there is a lot of epidemiological evidence to suggest that vitamin d has a role in glucose homeostasis , especially in maintaining normal glucose homeostasis and protecting against type 2 diabetes [ 1 , 2 ] .
however , controlled supplementation trials with vitamin d or its analogues have not been able to produce similar results . in a recent systematic review and meta - analysis of 15 trials lasting from two months to 7 years
two trials in subjects with impaired glucose tolerance ( igt ) did find a positive effect on glucose homeostasis with vitamin d3 supplementation [ 47 ] , whereas three trials in subjects with igt and one in subjects with normal glucose tolerance found no effects [ 810 ] .
insulin resistance , metabolic syndrome , and type 2 diabetes are associated with a systemic , chronic inflammation , where , for example , circulating levels of proinflammatory cytokines and c - reactive protein are elevated [ 11 , 12 ] . although vitamin d has immunomodulatory and anti - inflammatory properties , especially based on experimental studies , bodies of evidence from human studies in the prediabetic state assessing the role of moderate to large dose supplementation of vitamin d in inflammation and glucose metabolism are still relatively few . as more evidence is clearly needed , we decided to study the placebo - controlled effect of 5-month 40  g / d ( 1,600  iu ) or 80  g / d ( 3,200  iu ) oral vitamin d3 supplementation on glucose homeostasis in an eastern finnish population with either impaired fasting glucose ( ifg ) or igt during winter time when serum vitamin d3 concentrations drop without supplementation .
furthermore , we assessed the anti - inflammatory effect of the supplementation by measurements of serum high - sensitive c - reactive protein ( hscrp ) , plasma soluble tumor necrosis factor receptor type ii ( pstnfrii ) , plasma interleukin-6 ( pil-6 ) , plasma interleukin-1 receptor antagonist ( pil-1ra ) , and plasma interleukin-1 beta ( pil-1 ) concentrations .
the glucose metabolism effects of vitamin d supplementation in prediabetes ( vitdmet ) study was a 5-month randomized , double - blind , placebo - controlled supplementation trial that was conducted in winter 2011/2012 in eastern finland .
the primary aim was to assess the effects of vitamin d3 supplementation on glucose homeostasis in subjects with ifg or igt . for inclusion the subjects needed to be 60 years of age with evidence of disturbed glucose homeostasis , that is , either ifg ( fasting plasma glucose concentration 5.6 to 6.9  mmol / l ) or igt ( oral glucose tolerance test ( ogtt ) 120  min plasma glucose concentration 7.8 to 11.1  mmol / l ) , but not type 2 diabetes ( fasting plasma glucose concentration  7.0  mmol / l or ogtt 120  min plasma glucose concentration  11.1 
mmol / l ) , and to be overweight ( bmi > 25 ) , but not severely obese ( bmi < 35 ) .
furthermore , subjects needed to have their serum 25(oh)d3 concentration < 75  nmol / l .
exclusion criteria were any disease that could be affected by vitamin d , such as sarcoidosis , or a condition that could affect vitamin d metabolism , such as kidney disease .
the adult voluntary subjects were recruited by advertisements in regional newspapers and after a prescreening on the phone for inclusion and exclusion criteria ( self - reported elevated blood glucose , age , height , weight , and diseases ) were invited for the first screening at the study clinic ( visit 1 ) .
a written informed consent was collected , hba1c was measured with a point - of - care device , the subjects were interviewed , and their self - administered questionnaires were checked . if the subject 's hba1c was 5.57.0% , fasting venous blood samples were drawn and analyzed for plasma glucose and serum 25(oh)d3 concentrations .
those fulfilling the inclusion criteria were scheduled for the ogtt ( visit 2 ) , after which those fulfilling all criteria were randomized to receive either placebo , 40  g / d , or 80  g / d vitamin d3 daily .
each dose comprised a combination of placebo and 20  g tablets , two in the morning and two in the evening .
randomization was carried out by a statistician , in the order of entry after visit 2 .
study nurse distributed the containers according to the statistician - generated list and neither the nurse nor a participant knew the allocation group .
all tablets and containers were physically identical , the allocation arm code hidden in the serial number on the container label .
the average time between visit 2 and the beginning of the supplementation was approximately one week . during the 5-month supplementation period between october 2011 and april 2012 ,
the subjects visited the study site three times : 1 and 3 months after the start of the supplementation and at the end of the study ( figure 1 ) .
we aimed to recruit 102 participants , 34 into each of the three supplementation groups , but due to lower than expected public interest to participate and generally higher than anticipated serum 25(oh)d3 concentrations in the base population , at closing of the recruitment window we had 73 randomized subjects in the three supplementation arms ( 63 men and 10 women ) . according to power computations , to reach 80% power ( i.e. ,  = 0.20 ) at  = 0.05 , with a difference of 1 sd to 0.75 sd in the effect measure between the groups , there needed to be 16 to 28 subjects per group .
