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diabetes type 2 is a worldwide disease and it is estimated that at the end of 2030 more than 550 million people suffer from this disease ( 1 ) .
there are several studies showing that vitamin d deficiency may resulted in developing diabetes type 2 ( 24 ) .
vitamin d may facilitate insulin function by regulating its receptor expression , so it may be increases insulin sensitivity ( 5 ) .
vitamin d may also regulate glucose homeostasis by stimulating insulin release from pancreatic b - cells ( 6 , 7 ) .
therefore , the correction of vitamin d deficiency may result in improved glucose control and has beneficial effects on complications of diabetes type 2 ( 8) . lower circulating level of calcidiol was associated with the increased risk of coronary artery disease in diabetic patients ( 9 ) .
recently , malekshah et al . reported high prevalence of vitamin d deficiency in several cities of iran ( 11 ) .
because of the role of vitamin d in insulin function and the prevalence of vitamin d deficiency in iran , the aim of this study was to examine the effects of vitamin d supplementation on glucose control and insulin sensitivity and lipid profile in patients with type 2 diabetes .
the participants of this randomized double - blind placebo - controlled clinical trial study consist of 65 diabetic type 2 patients at the age range of 3060 years old who agreed to take place in our study between september 2012 and february 2013 .
seven patients discontinued the supplements consumption , three from placebo group and four from vitamin d group .
this study was approved by the tehran university of medical sciences ( tums ) ethical committee ( i d : 17112 ) and registered on www.clinicaltrial.org as nc 01876563 .
the exclusion criteria were consuming vitamin d supplements within 3 months before beginning of the study and having complications of diabetes , thyroid disorders , and using insulin , thiazolidindiones or anti- obesity drugs .
all patients used metformin and/or glibenclamide as anti - diabetic drugs and agreed to maintain their usual dietary and physical activity habits at the time of intervention .
participants of this study divided into two randomly allocated groups ( vitamin d and placebo ) by random permuted blocks within the strata ( bmi ) method .
the vitamin d group received 100 microgram or 4000 iu ( one tablet of vitamin d ) daily and placebo group received one placebo tablet per day for 2 months .
height , hip and waist circumference was measured to the nearest centimeter and weight to the nearest kilogram .
bmi was calculated as the weight divided by the square of height . in the beginning of the study and after 2 months , blood samples were taken after 1214 hours fasting overnight .
after centrifuging , serums were separated and stored at 80 c until measuring the concentration of serum calcidiol , lipid profile and insulin .
in addition , hba1c was measured with hplc columns as long - time control of glucose .
biochemical measurements : serum calcidiol was measured using chemiluminecense method with elecsys system 2010 with roche kit ( code number : 05894973 ) .
serum insulin was measured by human insulin elisa kit ( diametra , italy ) with the sensitivity of 0.25 mciu / ml and an intraassay and interassay of 5% and 10% respectively .
homa - ir was calculated using the formula of fasting glucose ( mmol / l )  fasting insulin ( iu / ml ) / 22.5 ( 12 ) .
the kolmogorov smirnoff test was used for determining normality of the parameters and wilcoxon test and mann - whitnes test were used to analysis of non - normal distribution variables within and between groups .
independent sample t - test and paired t - test were used for comparison between groups before and after supplementation and within groups for analysis of normal distribution variables . in all analysis , p - value <
vitamin d group consisted of 28 patients ( 15 men and 13 women ) and placebo group consisted of 30 patients ( 21 men and 9 women ) and there was not significant differences in the sex distribution between 2 groups ( p=0.154 ) .
in addition , there was no significant differences in times at sun exposure between two groups ( p=0.580 ) .
as it demonstrated , there are no significant differences between anthropometric parameters and duration of disease between 2 groups .
baseline characteristics of anthropometric parameters in vitamin d and placebo groups before the intervention data are expressed as mean  se , t - test was used to detect differences between the groups table 2 illustrates fasting biochemical characteristics of two groups at baseline and post intervention .
the results show that supplementation with vitamin d caused a significant decrease in hba1c ( p<0.001 ) and it was still significant after removing the baseline effect ( p<0.001 , ancova ) .
serum insulin concentration decreased significantly in vitamin d group ( p=0.048 ) , it was not significant between two groups at the end of the study but after adjusting for the baseline values the difference got statistically significant ( p<0.001 , ancova ) .
there was not any significant difference in homa - ir between two groups at the end of intervention , but it got statistically significant ( p=0.036 , ancova ) after adjusting for the baseline values .
