title: Megadose of vitamin C delays insulin response to a glucose challenge in normoglycemic adults.
abstract: 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 postprandial 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 postprandial but significantly elevated at 2 h postprandial. These data indicate that elevated plasma AA delays the insulin response to a glucose challenge in normoglycemic adults, thereby prolonging the postprandial hyperglycemia. These effects might be partially explained by the competitive inhibition of glucose transfer into pancreatic beta cells by high concentrations of circulating AA.
authors.affiliation: Department of Family Resources and Human Development, Arizona State University, Tempe 85287-2502.
authors.forename: C S
authors.initials: CS
authors.lastname: Johnston
authors.forename: M F
authors.initials: MF
authors.lastname: Yen
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D001205:Ascorbic Acid / Q000008:administration & dosage / Q000494:pharmacology*; D004311:Double-Blind Method; D004435:Eating; D005260:Female; D005951:Glucose Tolerance Test; D006801:Humans; D007328:Insulin / Q000097:blood*; D008297:Male; D012016:Reference Values; D013997:Time Factors
chemical_list: D007328:Insulin; D001205:Ascorbic Acid