DIETARY-REGULATION OF INTESTINAL ASCORBATE UPTAKE IN GUINEA-PIGS

We measured ascorbic acid (AA) uptake across the intestinal brush border in vitro in intact tissue from guinea pigs fed maintenance AA (200 mg/kg diet) or made hypervitaminotic (5,000 mg/kg diet) or hypovitaminotic (chronically and acutely). Total uptake per centimeter ileum was 25-50% lower in hypervitaminotic juvenile, adult male, and lactating guinea pigs compared with their respective controls, whereas carrier-mediated D-glucose uptake and Na+ - independent AA uptake were similar. High dietary ascorbate. specifically reduced the V(max) for carrier-mediated AA uptake. Hypovitaminosis had no significant effect on uptake of AA or other solutes. We performed diet-switching experiments (high-AA diet to maintenance diet) with young and adult guinea pigs to determine the reversibility of the downregulation. In adult guinea pigs, the downregulation of AA uptake was reversible within 7 days. In the young of mothers fed high AA during pregnancy and lactation, and which fed on high AA for 14 days after weaning, the downregulation was revesible within 14 days. Thus regulation of AA uptake is reversible and therefore probably does not play a significant role in the development of vitamin C dependency in human adults, or their young, after ingestion of megadoses of ascorbic acid.