Megadose of vitamin C delays insulin response to a glucose challenge in normoglycemic adults. Supplementation of Vitamin C Reduces Blood Glucose and Improves Glycosylated Hemoglobin in Type 2 Diabetes Mellitus: A Randomized, Double-Blind Study The Effect of Vitamin C Supplementation on Diabetic Hyperlipidaemia: a Double Blind, Crossover Study High-dose oral vitamin C partially replenishes vitamin C levels in patients with Type 2 diabetes and low vitamin C levels but does not improve endothelial dysfunction or insulin resistance. FFA-induced endothelial dysfunction can be corrected by vitamin C. Effect of daily supplementation of fruits on oxidative stress indices and glycaemic status in type 2 diabetes mellitus. Vitamin C: an aldose reductase inhibitor that normalizes erythrocyte sorbitol in insulin-dependent diabetes mellitus. Vitamin D supplementation affects serum high-sensitivity C-reactive protein, insulin resistance, and biomarkers of oxidative stress in pregnant women. Effects of vitamins C and E and beta-carotene on the risk of type 2 diabetes in women at high risk of cardiovascular disease: a randomized controlled trial. Study protocol: a randomised placebo-controlled clinical trial to study the effect of vitamin D supplementation on glycaemic control in type 2 Diabetes Mellitus SUNNY trial Vitamin C improves endothelium-dependent vasodilation in patients with non-insulin-dependent diabetes mellitus. Supplementation with cholecalciferol does not improve glycaemic control in diabetic subjects with normal serum 25-hydroxyvitamin D levels Effects of vitamin D supplementation on markers of vascular function after myocardial infarction--a randomised controlled trial. Effect of vitamin D supplementation on glycemic control in patients with type 2 diabetes: a systematic review of interventional studies Vitamin C affects thrombosis/ fibrinolysis system and reactive hyperemia in patients with type 2 diabetes and coronary artery disease. Vitamin C supplementation affects oxidative-stress blood markers in response to a 30-minute run at 75% VO2max. Effects of a 'healthy' diet and of acute and long-term vitamin C on vascular function in healthy older subjects.
The effect of a megadose of ascorbic acid ( AA ) on glucose and insulin responses after an oral-glucose-tolerance test ( OGTT ) is unknown . With a double-blind , placebo-controlled design , nine normoglycemic subjects ( 22 + /- 1 y , mean + /- SEM ) consumed AA  ( 2 g/d ) or placebo for 2 wk after a 2-wk washout period with placebo , and an OGTT was performed after an overnight fast . This 4-wk protocol was repeated in a crossover fashion . Plasma glucose was significantly elevated 1-h postpr and ial in vitamin C-saturated subjects vs those taking a placebo . The plasma insulin response curve was shifted rightward in vitamin C-saturated subjects relative to baseline : plasma insulin was significantly depressed at 0.5 h postpr and ial but significantly elevated at 2 h postpr and ial . These data indicate that elevated plasma AA delays the insulin response to a glucose challenge in normoglycemic adults , thereby prolonging the postpr and ial hyperglycemia . These effects might be partially explained by the competitive inhibition of glucose transfer into pancreatic beta cells by high concentrations of circulating AA No study has ever examined the effect of vitamin C with metformin on fasting ( FBS ) and postmeal blood glucose ( PMBG ) as well as glycosylated hemoglobin ( HbA1c ) in the treatment of type 2 diabetes mellitus ( DM ) . The goal was to examine the effect of oral vitamin C with metformin on FBS , PMBG , HbA1c , and plasma ascorbic acid level ( PAA ) with type 2 DM . Seventy patients with type 2 DM participated in a prospect i ve , double-blind , placebo-controlled , 12-week study . The patients with type 2 DM were divided r and omly into placebo and vitamin C group of 35 each . Both groups received the treatment for twelve weeks . Decreased PAA levels were found in patients with type 2 diabetes mellitus . This level was reversed significantly after treatment with vitamin C along with metformin compared to placebo with metformin . FBS , PMBG , and HbA1c levels showed significant improvement after 12 weeks of treatment with vitamin C. In conclusion , oral supplementation of vitamin C with metformin  reverses ascorbic acid levels , reduces FBS , PMBG , and improves HbA1c . Hence , both the drugs in combination may be used in the treatment of type 2 DM to maintain good glycemic control Fifty diabetic patients took part in a four‐month , double‐blind crossover study comparing 500 mg of vitamin C daily with placebo . No significant difference was observed between vitamin C and placebo therapy in fasting whole blood glucose , serum cholesterol , triglycerides , and glycosylated haemoglobin levels Endothelial dysfunction is a hallmark of Type 2 diabetes related to hyperglycemia and oxidative stress . Nitric oxide-dependent vasodilator actions of insulin may augment glucose disposal . Thus endothelial dysfunction may worsen insulin resistance . Intra-arterial administration of vitamin C improves endothelial dysfunction in diabetes . In the present study , we investigated effects of high-dose oral vitamin C to alter endothelial dysfunction and insulin resistance in Type 2 diabetes .  Plasma vitamin C levels in 109 diabetic subjects were lower than healthy ( 36 + /- 2 microM ) levels . Thirty-two diabetic subjects with low plasma vitamin C ( < 40 microM ) were subsequently enrolled in a r and omized , double-blind , placebo-controlled study of vitamin C ( 800 mg/day for 4 wk ) . Insulin sensitivity ( determined by glucose clamp ) and forearm blood flow in response to ACh , sodium nitroprusside ( SNP ) , or insulin ( determined by plethysmography ) were assessed before and after 4 wk of treatment . In the placebo group ( n = 17 subjects ) , plasma vitamin C ( 22 + /- 3 microM ) , fasting glucose ( 159 + /- 12 mg/dl ) , insulin ( 19 + /- 7 microU/ml ) , and SI(Clamp ) [ 2.06 + /- 0.29 x 10(-4 ) dl x kg(-1 ) x min(-1)/(microU/ml ) ] did not change significantly after placebo treatment . In the  vitamin C  group ( n = 15 subjects ) , basal plasma vitamin C ( 23 + /- 2 microM ) increased to 48 + /- 6 microM ( P < 0.01 ) after treatment , but this was significantly less than that expected for healthy subjects ( > 80 microM ) . No significant changes in fasting glucose ( 156 + /- 11 mg/dl ) , insulin ( 14 + /- 2 microU/ml ) , SI(Clamp ) [ 2.71 + /- 0.46 x 10(-4 ) dl x kg(-1 ) x min(-1)/(microU/ml ) ] , or forearm blood flow in response to ACh , SNP , or insulin were observed after vitamin C treatment . We conclude that high-dose oral vitamin C therapy , result ing in incomplete replenishment of vitamin C levels , is ineffective at improving endothelial dysfunction and insulin resistance in Type 2 diabetes Insulin resistance is associated with an inappropriate elevation of plasma FFA and endothelial dysfunction . FFA could stimulate formation of reactive oxygen species , which could be responsible for vascular impairment . In this r and omized , double-blind , cross-over study in 10 healthy volunteers ( 24 + /- 3 yr old ) , forearm blood flow ( FBF ) responses to intraarterial acetylcholine ( ACh ) and glyceryl trinitrate were assessed with coadministration of vitamin C ( 24 mg/ml ) or placebo , respectively , in the presence of increased plasma FFA induced by Intralipid/heparin infusion . The rise in plasma FFA from 320 + /- 64 to 1852 + /- 232 micromol/liter was associated with a reduced response of FBF to ACh by 55 % ( P < 0.01 ) . During coadministration of vitamin C , the impaired responsiveness of FBF to ACh was completely reversed and not different from that observed under baseline conditions . Vitamin C did not affect plasma FFA concentrations . Glyceryl trinitrate responsiveness was unchanged during FFA elevation , with or without vitamin C. These data suggest that FFA-induced vascular oxidative stress could contribute to endothelial dysfunction in insulin-resistant patients . High concentrations of antioxidants are able to reverse the local effects of FFA on endothelium-dependent vasodilation This study sought to examine whether consumption of two low-calorie fruit/day for 3-months can effectively improve oxidative stress , anthropometry , blood pressure and glycaemic control in type 2 diabetes mellitus . Study involved 123 patients who were assigned to receive either st and ard care or with additional dietary therapy .  