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 .
Molecular detection of exercise-induced free radicals following ascorbate prophylaxis in type 1 diabetes mellitus: a randomised controlled trial
Abstract Aims /hypothesis Patients with type 1 diabetes mellitus are more susceptible than healthy individuals to exercise-induced oxidative stress and vascular endothelial dysfunction , which has important implication s for the progression of disease . Thus , in the present study , we design ed a r and omised double-blind , placebo-controlled trial to test the original hypothesis that oral prophylaxis with vitamin C attenuates rest and exercise-induced free radical-mediated lipid peroxidation in type 1 diabetes mellitus . Methods All data were collected from hospitalised diabetic patients . The electron paramagnetic resonance spectroscopic detection of spin-trapped α-phenyl-tert-butylnitrone ( PBN ) adducts was combined with the use of supporting markers of lipid peroxidation and non-enzymatic antioxidants to assess exercise-induced oxidative stress in male patients with type 1 diabetes ( HbA1c 7.9 ± 1 % , n = 12 ) and healthy controls ( HbA1c 4.6 ± 0.5 % , n = 14 ) . Following participant r and omisation using numbers in a sealed envelope , venous blood sample s were obtained at rest , after a maximal exercise challenge and before and 2 h after oral ingestion of 1 g ascorbate or placebo . Participants and lead investigators were blinded to the administration of either placebo or ascorbate treatments . Primary outcome was the difference in changes in free radicals following ascorbate ingestion . Results Six diabetic patients and seven healthy control participants were r and omised to each of the placebo and ascorbate groups . Diabetic patients ( n = 12 ) exhibited an elevated concentration of PBN adducts ( p < 0.05 vs healthy , n = 14 ) , which were confirmed as secondary , lipid-derived oxygen-centred alkoxyl ( RO · ) radicals ( anitrogen = 1.37 mT and aβhydrogen = 0.18 mT ) . Lipid hydroperoxides were also selectively elevated and associated with a depression of retinol and lycopene ( p < 0.05 vs healthy ) . Vitamin C supplementation  increased plasma vitamin C concentration to a similar degree in both groups ( p < 0.05 vs pre-supplementation ) and attenuated the exercise-induced oxidative stress response ( p < 0.05 vs healthy).There were no selective treatment differences between groups in the primary outcome variable . Conclusions /interpretationThese findings are the first to suggest that oral vitamin C supplementation provides an effective prophylaxis against exercise-induced free radical-mediated lipid peroxidation in human diabetic blood . Clinical trials registration number : IS RCT N96164937 Funding : No external funding
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 .