creatinine , and egfr ( p > 0,05 ) ( table 3 ) .   while in 72% of the inpatients ( n : 90 ) serum 25(oh)d levels were less than 15  ng / ml ( 37.4  nm / l ) , in 54.2% of the outpatients ( n : 86 ) serum 25(oh)d levels were less than 15  ng / ml ( 37.4  nm / l ) ( p < 0.05 ) . while age , bun , serum creatinine , and phosphorus levels of inpatients were higher , serum 25(oh)d , serum calcium , albumin levels , and egfr of inpatients were lower than those of outpatients ( p < 0,05 ) . of 280 patients , 156 ( 55.7% ) had at least one measurement of 25(oh)d levels during the previous followup in our nephrology unit . of these 280 patients , 55.7% ( 156 ) were previously supplemented with cholecalciferol according to vitamin d supplementation protocol of our nephrology unit . the mean duration between recent and preceding measurements of serum 25(oh)d levels and initiation of vitamin d supplementation was 9.9  3.9 months . the preceding mean serum 25(oh)d level was 12.4  6.8  ng / ml ( 30.9  16.7 nm / l ) . of 174 patients with vitamin d deficiency , 83 ( 47.7% ) and 73 ( 68.9% ) of 106 patients without vitamin d deficiency had vitamin d supplementation history . of 125 inpatients , 57 ( 45.6% ) and 99 ( 63.9% ) of 155 outpatients had vitamin d supplementation history ( p < 0,05 ) . no difference was found between patients supplemented with vitamin d previously or not according to bun , serum albumin , and phosphorus . while inpatients supplemented with vitamin d previously , age ( 50,8  16,6 versus 58,1  16,7 years ) , hospitalization days ( 4,7  9,2 versus 7,9  11,1 days ) , and egfr ( 15,2  21,1 versus 36,5  38,4 ml / min/1.73  m ) were significantly lower , and serum 25(oh)d ( 20,2  18,3 versus 11,9  10,8 mg / dl ) , and pth ( 443,9  468,2 versus 289,6  346,5 serum 25(oh)d levels were statistically lower in females than males ( 15,2  16,8 versus 18,2  14,7  ng / ml ; 45.4  42 versus 37.9  41.9  nm / l , resp . ) ( p < 0.05 ) . the prevalence of vitamin d deficiency among these 153 women was 69.3% ( 106 patients ) and 43.5% in men ( 68 patients ) ( p < 0,05 ) . of 153 female patients with vitamin d deficiency , 91 ( 56.8% ) and 65 ( 59.3% ) of 127 male patients with vitamin d deficiency had vitamin d supplementation history ( p > 0,05 ) . the mean duration between recent and preceding measurements of 25(oh)d levels and initiation of vitamin d supplementation was 10.3  3.6 months in female patients and 9.7  4.1 months in male patients ( p > 0.05 ) . no difference was found between males and females according to age , bun , serum albumin , calcium , phosphorus , pth , egfr , and hospitalization days ( p > 0,05 ) . serum creatinine was lower in female patients ( 5,3  3,3 versus 6,6  4,6  mg / dl , resp . ) female gender , not having vitamin d supplementation history , low serum albumin , and low bun levels were significant independent predictors of vitamin d deficiency , while we were unable to demonstrate any relationship with vitamin d deficiency and presence of diabetes mellitus , serum creatinine , egfr , being on hemodialysis , and being hospitalized ( table 4 ) . malatya city , which gets lots of sunlight , is at a latitude of 38  :  21 north , longitude 38  :  19 east , and an altitude of 998  m . however , 62.1% of our patients had vitamin d deficiency , 23.6%vitamin d insufficiency , while 14.3% of the patients had adequate vitamin d stores . the results of a study reported a 57% prevalence of vitamin d deficiency in 290 patients hospitalized in the general medical wards at massachusetts general hospital . sixty - three percent of the patients studied in march and 49% of those studied in september had serum 25(oh)d concentrations of 15  ng / ml ( 37.4  nm / l ) or less . these authors reported that anticonvulsant - drug therapy , renal dialysis , nephrotic syndrome , and winter season were significantly associated with hypovitaminosis d . daoudi et al .   