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 subject s (22 +/- 1 y, ($) over bar +/- SEM) consumed AA (2 g/d) or placebo fo r 2 wk after a 2-wk washout period with placebo, and an OGTT was perfo rmed after an overnight fast. This 4-wk protocol was repeated in a cro ssover fashion. Plasma glucose was significantly elevated 1-h postpran dial in Vitamin C-saturated subjects vs those taking a placebo. The pl asma insulin response curve was shifted rightward in vitamin C-saturat ed subjects relative to baseline: plasma insulin was significantly dep ressed at 0.5 h postprandial but significantly elevated at 2 h postpra ndial. These data indicate that elevated plasma AA delays the insulin response to a glucose challenge in normoglycemic adults, thereby prolo nging the postprandial hyperglycemia. These effects might be partially explained by the competitive inhibition of glucose transfer into panc reatic beta cells by high concentrations of circulating AA.