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anemia is a global health problem   and an independent risk factor for increased morbidity and mortality in various groups of patients , especially in patients with chronic diseases and in the elderly .
low levels of 25-hydroxyvitamin d ( 25ohd ) are also highly prevalent among these patients .
recent epidemiological evidence suggests that circulating 25ohd is inversely associated with hemoglobin ( hb ) levels [ 415 ] .
the risk of anemia is highest at deficient 25ohd levels ( i.e. , < 30  nmol / l ) and lowest at 25ohd levels of 50 to 125  nmol / l [ 4 , 6 , 15 ] . in chronic kidney disease ( ckd )
patients , some nonrandomized intervention studies could already show that intravenous administration of activated vitamin d ( 1,25-dihydroxyvitamin d3 = 1,25(oh)2d3 ) is associated with an increase in hb levels within 12 months of treatment and a reduced need for erythropoietin ( epo ) [ 16 , 17 ] .
moreover , intravenous 1,25(oh)2d3 administration was associated with a decreased weekly epo dose of 50% . regarding hb levels ,
similar results have been obtained in hemodialysis patients with oral alfacalcidol ( 1-vitamin d3 ) after 8 weeks   and also after 12 and 18 months of treatment . in hemodialysis patients , high - dose oral vitamin d2 ( 50,000  iu monthly ) was associated with dose - reductions in erythropoiesis - stimulating agents ( esa ) , while hb concentrations remained unchanged [ 21 , 22 ] . in children with ckd stage 5 and 25ohd levels
< 75  nmol / l , 12 weeks of vitamin d2 treatment in conjunction with 1,25(oh)2d3 was associated with a significantly reduced dose of esa required to treat the children . in anemic patients with preserved kidney function , however , one single intramuscular bolus of 600,000  iu vitamin d3 did not influence hb levels . nevertheless , it is noteworthy that in general populations the effect of high - dose bolus administration of vitamin d on clinical outcomes has been questioned .
the purpose of the present study was to determine the effect of a daily vitamin d3 supplement on hb levels in a group of hypertensive patients with preserved kidney function and inadequate 25ohd levels .
this is a secondary analysis of the styrian vitamin d hypertension trial of a postspecified endpoint .
the study was a randomized , double - blind , placebo - controlled , single - center trial which took place at the outpatient clinic at the division of endocrinology and metabolism , medical university of graz , austria .
the study was registered at eudract ( number 2009 - 018125 - 70 ) and clinicaltrials.gov ( nct02136771 ) .
the study was approved by the ethics committee of the medical university of graz , austria .
two hundred study participants ( 106 men and 94 women ) were recruited from the clinics of the department of cardiology and the department of internal medicine , division of endocrinology and metabolism , medical university of graz , austria , from june 2011 to august 2014 .
eligible study participants were adults aged 18 years or older with a serum 25ohd concentration below 75  nmol / l and arterial hypertension .
pregnant or lactating women were excluded as well as patients with hypercalcemia ( serum calcium > 2.65 
mmol / l ) , regular vitamin d intake > 880  iu per day during the last four weeks before the study , estimated glomerular filtration rate ( egfr ) < 15  ml / min per 1.73  m , drug intake as part of another clinical study , acute coronary syndrome or cerebrovascular event in the previous two weeks , 24-hour systolic bp > 160  mm  hg or < 120  mm  hg , 24-hour diastolic bp > 100  mm  hg , change in hypertensive treatment ( drugs or lifestyle ) in the previous four weeks or planned changes in antihypertensive treatment during the study , any disease with an estimated life expectancy of < 1 year , any clinically significant acute disease requiring drug treatment , and chemotherapy or radiation therapy .
eligible study participants were randomly allocated to receive 2800  iu ( 70  g ) cholecalciferol as seven oily drops per day ( oleovit d3 , fresenius kabi austria , graz , austria ) or a matching placebo for eight weeks .
the dose of 2,800  iu vitamin d per day was chosen because a rule of thumb suggests that vitamin d supplementation of 1,000  iu increases 25ohd levels by approximately 25  nmol / l .