from belgium reported that more than 60% of 332 patients hospitalized presented with serum concentrations below the lower limit of normal ( 12  ng / ml , 29.9  nm / l ) , whereas less than 5% reached 30  ng / ml ( 74.9  nm / l ) , a level generally recommended to avoid vitamin d insufficiency . these authors reported that young patients are not at a lesser risk of vitamin d deficiency than the older patients and women showed a lesser deficiency than men . we found a higher prevalence of vitamin d deficiency in inpatients ( 63.9% ) than those of outpatients ( 45.6% ) in the nephrology department during early winter season ( p < 0,05 ) . however , in logistic regression analysis being female gender , not having vitamin d replacement previously , low serum albumin , and low bun levels were significant independent predictors of vitamin d deficiency , while we were unable to demonstrate any relationship with vitamin d deficiency and presence of diabetes mellitus , serum creatinine , egfr , and being hospitalized . in our department we follow a vitamin d supplementation protocol which is similar to that recommended by k / doqi guidelines   except cholecalciferol usage . if serum level of 25(oh)d < 5  ng / ml ( 12.5  nm / l ) , we prescribe 50000  iu of vitamin d3 ( cholecalciferol)/week orally/12 weeks and then monthly for six months . if serum level of 25(oh)d is between 5 and 15  ng / ml ( 12.537.4  nm / l ) , we prescribe 50000  iu of vitamin d3 ( cholecalciferol)/weekly orally/4 weeks and then monthly for six months of all patients , 55.7% were supplemented with vitamin d previously during followup in our nephrology unit . vitamin d deficiency was less common in patients supplemented with cholecalciferol previously ; even vitamin d deficiency was also found among patients supplemented with cholecalciferol during followup . however , we do not have any information on compliance with vitamin d supplements .   in a retrospective study of 88 patients with ckd stages 15 and baseline 25-hydroxyvitamin d level < 30  ng / ml ( < 75 treatment with ergocalciferol as recommended by k / doqi guidelines managed to achieve in only 25% > or = 30  ng / ml ( 75  nmol / l ) . these authors reported that current k / doqi guidelines are inadequate for correcting vitamin d deficiency in ckd patients and recommended to monitor serum 25(oh)d levels regularly and to give appropriate vitamin d supplementation in order to achieve normal vitamin d status . it seems that our findings are consistent with those of qunibi et al . in our study during fall vitamin d deficiency was more common among women ( 69.3% versus 43.5% ) ; even percentage of the women supplemented with cholecalciferol previously was similar to that of men . in another a cross - sectional study from turkey hatun et al .   demonstrated that vitamin d insufficiency ( 43.8% ) and deficiency ( 21% ) are common among turkish adolescent girls at the end of the winter ( in april ) . this finding is more striking in girls who wear concealing clothing , and they did not improve significantly during the summer , whereas the vitamin d status of girls in the other groups did .   in turkey , the main source of vitamin d is cutaneous synthesis because there is no food fortification with vitamin d , and supplementation of vitamin d is not a routine practice . veiling or staying indoors is common in our female patients , although we did not have any information about these in this study . it is possible that these women benefited less from the decreased sunlight through the fall months than men do , possibly by exposing their skin less to the sun . firstly , this study suffers from all the limitations of retrospective observational studies that rely on existent databases , and the information was collected only at one point in time . as a conclusion , vitamin d deficiency seems to be an important problem in both inpatients and outpatients of nephrology in turkey . women are at more risk of deficiency and may therefore need to consume higher doses of the vitamin d. monitoring serum 25(oh)d levels regularly and replacement of vitamin d are important . a governmental mandate about the supplementation of foods with vitamin d