1 sd equals an effect size of 10% to 50% in most of the outcome parameters .
the research ethics committee of the northern savo hospital district has approved the study protocol .
serum 25(oh)d3 concentration was measured from venous blood samples by a high performance liquid chromatography with coulometric electrode array ( hplc - cead ) , as described .
the interassay cv% of the method is < 8.0% and the intra - assay cv% is < 7.3% .
plasma glucose concentration was assayed by the photometric hexokinase method ( konelab 20xt , thermo fischer scientific , vantaa , finland ) and plasma insulin concentration by the chemiluminescence assay ( diasorin liaison , diasorin , dietzenbach , germany ) .
plasma glucose and plasma insulin concentrations were measured at three time points : at start ( i.e. , fasting ) , at 30  min , and at 120  min .
homeostatic modeling assessment ( homa2 ) indices were computed according to the nonlinear function presented by wallace and coworkers . the insulinogenic index ( igi )
was computed as follows : igi = ( 30  min insulin  0  min insulin)/(30  min glucose  0  min glucose ) in the ogtt , as described by phillips and coworkers , and the insulin sensitivity index ( isi ) was computed as follows : isi = 10000/sqr[(fasting glucose  fasting insulin )  ( mean glucose  mean insulin ) ] , as described by matsuda and defronzo .
hba1c was measured with a point - of - care device from a capillary blood sample as instructed by the manufacturer ( dca vantage , siemens healthcare diagnostics , deerfield , illinois , usa ) .
serum liver enzymes gamma - glutamyl transpeptidase ( sggt ) and alanine aminotransferase ( salat ) were determined with enzymatic photometric ifcc methods , kidney function marker serum creatinine ( screa ) with an enzymatic photometric test , and serum total calcium ( sca ) with colorimetric arsenazo iii test at site with a clinical chemistry analyzer ( konelab 20xt , thermo fisher scientific , vantaa , finland ) .
serum parathyroid hormone ( spth ) and serum thyroid stimulating hormone ( stsh ) were assayed by chemiluminescence methods ( diasorin liaison , diasorin , dietzenbach , germany ) .
total blood count ( tbc ) and inflammatory cytokines hscrp , pstnfrii , pil-6 , pil-1ra , and pil-1 were measured in eastern finland laboratory centre , kuopio , finland .
plasma il-6 , il-1ra , soluble tnfrii , and il-1 were measured with elisa from r&d systems ( minneapolis , mn , usa ) .
body weight was measured to 0.1  kg with subjects wearing light underwear , with a recently calibrated vetek ti-1200 scale ( vetek , sweden ) , and height in a frankfurt position to 0.5  cm by a wall - mounted device ( kawe , germany ) .
body mass index ( bmi ) was computed as weight / height squared ( kg / m ) .
blood pressure was measured after a 5  min rest from the upper arm at a supine position , using a recently calibrated semiautomatic device ( omron intellisense m7 , omron healthcare , netherlands ) .
systolic and diastolic blood pressure values were computed as the mean of the last two of three measurements .
participant compliance in taking supplements as instructed was assessed by counting the study supplements not consumed at the last study visit . from the statistical analyses we excluded two subjects who had an increase in their bmi to > 35  kg / m between the randomization and the start of the supplementation ( visit 3 ) , two subjects who reported at the end of the study that they had started to use a high - dose ( 50  g / day and 125  g / day ) vitamin d3 supplement in addition to the study supplements during the study period , and one subject due to low compliance .
the subject with the low compliance in the 80  g / day group had used only 17% of the study tablets . after the exclusions , the number of subjects in the analyses with the baseline data was 68 ( table 1 ) and in the analyses with the change variables 66 ( table 2 ) , after the two dropouts ( figure 1 ) were excluded .
