American Journal of Clinical Nutrition, Vol 60, 735-738, Copyright © 1994 by The American Society for Clinical Nutrition, Inc

ORIGINAL RESEARCH COMMUNICATIONS

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

CS Johnston and MF Yen
Department of Family Resources and Human Development, Arizona State University, Tempe 85287-2502.

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.