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 .
Influence of vitamin C supplementation on oxidative and immune changes after an ultramarathon.
The purpose of this r and omized study was to measure the influence of  vitamin C ( n = 15 runners ) compared with  placebo ( n = 13 runners )  supplementation on oxidative and immune changes in runners competing in an ultramarathon race . During the 7-day period before the race and on race day , subjects ingested in r and omized , double-blind fashion 1,500 mg/day vitamin C or placebo . On race day , blood sample s were collected 1 h before race , after 32 km of running , and then again immediately after race . Subjects in both groups maintained an intensity of approximately 75 % maximal heart rate throughout the ultramarathon race and ran a mean of 69 km ( range : 48 - 80 km ) in 9.8 h ( range : 5 - 12 h ) . Plasma ascorbic acid was markedly higher in the vitamin C compared with placebo group prerace and rose more strongly in the vitamin C group during the race ( postrace : 3.21 + /- 0.29 and 1.28 + /- 0.12 microg/100 microl , respectively , P < 0.001 ) . No significant group or interaction effects were measured for lipid hydroperoxide , F2-isoprostane , immune cell counts , plasma interleukin (IL)-6 , IL-10 , IL-1-receptor antagonist , or IL-8 concentrations , or mitogen-stimulated lymphocyte proliferation and IL-2 and IFN-gamma production . These data indicate that vitamin C supplementation in carbohydrate-fed runners does not serve as a countermeasure to oxidative and immune changes during or after a competitive ultramarathon race
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 .