glucose homeostasis was assessed by fasting glucose , insulin , and hba1c measurements and ogtt glucose and insulin measurements .
the derived variables to be tested were the change in the chosen variables over the supplementation period .
these were as follows : serum hba1c ( concentration and % ) , fasting , 30  min and 120  min glucose and insulin concentrations , homa2 insulin resistance ( homa2-ir ) , insulin sensitivity ( homa2-is ) and beta cell function ( homa2-b% ) , igi , and isi .
inflammatory effects were assessed by analyzing the change in the hscrp , pstnfrii , pil-6 , pil-1ra , and pil-1 concentrations over the supplementation period .
physiological and safety indicators were changes in the 25(oh)d3 , spth , sca , screa , sggt , salat , and stsh .
as a large proportion of the variables did not follow the normal distribution , we used the kruskal - wallis test ( pk - w ) to test the intergroup differences , mann - whitney u test ( pm - wu ) to test the pairwise intergroup differences ( placebo versus 40  g / d and placebo versus 80  g / d ) , and the jonckheere - terpstra test ( pj - t ) for trend across the groups .
related - samples wilcoxon signed rank test ( pwsr ) was used to study the intragroup change over time .
a two - sided p < 0.05 was considered statistically significant for the kruskal - wallis test , a 1-sided p < 0.05 for the jonckheere - terpstra test , and a two - sided p < 0.025 for the mann - whitney u test ( i.e. , with the bonferroni correction ) .
all statistical analyses were computed with the spss statistical package for windows , version 21.0 ( armonk , ny : ibm corp . ) .
the supplementation effect analyses were conducted per protocol ; as for the dropouts there were no data available at the end of the study to compute the change variables .
despite the moderately small sample size , the randomization yielded reasonably identical groups with regard to vitamin d and glucose metabolism - related parameters . characteristics of the 68 study subjects at baseline are presented in table 1 .
the median age of the subjects at entry was 65.7 ( interquartile range 62.7 to 69.7 ) years , and the median serum 25(oh)d3 concentration was 57.2 ( interquartile range 47.2 to 67.2 )
the arithmetic mean 25(oh)d3 concentration was 57.0  nmol / l with a standard deviation of 11.0  nmol / l .
of the 68 subjects that entered the supplementation period , 66 completed the study ( figure 1 ) .
there was a marked dose - dependent increase in the average serum 25(oh)d3 concentration in the three study groups , from 55.3 to 59.0  nmol / l , from 57.7 to 85.4  nmol / l , and from 58.1 to 103.1  nmol / l , in the placebo , 40  g / d , and 80  g / d groups , respectively ( mean ranks for change 17.1 , 35.4 , and 47.8 , resp .
, pk - w < 0.001 ; see table 2 ) . at baseline , one of the subjects in the 80  g / day group had serum 25(oh)d3 > 75 
nmol / l . at the end of the study , 3/21 ( 14% ) subjects in the placebo group , 16/24 ( 67% ) subjects in the 40  g / d group , and 18/21 ( 86% ) subjects in the 80  g / d group had serum 25(oh)d3 > 75 
concomitant with the increase in the serum 25(oh)d3 concentrations , there was a dose - dependent decrease in the serum pth concentrations ( table 2 ) .
changes in the tested glucose homeostasis parameters over the supplementation period are given in table 2 .
the only statistically significant effect between the groups was an increase in the 120  min plasma glucose concentration , that is , opposite to expected ( pk - w = 0.039 , pj - t = 0.021 ) , although only the pairwise group difference for the placebo versus 40  g / d was statistically significant after bonferroni correction ( p = 0.022 ) , and a decreasing trend in the hba1c concentration ( pj - t = 0.024 ) and in the 30  min insulin concentration ( pj - t = 0.030 ) .
borderline statistically significant trend was observed in the igi ( pj - t = 0.063 ) . in the tested inflammation markers , shown in table 2 , the only borderline statistically significant finding was a decreasing trend in the plasma il-1ra concentration ( pj - t = 0.070 ) .
there were detectable concentrations of pil-1 only for four subjects at entry and for two subjects at the end of the study and therefore the p values can not be considered reliable .
no statistically significant changes were observed in sca ( table 2 ) or in the parameters of liver or kidney function or in the tbc ( data not shown ) .