given that a commonly used normal range of 25ohd is 75 to 150  nmol / l   we conclude that a supplementation of 2,800  iu daily may be sufficient to increase the 25ohd level of most study participants to target ranges without causing supraphysiological 25ohd levels .
one hundred patients were assigned to the intervention group and 100 patients to the control group .
randomization was performed by web - based software ( http://www.randomizer.at/ ) with a permuted block randomization ( block size of 10 and stratification according to sex ) .
the primary outcome measures were hb levels and anemia . in accordance with earlier classifications [ 4 , 6 , 29 ] , hb concentrations <
12.5  g / dl were considered as anemic , which corresponded to the average threshold value of the world health organization gender - based definition ( < 13.0  g / dl in men and < 12.0  g / dl in women ) .
blood sampling was performed in the morning between 7 and 11 am after an overnight fast .
all blood samples were either measured at least four hours after blood collection or immediately stored at 20c until analysis .
measurement of 25ohd was performed by chemiluminescence assay ( ids - isys 25-hydroxyvitamin d assay ; immunodiagnostic systems ltd . , boldon , uk ) on an ids - isys multidiscipline automated analyzer .
all hematological parameters were measured on a sysmex xe-5000 automated hematology analyzer ( sysmex america , inc . ,
the measurements of other biochemical parameters have been described elsewhere . in accordance with published data
[ 31 , 32 ] we categorized 25ohd levels < 30  nmol / l as deficient , 3049.9  nmol / l as insufficient , and 50 to 74.9  nmol / l as borderline .
change from baseline data is shown as means and 95% confidence interval ( ci ) .
we used the mcnemar test and fisher 's exact test , respectively , to assess differences in anemic status within and between groups . the unpaired t - test or chi - squared test
ancova with adjustments for baseline values was used to test for differences in the outcome variables between the vitamin d and the placebo group at the follow - up visit .
analyses were performed using spss version 21.0 ( spss , inc . , chicago , il , usa ) .
baseline characteristics of the study participants are shown in table 1 . in both groups ,
mean hb values were clearly above the anemia threshold of 12.5  g / dl . at baseline , the proportion of anemia in the vitamin d and placebo groups was 9.0% and 4.0% , respectively ( p = 0.152 ) . regarding 25ohd levels ,
6.0% of the vitamin d group and 9.0% of the placebo group had deficient levels .
the proportion of insufficient 25ohd levels was 27.0% and 36.0% , respectively . in the remaining 67.0% and 55.0% , the vitamin d status was classified as borderline .
circulating 25ohd increased on average by 35.4 nmo / l ( 95% ci : 31.2 to 39.6 
nmol / l ) in the treatment group and 8.1  nmol / l ( 95% ci : 4.6 to 11.7 
moreover , vitamin d treatment did not influence anemic status significantly ( p > 0.999 ) . in detail , the percentage of anemic subjects remained constant in the vitamin d group ( p > 0.999 ) and increased only slightly in the placebo group ( p > 0.999 ) ( figure 1 ) .
vitamin d treatment had no effect on hb levels and other hematological parameters in anemic patients ( see table s1 in supplementary material available online at http://dx.doi.org/10.1155/2016/6836402 ) .
moreover , there was no vitamin d effect on hb levels and other hematological parameters in the group of subjects with initial 25ohd levels < 30  nmol / l ( table s2 ) .
there was , however , a significant vitamin d effect on pth levels , with suppressed pth levels in the vitamin d group ( table 2 ) .
this study showed that a daily vitamin d supplement of 2800  iu for 8 weeks had no effect on hb levels or anemia risk in hypertensive patients with 25ohd levels
in addition , compared with previous interventional studies we were able to include a large number of patients .
moreover , the daily vitamin d dose was sufficient to increase in - study 25ohd levels on average clearly above 75 
this value is recommended by many endocrinologists and vitamin d researchers as the lower target level for various clinical outcomes [ 33 , 34 ] .