Dietary intervention result ed in significant reduction in malondialdehyde , plasma glucose , glycated haemoglobin and improvement in antioxidants like vitamin C and reduced glutathione when compared to controls . Mean plasma levels of vitamin C increased by 64 % ( p < 0.001 ) . There were no differences in waist circumference , waist-to-hip ratio , blood pressure , vitamin E and superoxide dismutase in the intervention group at follow-up . Diet rich in fruits can improve some antioxidants which are likely to reduce oxidative stress in type 2 diabetes . Regular consumption of fruits can lower the glycaemic status in these patients . The study supports the usefulness of plasma vitamin C as a biomarker for fruit intake OBJECTIVE Diabetic hyperglycemia promotes sorbitol production from glucose via aldose reductase . Since the intracellular accumulation of sorbitol , or its sequelae , are postulated to contribute to the progression of chronic diabetic complications , aldose reductase inhibitors ( ARI ) offer therapeutic promise . Others have shown that vitamin C at pharmacologic doses decreases erythrocyte ( RBC ) sorbitol . We examined whether smaller , physiologic doses of vitamin C were also effective in individuals with insulin-dependent diabetes mellitus ( IDDM ) and whether vitamin C was an ARI in vitro . METHODS Vitamin C supplements ( 100 or 600 mg ) were taken daily for 58 days by young adults with IDDM and nondiabetic adults in an otherwise free-living design . Diabetic control was monitored by fasting plasma glucose , glycosylated hemoglobin , and glycosuria and was moderate to poor throughout the study . RBC sorbitol was measured at baseline and again at 30 and 58 days . Three-day dietary records and 24-hour urine collection s were performed for each sampling day . RESULTS RBC sorbitol levels were significantly elevated in IDDMs , on average doubled , despite their more than adequate dietary intakes of vitamin C and normal plasma concentrations . Vitamin C supplementation at either dose normalized the RBC sorbitol in IDDMs within 30 days . This correction of sorbitol accumulation was independent of changes in diabetic control . Furthermore , our in vitro studies show that ascorbic acid inhibited aldose reductase activity . CONCLUSIONS Vitamin C supplementation is effective in reducing sorbitol accumulation in the erythrocytes of diabetics . Given its tissue distribution and low toxicity , we suggest a superiority for vitamin C over pharmaceutic ARIs Unfavorable metabolic profiles and oxidative stress in pregnancy are associated with several complications . This study was conducted to determine the effects of vitamin D supplementation on serum concentrations of high-sensitivity C-reactive protein ( hs-CRP ) , metabolic profiles , and biomarkers of oxidative stress in healthy pregnant women . This r and omized , double-blind , placebo-controlled clinical trial was conducted in 48 pregnant women aged 18 - 40 y old at 25 wk of gestation . Participants were r and omly assigned to receive either 400 IU/d cholecalciferol supplements ( n = 24 ) or placebo ( n = 24 ) for 9 wk . Fasting blood sample s were taken at study baseline and after 9 wk of intervention to quantify serum concentrations of hs-CRP , lipid concentrations , insulin , and biomarkers of oxidative stress . After 9 wk of intervention , the increases in serum 25-hydroxyvitamin D and calcium concentrations were greater in the vitamin D group ( + 3.7 μg/L and + 0.20 mg/dL , respectively ) than in the placebo group ( -1.2 μg/L and -0.12 mg/dL , respectively ; P < 0.001 for both ) . Vitamin D supplementation result ed in a significant decrease in serum hs-CRP ( vitamin D vs. placebo groups : -1.41 vs. + 1.50 μg/mL ; P-interaction = 0.01 ) and insulin concentrations ( vitamin D vs. placebo groups : -1.0 vs. + 2.6 μIU/mL ; P-interaction = 0.04 ) and a significant increase in the Quantitative Insulin Sensitivity Check Index