no statistically significant differences between groups were observed in changes in waist circumference or bmi , in blood pressure , or in circulating lipids ( data not shown ) .
we repeated the analyses by including also those with bmi > 35 at the start of the supplementation period , those who started their own high - dose vitamin d supplementation during the trial , or the one with low compliance ( n = 71 ) .
the results were generally similar to those with the exclusions , except for the p for trend for il1-ra , which was statistically significant ( pj - t = 0.027 ) .
our study showed that oral supplementation with daily 40  g or 80  g doses of vitamin d3 for five months markedly increases the circulating concentrations of 25(oh)d3 , even over the winter months that usually are associated with a decrease in the 25(oh)d3 in this population .
we also showed that the 80  g dose is more potent than the 40  g dose in increasing serum 25(oh)d3 and that a considerably large proportion of the subjects in both supplementation groups reached serum 25(oh)d3 concentrations above 75  nmol / l .
furthermore , we showed that the supplementation also resulted in a decrease in serum pth concentration but did not induce a rise in serum total calcium concentration .
vitamin d3 supplementation , even at 80  g per day , was also well tolerated and safe .
the primary outcome of the study , body glucose homeostasis , remained for the most part practically unchanged , thus speaking against the role of vitamin d3 as an important regulator of glucose homeostasis in an ageing finnish population with prediabetes . among the 13 glucose homeostasis indices analyzed , the 120  min plasma glucose was the only index that was statistically significant also in the pairwise analyses .
we believe that other changes observed ( 30  min insulin , hba1c ) may be related to random fluctuation of glucose metabolism in individuals with impaired glucose metabolism . among the interventional studies on glucose homeostasis indicators in populations with prediabetes , an open - label study by nazarian et al . showed an improvement in insulin sensitivity in 11 ifg subjects after a 4-week 10,000  iu / d ( 250  g / d ) vitamin d3 intervention . in an earlier post
, there was a lower rise in fasting plasma glucose concentration and lower increase in homa - ir in 3 years in the group taking 700  iu ( 17.5  g ) vitamin d3 plus 500  mg calcium citrate daily as compared with subjects taking placebo .
glucose homeostasis was also improved in the study by mitri et al . , where 16-week vitamin d3 supplementation of 2,000  iu / d ( 50  g / d ) improved -cell function and attenuated the rise in hba1c .
in an open - label study by dutta et al . in subjects with igt or ifg , vitamin d3 supplementation of 60,000  iu ( 1500  g )
once per week for 8 weeks and then monthly along with 1250  mg of calcium carbonate / d resulted in lower fasting blood glucose and improvement in insulin resistance during an average follow - up of 28 months , when compared with subjects taking only calcium carbonate . in a recent placebo - controlled study by gagnon et al
, vitamin d3 upplementation for six months with a dose aimed at increasing the serum 25(oh)d3 to 75  nmol / l ( 20006000  iu / d , 50150  g / d ) improved insulin sensitivity in subjects with prediabetes , but not in subjects without prediabetes .
in contrast , in a study by harris et al . in 89 overweight or obese african american subjects , 12-week supplementation with 4,000  iu / d ( 100  g / d ) of vitamin d3 , compared with placebo , led to reduced insulin sensitivity and increased insulin secretion . in the 12-month supplementation trial by davidson et al . in 117 latino and african american subjects in california with hypovitaminosis d3 ( i.e. , serum 25(oh)d3 concentration < 30  ng / ml ( 75  nmol / l ) ) , the average weekly dose of 88,865  iu ( 2,222  g , or about 317  g / day ) of vitamin d3 had no effect on insulin secretion , insulin sensitivity , or development of type 2 diabetes , as compared with placebo .
our results support these null findings on glucose homeostasis in finns , a caucasian population at northern latitudes , even though we have earlier observed vitamin d3 sufficiency to associate with the lower risk of type 2 diabetes in an observational study of the same source population .
the observed increase in the 120  min plasma glucose is somewhat unexpected , but one explanation may simply be a chance finding due to a largish number of tests that were carried out .
the increase was larger in the 40  g group than in the 80  g group , which supports that the finding occurred by chance as we believe is the case with other variables we monitored , that is , 30  min insulin and hba1c . taken together ,
the evidence from the observational studies supports the association between low vitamin d status and impaired glucose metabolism , but the results from the experimental studies have not found improvements in glucose metabolism with vitamin d supplementation .
