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 .
Ascorbic acid supplementation improves skeletal muscle oxidative stress and insulin sensitivity in people with type 2 diabetes: Findings of a randomized controlled study.
AIM /HYPOTHESIS Skeletal muscle insulin resistance and oxidative stress are characteristic metabolic disturbances in people with type 2 diabetes . Studies in insulin resistant rodents show an improvement in skeletal muscle insulin sensitivity and oxidative stress following antioxidant supplementation . We therefore investigated the potential ameliorative effects of antioxidant ascorbic acid ( AA ) supplementation on skeletal muscle insulin sensitivity and oxidative stress in people with type 2 diabetes . METHODS Participants with stable glucose control commenced a r and omized cross-over study involving four months of AA ( 2 × 500 mg/day ) or placebo supplementation . Insulin sensitivity was assessed using a hyperinsulinaemic , euglycaemic clamp coupled with infusion of 6,6-D2 glucose . Muscle biopsies were measured for AA concentration and oxidative stress markers that included basal measures ( 2',7'-dichlorofluorescin [ DCFH ] oxidation , ratio of reduced-to-oxidized glutathione [ GSH/GSSG ] and F2-Isoprostanes ) and insulin-stimulated measures ( DCFH oxidation ) . Antioxidant concentrations , citrate synthase activity and protein abundances of sodium-dependent vitamin C transporter 2 ( SVCT2 ) , total Akt and phosphorylated Akt ( ser473 ) were also measured in muscle sample s. RESULTS AA supplementation significantly increased insulin-mediated glucose disposal ( delta rate of glucose disappearance ; ∆Rd ) ( p=0.009 ) , peripheral insulin-sensitivity index ( p=0.046 ) , skeletal muscle AA concentration ( p=0.017 ) and muscle SVCT2 protein expression ( p=0.008 ) ; but significantly decreased skeletal muscle DCFH oxidation during hyperinsulinaemia ( p=0.007 ) when compared with  placebo . Total superoxide dismutase activity was also lower following AA supplementation when compared with placebo ( p=0.006 ) . Basal oxidative stress markers , citrate synthase activity , endogenous glucose production , HbA1C and muscle Akt expression were not significantly altered by AA supplementation . CONCLUSIONS /INTERPRETATION In summary , oral AA supplementation ameliorates skeletal muscle oxidative stress during hyperinsulinaemia and improves insulin-mediated glucose disposal in people with type 2 diabetes . Findings implicate AA supplementation as a potentially inexpensive , convenient , and effective adjunct therapy in the treatment of insulin resistance in people with type 2 diabetes
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 .