score ( vitamin D vs. placebo groups : + 0.02 vs. -0.02 ; P-interaction = 0.006 ) , plasma total antioxidant capacity ( vitamin D vs. placebo groups : + 152 vs. -20 mmol/L ; P-interaction = 0.002 ) , and total glutathione concentrations ( vitamin D vs. placebo groups : + 205 vs. -32 μmol/L ; P-interaction = 0.02 ) compared with placebo . Intake of vitamin D supplements led to a significant decrease in fasting plasma glucose ( vitamin D vs. placebo groups : -0.65 vs. -0.12 mmol/L ; P-interaction = 0.01 ) , systolic blood pressure ( vitamin D vs. placebo groups : -0.2 vs. + 5.5 mm Hg ; P-interaction = 0.01 ) , and diastolic blood pressure ( vitamin D vs. placebo groups : -0.4 vs. + 3.1 mm Hg ; P-interaction = 0.01 ) compared with placebo . In conclusion , vitamin D supplementation for 9 wk among pregnant women has beneficial effects on metabolic status BACKGROUND Vitamin C , vitamin E , and beta-carotene are major antioxidants and as such may protect against the development of type 2 diabetes via reduction of oxidative stress . OBJECTIVE The purpose of this study was to investigate the long-term effects of supplementation with vitamin C , vitamin E , and beta-carotene for primary prevention of type 2 diabetes . DESIGN In the Women 's Antioxidant Cardiovascular Study , a r and omized trial that occurred between 1995 and 2005 , 8171 female health professionals aged > or = 40 y with either a history of cardiovascular disease ( CVD ) or > or =3 CVD risk factors were r and omly assigned to receive vitamin C ( ascorbic acid , 500 mg every day ) , vitamin E ( RRR-alpha-tocopherol acetate , 600 IU every other day ) , beta-carotene ( 50 mg every other day ) , or their respective placebos . RESULTS During a median follow-up of 9.2 y , a total of 895 incident cases occurred among 6574 women who were free of diabetes at baseline . There was a trend toward a modest reduction in diabetes risk in women assigned to receive vitamin C compared with those assigned to receive placebo [ relative risk ( RR ) : 0.89 ; 95 % CI : 0.78 , 1.02 ; P = 0.09 ] , whereas a trend for a slight elevation in diabetes risk was observed for vitamin E treatment ( RR : 1.13 ; 95 % CI : 0.99 , 1.29 ; P = 0.07 ) . However , neither of these effects reached statistical significance . No significant effect was observed for beta-carotene treatment ( RR : 0.97 ; 95 % CI : 0.85 , 1.11 ; P = 0.68 ) . CONCLUSION Our r and omized trial data showed no significant overall effects of vitamin C , vitamin E , and beta-carotene on risk of developing type 2 diabetes in women at high risk of CVD . This trial was registered at clinical trials.gov as NCT00000541 Background Besides the classical role of vitamin D on calcium and bone homeostasis , vitamin D deficiency has recently been identified as a contributing factor in the onset of insulin resistance in type 2 diabetes mellitus . However , it is uncertain whether vitamin D deficiency and poor glycaemic control are causally interrelated or that they constitute two independent features of type 2 diabetes mellitus . There are limited clinical trials carried out which measured the effect of vitamin D supplementation on glycaemic control . The objective of this study is to investigate the effect of vitamin D supplementation on glycaemic control and quality of life in patients with type 2 diabetes mellitus . Methods / design In a r and omised double-blind placebo-controlled trial conducted in five general practice s in the Netherl and s three hundred patients with type 2 diabetes mellitus treated with lifestyle advises or metformin or sulphonylurea-derivatives are r and omised to receive either placebo or 50,000 IU Vitamin D3 at monthly intervals . The primary outcome measure is the change in glycated haemoglobin level between baseline and six months . Secondary outcome measures include blood pressure , anthropometric parameters , lipid profile , insulin resistance , quality of life , advanced glycation end products and safety profiles . Quality of life will be measured by The Short Form ( SF-36 ) Health Survey question naire . Advanced glycation end products are measured by an AGE-reader . Discussion