second , the study duration of 8 weeks was relatively short , given that the half - life of red blood cells in circulation is 3 months .
third , we have no data on circulating levels of 1,25(oh)2d , which is the active , hormonal form of vitamin d. generally , evidence is increasing that 1,25(oh)2d can stimulate erythropoiesis in red blood cell precursor cells by increasing epo sensitivity .
furthermore , 1,25(oh)2d can upregulate proliferation of hematopoietic progenitor cells [ 35 , 36 ] .
nevertheless , our findings with plain vitamin d in patients with preserved kidney function do not support the beneficial impact on hb levels of earlier studies with activated vitamin d in ckd patients [ 16 , 17 ] .
it is , however , noteworthy that in ckd patients the prevalence of anemia is high and circulating 1,25(oh)2d levels are low .
moreover , a recent meta - analysis of randomized controlled trials has demonstrated that , after administration of vitamin d or activated vitamin d , the increase in circulating 1,25(oh)2d tends to be much higher in ckd patients than in non - ckd patients . in our study , a vitamin d - induced increase in circulating 1,25(oh)2d levels may have been blunted by the suppressed pth levels , and this may have contributed , at least in part , to the null effect on hb levels . in the present study ,
moreover , it was a surprising finding that all 13 anemic patients had initial 25ohd levels > 50  nmol / l . generally , epidemiological evidence suggests that circulating 25ohd levels are inversely associated with hb levels and anemia risk [ 4 , 6 ] .
a recent meta - analysis of retrospective observational studies showed that , compared with individuals with adequate 25ohd levels , low 25ohd levels were associated with an odds ratio for anemia of 2.25 ( 95% ci : 1.473.44 ) . the cut - off for low 25ohd levels was 50 
nmol / l in 5 out of the 7 included studies and 75  nmol / l in the remaining 2 studies . only individuals without chronic diseases were included in that meta - analysis .
however , two large studies in patients with cardiovascular disease [ 4 , 6 ] indicate that hb levels are only significantly affected if circulating 25ohd levels are in the deficiency range . also of note
is the fact that observational studies have shown circulating 1,25(oh)2d to be a better predictor of anemia than circulating 25ohd [ 4 , 5 , 39 ] .
altogether , data indicate that especially anemic patients with deficient 25ohd levels and low circulating 1,25(oh)2d levels may benefit from improved vitamin d status .
in conclusion , our data demonstrate that a daily vitamin d supplement of 2800  iu for 8 weeks does not increase hb levels in nonanemic hypertensive patients with 25ohd levels
epidemiological evidence suggests that circulating 25-hydroxyvitamin d ( 25ohd ) levels are inversely associated with hemoglobin ( hb ) levels and anemia risk . 
 we evaluated whether vitamin d supplementation improves hb levels and reduces anemia risk in hypertensive patients . 
 two hundred patients with 25ohd levels < 75  nmol / l who attended the styrian vitamin d hypertension trial were included , of whom 188 completed the trial . 
 patients randomly received 2800  iu vitamin d3 daily or a matching placebo for eight weeks . 
 initially , the prevalence of anemic status ( hb levels < 12.5  g / dl ) and deficient 25ohd levels ( < 30  
 nmol / l ) was 6.5% and 7.5% , respectively . 
 all anemic patients had 25ohd levels > 50  nmol / l . 
 the mean ( 95% confidence interval ) vitamin d effect on hb levels was 0.04 ( 0.14 to 0.22 ) g / dl ( p = 0.661 ) . 
 moreover , vitamin d treatment did not influence anemic status significantly ( p > 0.999 ) . likewise , vitamin d had no significant effect on hb levels in the subgroups of anemic patients or in patients with initial 25ohd levels < 30  nmol / l . in conclusion , 
 a daily vitamin d supplement of 2800  iu for eight weeks did not improve hb levels or anemic status in hypertensive patients . 
 future trials should focus on anemic patients with deficient 25ohd levels ( e.g. , < 30  nmol / l ) . 
 this trial is registered with clinicaltrials.gov [ nct02136771 ] .