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 .
Lowering of oxidative stress improves endothelial function in healthy subjects with habitually low intake of fruit and vegetables: a randomized controlled trial of antioxidant- and polyphenol-rich blackcurrant juice.
Inadequate intake of the recommended five-a-day fruit and vegetable portions might contribute to increased cardiovascular disease risk . We assessed the effects of dietary intake of a blackcurrant juice drink , rich in vitamin C and polyphenols , on oxidative stress and vascular function . This was a double-blind , placebo-controlled , parallel group study of 66 healthy adults who habitually consume < 2 portions of fruit and vegetables per day . Participants were r and omly allocated to consume 250ml of placebo ( flavored water ) or low or high blackcurrant juice drink four times a day for 6 weeks . Flow-mediated dilation ( FMD ) and plasma concentrations of F2-isoprostanes and vitamin C were measured . In the high blackcurrant juice drink group FMD increased significantly ( 5.8±3.1 to 6.9±3.1 % , P=0.022 ) compared with the placebo group ( 6.0±2.2 to 5.1±2.4 % ) . Plasma vitamin C concentration increased significantly in the low ( 38.6±17.6 to 49.4±21.0µmol/L , P<0.001 ) and high ( 34.6±20.4 to 73.8±23.3µmol/L , P<0.001 ) blackcurrant juice drink groups compared with the placebo group ( 38.1±21.0 to 29.0±17.6µmol/L ) . F2-isoprostane concentrations were significantly lower in the high blackcurrant juice drink group ( 225±64pg/ml ) compared with the low blackcurrant juice drink ( 257±69pg/ml , P=0.002 ) and placebo group ( 254±59pg/ml , P=0.003 ) . At follow-up , changes in plasma vitamin C correlated significantly with changes in FMD ( r=0.308 , P=0.044 ) . Consumption of blackcurrant juice drink high in vitamin C and polyphenols can decrease oxidative stress and improve vascular health in individuals with habitually low dietary fruit and vegetable intake
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 .