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 .
Megadose of vitamin C delays insulin response to a glucose challenge in normoglycemic adults.
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
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 .