This trial will be the first study exploring the effect of vitamin D supplementation on both glycaemic control and quality of life in patients with type 2 diabetes mellitus . Our findings will contribute to the knowledge of the relationship between vitamin D status and insulin resistance in patients with type 2 diabetes mellitus . Trial registration The Netherl and s trial register : Endothelium-dependent vasodilation is impaired in humans with diabetes mellitus . Inactivation of endothelium-derived nitric oxide by oxygen-derived free radicals contributes to abnormal vascular reactivity in experimental models of diabetes . To determine whether this observation is relevant to humans , we tested the hypothesis that the antioxidant , vitamin C , could improve endothelium-dependent vasodilation in forearm resistance vessels of patients with non-insulin-dependent diabetes mellitus . We studied 10 diabetic subjects and 10 age-matched , nondiabetic control subjects .  Forearm blood flow was determined by venous occlusion plethysmography . Endothelium-dependent vasodilation was assessed by intraarterial infusion of methacholine ( 0.3 - 10 micrograms/min ) . Endothelium-independent vasodilation was measured by intraarterial infusion of nitroprusside ( 0.3 - 10 micrograms/min ) and verapamil ( 10 - 300 micrograms/min ) . Forearm blood flow dose-response curves were determined for each drug before and during concomitant intraarterial administration of vitamin C ( 24 mg/min ) . In diabetic subjects , endothelium-dependent vasodilation to methacholine was augmented by simultaneous infusion of vitamin C ( P = 0.002 ) ; in contrast , endothelium-independent vasodilation to nitroprusside and to verapamil were not affected by concomitant infusion of vitamin C ( P = 0.9 and P = 0.4 , respectively ) . In nondiabetic subjects , vitamin C administration did not alter endothelium-dependent vasodilation ( P = 0.8 ) . We conclude that endothelial dysfunction in forearm resistance vessels of patients with non-insulin-dependent diabetes mellitus can be improved by administration of the antioxidant , vitamin C. These findings support the hypothesis that nitric oxide inactivation by oxygen-derived free radicals contributes to abnormal vascular reactivity in diabetes Background Studies have suggested that vitamin D may be important for both insulin sensitivity and insulin secretion , and that supplementation with vitamin D may subsequently prevent development of type 2 diabetes . Aim of the study The objective of the current study was to test the hypothesis that supplementation with vitamin D would improve glycaemic control in subjects with type 2 diabetes . Methods Thirty-six subjects with type 2 diabetes , treated with metformin and bed-time insulin  , were r and omised to supplementation with cholecalciferol ( 40,000 IU per week ) versus placebo for 6 months . Thirty-two subjects participated throughout the entirety of the study . Fasting blood sample s were drawn before and at the end of the 6 month study without the previous bed-time insulin injection . The insulin and metformin doses were not changed throughout the study . Results After 6 months , the fasting glucose , insulin , C-peptide , fructosamine , and HbA1c levels were not significantly different from baseline values . In addition , changes in these parameters ( values at 6 months minus values at baseline ) did not differ between the vitamin D and the placebo group . Conclusions We were not able to demonstrate that vitamin D supplementation had a significant effect on glucose metabolism in subjects with type 2 diabetes but without vitamin D deficiency . Further studies are needed in larger groups of subjects with type 2 diabetes or impaired glucose tolerance , who also exhibit low serum 25-hydroxyvitamin D levels BACKGROUND Low vitamin D levels are common , and are associated with a higher incidence of future vascular events . We tested whether vitamin D supplementation could improve endothelial function and other markers of vascular function in patients with a history of myocardial infa rct ion . METHODS Parallel group , placebo-controlled , double-blind r and omised trial . Patients with a history of myocardial infa rct ion were r and omised to receive 100,000 units of oral vitamin D3 or placebo at baseline , 2 months and 4 months . Outcomes were measured at baseline , 2 and 6 months . Reactive hyperaemia index on fingertip plethysmography was the primary outcome . Secondary outcome measures included blood pressure , cholesterol , C-reactive protein , von Willebr and factor , tumour necrosis factor alpha , E-selectin , B-type natriuretic peptide , thrombomodulin and 25-hydroxyvitamin D levels . RESULTS 75 patients were r and omised , mean age 66 years . 