a recent mendelian randomisation study that looked into the association between body vitamin d status and glucose homeostasis and type 2 diabetes with four snps and four glycaemic traits showed an association in two snps near genes related to 25(oh)d synthesis with fasting insulin , but not in any of the other eleven tests conducted .
however , if a limitation is to be pointed out , many of the trials have been conducted in subjects with healthy glucose homeostasis or in subjects with diabetes , and only few have been carried out in subjects with prediabetes , the subjects that would probably show any effect on glucose homeostasis most easily .
therefore , well - designed randomized trials with large sample sizes and in people with insufficient vitamin d status may be needed to elucidate the role of vitamin d supplementation in glucose homeostasis and in prevention of type 2 diabetes .
we found supplemental vitamin d3 to exert some anti - inflammatory action , when a borderline statistically significant decrease in the il-1ra concentration was found .
il-1ra is regarded as one of the most sensitive markers of inflammation , as it is readily secreted by many cell types and plasma levels are high enough to be reliably measured .
elevated values of il-1ra have also been shown to predict the onset of type 2 diabetes [ 2224 ] .
the results from previous studies of the anti - inflammatory effects of vitamin d supplementation have been inconsistent .
some studies have found vitamin d supplementation to improve the levels of certain inflammatory markers , such as il-6 , il-10 , crp , or tnf- [ 6 , 10 , 2527 ] , whereas others have found no effects on any of the studied markers [ 19 , 2830 ] .
however , only a few studies have been conducted in subjects with prediabetes [ 6 , 19 , 31 ] and , to our knowledge , no study thus far has investigated the impact of vitamin d supplementation on il-1ra .
our study was conducted during october to april , the period of low uvb light exposure from the sun in finland , to allow as unbiased supplemental vitamin d effect as possible .
strengths of the study were the homogenous study population and use of two different , sufficiently large vitamin d3 doses , which enabled studying possible dose response .
a limitation of the study was the unbalanced gender distribution , especially the low number of females .
another limitation is the lower than planned total number of study subjects , which was due to the need to close recruitment to allow a sufficiently long supplementation during the low uvb exposure period , and the higher than anticipated average 25(oh)d3 concentration of the study population at the start of the study .
however , the fairly consistent results of our study do not suggest a marked change in observed effects even if some more subjects would have been included , or if intention - to - treat analysis had been carried out instead of per - protocol analysis , given that per - protocol tends to exaggerate the treatment effects , if anything .
in conclusion , our study does not support the role of winter - time relatively high - dose vitamin d3 supplementation as a means to improve glucose homeostasis in a general ageing population with prediabetes but suggests a modest anti - inflammatory effect .
 epidemiological evidence suggests a role for vitamin d in type 2 diabetes prevention . 
 we investigated the effects of vitamin d3 supplementation on glucose metabolism and inflammation in subjects with prediabetes . a 5-month randomized , 
 double - blind , placebo - controlled intervention with three arms ( placebo , 40  g / d , or 80  g / d vitamin d3 ) was carried out among sixty - eight overweight ( bmi 2535 ) and aging ( 60 years ) subjects from finland , with serum 25-hydroxyvitamin d3 [ 25(oh)d3 ] < 75  nmol / l and either impaired fasting glucose or impaired glucose tolerance . 
 analyses included 66 subjects who completed the trial . 
 glucose metabolism was evaluated by fasting and 2-hour oral glucose tolerance test - derived indices and glycated hemoglobin . 
 inflammation was evaluated by high - sensitive c - reactive protein and five cytokines . 
 although a dose - dependent increase in serum 25(oh)d3 over the supplementation period was observed ( p trend < 0.001 ) , there were no other statistically significant differences in changes in the 13 glucose homeostasis indicators between the study groups other than increase in the 120  min glucose concentration ( p trend = 0.021 ) and a decreasing trend both in 30  min plasma insulin ( p trend = 0.030 ) and glycated hemoglobin ( p trend = 0.024 ) concentrations . 
 a borderline statistically significant decreasing trend in interleukin-1 receptor antagonist concentration was observed ( p = 0.070 ) . 
 vitamin d3 supplementation does not improve glucose metabolism in ageing subjects with prediabetes but may have modest anti - inflammatory effects . 