hdl - c concentration increased significantly in both vitamin d ( p=0.046 ) and placebo receiving groups ( p=0.028 ) .
in addition , mean concentration of fbs and tc increased significantly in placebo group . for tc concentration ,
the difference between two groups was statistically significant after removing the effect of baseline values ( p=0.021 , ancova ) .
serum calcidiol was still significantly different between two groups after removing baseline amounts ( p<0.001 , ancova ) .
fasting biochemical characteristics of vitamin d and placebo groups at baseline and post intervention data are expressed as mean  se , independent samples t - test ,
the results of this study showed that vitamin d supplementation decreased serum insulin concentration and had beneficial effects in decreasing hba1c in diabetic type 2 patients .
there are several studies with similar results supporting this idea that vitamin d is an important nutrient in control of glucose homeostasis ( 13 , 14 ) . vitamin d intake decreased prevalence of diabetes type 2 in long - time ( 15 ) .
one mechanism that plausible for relating vitamin d to diabetes may be its action on insulin receptor in beta cells .
vitamin d can stimulate gene expression of insulin receptor and increases glucose transport from the intestine ( 5 ) .
another mechanism is that 1,25 ( oh)2d3 involves in calcium absorption from the gut and calcium is necessary for insulin release from beta - cells ( 16 , 17 ) .
recently , it has been cleared that beta - cells have receptors for 1,25 ( oh)2d3 , the active form of vitamin d and this cells can convert calcidiol to this form of vitamin ( 17 ) .
one study showed that whether calcium is used or not , vitamin d supplementation improves glucose control in adults at high risk of diabetes type 2 ( 18 ) .
supplementation with vitamin d was only related to improved glucose control in diabetic patients with vitamin d deficiency and this nutrient had not any beneficial effect in patients with normal range of serum calcidiol ( 19 ) .
supplementation of 1000 iu vitamin d combined with or without calcium in the form of fortified yogurt resulted in decreased hba1c ( 20 ) .
however , vitamin d supplementation although can normalize the calcidiol concentration in diabetic patients ; it has no long - term effect on glycemic control in this people ( 21 ) . in our study
, vitamin d supplementation did not changed serum lipid profile in diabetic patients significantly except for hdl - c .
however , the concentration of ldl - c and fbs increased significantly in placebo receiving group .
one previous study showed that supplementation with vitamin d did not result in significant reduction in plasma glucose , tc , ldl - c and tg ( 19 ) .
moreover , vitamin d supplementation had not significant effect in reducing serum fbs or insulin resistance in one systematic review ( 22 ) . unlike our results , in the study of al - daghri et al .
supplementation with vitamin d had decreased significantly serum tc and ldl - c concentration ( 23 ) .
one of limitations of our study is the number of patients participated in the study .
maybe , if we used more patients , we could achieve better results such as reduction in some fractions of lipid profile .
supplementation with ul dose of vitamin d could improve glucose control in diabetic type 2 patients , but did not exhibit any beneficial change in lipid profile except for hdl - c concentration .
therefore , it seems that vitamin d supplementation might be used as a strategy for control of glucose control in these people .
ethical issues ( including plagiarism , informed consent , misconduct , data fabrication and/or falsification , double publication and/or submission , redundancy , etc . ) have been completely observed by the authors .
background : vitamin d deficiency is prevalent in diabetes type 2 and this vitamin may be related to insulin action . 
 this randomized controlled trial study was done to evaluate the effect of vitamin d supplementation on glucose control and insulin resistance in patients with diabetes type 2.methods:participants of this randomized clinical trial study consisted of 28 patients with type 2 diabetes who received 100 microgram ( 4000 iu ) vitamin d and 30 diabetic patients who received placebo for 2 months between september 2012 and february 2013 . 
 the effect of vitamin d on glucose control was assessed by measuring hba1c and insulin resistance as homa - ir at the baseline and the end of the intervention.results:the results showed a significant decrease in hba1c ( from 7.29  0.22 % to 6.76  0.18 % , p<0.001 ) and insulin concentration ( from 8.24  0.97 iu / ml to 6.55  0.28 iu / ml , p=0.048 ) , but a non - significant decrease in homa - ir in vitamin d group . 
 also , hdl - c level increased significantly in both of vitamin d ( p=0.046 ) and placebo groups ( p=0.028).conclusion : it seems that vitamin d supplementation has beneficial effects on glucose homeostasis and can increases insulin sensitivity in diabetic 2 patients .