74/75 ( 99 % ) completed 6 month follow-up . 25 hydroxyvitamin D levels increased in the intervention group relative to placebo ( + 13 vs + 1 nmol/L , p=0.04 ) . There was no between-group difference in change in reactive hyperaemia index between baseline and 6 months ( -0.18 vs -0.07 , p=0.40 ) . Of the secondary outcomes , only C-reactive protein showed a significant decline in the intervention arm relative to placebo at 6 months ( -1.3 vs 2.0mg/L , p=0.03 ) . Systolic blood pressure ( + 1.4 vs + 2.3 mmHg , p=0.79 ) , diastolic blood pressure ( + 2.0 vs + 0.8 mmHg , p=0.54 ) and total cholesterol ( + 0.26 vs + 0.24 mmol/L , p=0.88 ) showed no between-group difference at 6 months . CONCLUSIONS Supplementation with vitamin D did not improve markers of vascular function in patients with a history of myocardial infa rct ion Background Diabetes and vitamin D deficiency are global epidemics . Research ers have long been exploring the role of potentially modifiable factors to manage type 2 diabetes . We conducted a systematic review of prospect i ve studies and r and omized controlled trials that involved vitamin D supplementation and specifically intended to study glycemic outcomes related to type 2 diabetes . Methods Two authors independently search ed Medline and PubMed for longitudinal studies that had assessed the effect of vitamin D supplements on glycemic control , insulin resistance and beta-cell dysfunction in patients with diabetes . Results Seventeen r and omized control trials and seven longitudinal studies with a minimum follow-up of one month were included . Results of the various short-term studies ( follow up ≤ 3 months ) suggested that vitamin D supplementation had a positive impact on glycemic control and metabolic parameters such as insulin resistance and beta cell dysfunction . However , the evidence was weak due to the low method ological quality of the studies . There was no significant effect on HbA1c , beta cell function and insulin resistance in the long-term studies ( follow up > 3 months ) . There existed heterogeneity in the methodology of the studies , inclusion criteria , mode of supplementation of vitamin D and the duration of follow up . Conclusions Current evidence based on r and omized controlled trials and longitudinal studies do not support the notion that vitamin D supplementation can improve hyperglycemia , beta cell secretion or insulin sensitivity in patients with type 2 diabetes . Large-scale trials with proper study design , optimal vitamin D supplementation and longer follow up need to be conducted OBJECTIVE To examine the effect of vitamin C on forearm vasodilatory response to reactive hyperemia and on plasma level of plasminogen activator inhibitor 1 ( PAI-1 ) , von Willebr and factor ( vWF ) , tissue plasminogen activator ( tPA ) , antithrombin III ( ATIII ) , proteins C and S , and factors V ( fV ) and VII ( fVII ) in patients with both type 2 diabetes and CAD . RESEARCH DESIGN AND METHODS A total of 39 patients with type 2 diabetes and CAD were divided into two groups and received vitamin C ( 2 g/day ) or no antioxidant for 4 weeks . Forearm blood flow was determined using venous occlusion gauge-strain plethysmography at baseline and after treatment . Forearm vasodilatory response to reactive hyperemia ( RH% ) or nitrate ( NTG% ) was defined as the percent change of flow from baseline to the maximum flow during reactive hyperemia or after administration of nitrate , respectively . Biochemical markers were determined by enzyme-linked immunosorbent assay ( ELISA ) or other st and ard methods . RESULTS RH% was significantly increased after treatment with vitamin C ( from 62.4 + /- 7.2 to 83.1 + /- 9.3 % , P = 0.024 ) but remained unaffected in the control group . Vitamin C decreased plasma levels of fV ( from 143 + /- 5.4 to 123 + /- 6.03 % , P = 0.038 ) , vWF ( from 133.5 + /- 14.5 to 109.5 + /- 11.4 % , P = 0.016 ) , and tPA ( from 12.3 + /- 0.99 to 8.40 + /- 0.60 ng/ml , P = 0.001 ) , whereas these levels remained unaffected in the control group . The changes in RH% , vWF , and tPA were significantly greater ( P = 0.028 , 0.036 , and 0.007 , respectively ) in the vitamin C-treated group than in the control group . Levels of ATIII , proteins S and C , fVII , and PAI-1 remained unchanged in all groups . CONCLUSIONS Short-term treatment with high doses of vitamin C improved RH% and decreased plasma levels of tPA and vWF in patients with type 2 diabetes and CAD Vitamin C supplementation ( VC ) ( either 500 or 1000 mg/d for 2 wk ) was compared to a placebo treatment ( P ) to ascertain if VC could influence oxidative stress . Twelve healthy males ( 25 + /- 1.4 y ) were r and omly assigned in a counter-balanced design with a 2-wk period between treatments . Data were analyzed using repeated measures ANOVA . Exercise intensity measures ( VO(2 ) , RER , RPE , HR , lactate ) were similar across treatments . Resting blood oxidative-stress markers were unaffected by treatment . Exercise decreased total blood glutathione ( TGSH ) and reduced glutathione ( GSH ) and increased oxidized glutathione ( GSSG ) ( P < 0.01 ) independent of treatment . Protein carbonyls ( PC ) increased 3.8 fold in the P ( P < 0.01 ) . VC attenuated the PC exercise response in a dose-dependent manner ( P < 0.01 ) . Thiobarbituric acid reactive substances ( TBARS ) was not influenced by exercise ( P = 0.68 ) or VC . These data suggest that VC supplementation can attenuate exercise-induced protein oxidation in a dose-dependent manner with no effect on lipid peroxidation and glutathione status OBJECTIVE Aging is associated with endothelial dysfunction . We studied the acute and longer-term effects of vitamin C compared to a ' Mediterranean-type ' diet on endothelial function in healthy older subjects . METHODS Bilateral venous occlusion plethysmography was used to measure forearm blood flow in subjects aged 57 - 80 years . Responses to cumulative intra-arterial doses of the endothelium-dependent dilator bradykinin ( BK ; n=56 ; 20 , 40 , 80 pmol/min ) and the nitric oxide donor glyceryl trinitrate ( GTN ; n=54 ; 4 , 8 , 16 nmol/min ) , were determined alone and in the presence of vitamin C ( 25 mg/min ) . We then r and omised 54 subjects to a  ' healthy ' diet  ( n=18 ) , vitamin C ( 1 g/day ; n=18 ) or placebo for 6 weeks and reassessed endothelial and smooth muscle function . RESULTS Acute intra-arterial vitamin C did not alter dilatation to BK or GTN . Similar increases in plasma vitamin C occurred on oral vitamin C ( 83+/-4 to 135+/-8 micromol/l ) and ' healthy ' diet ( 84+/-5 to 135+/-27 micromol/l ; P<0.01 for both ) , with no change seen on placebo . Treatment with a ' healthy ' diet but not oral vitamin C improved endothelium-dependent ( P=0.043 ) and endothelium-independent dilatation ( P=0.011 ) . CONCLUSIONS A ' Mediterranean-type ' diet rich in vitamin C improves vascular function . Neither acute intra-arterial nor sustained administration of oral vitamin C improves vascular function in healthy older subjects
Overall , vitamin C did not modify glucose , HbA1c and insulin concentrations .
Meta-regression analyses showed that age was a modifier of the effect of vitamin C on insulin concentration .
Furthermore , the effect size was associated with baseline BMI and plasma glucose levels , and with the duration of the intervention .
In conclusion , greater reduction in glucose concentrations observed in patients with diabetes , older individuals and with more prolonged supplementation .
R and omised controlled trials ( RCTs ) have observed contrasting results on the effects of vitamin C on circulating biomarkers of glycaemic and insulin regulation .