Ascorbic acid safety: analysis of factors affecting iron absorption.

The potential for excessive iron absorption by subjects ingesting ascorbic acid doses above the recommended dietary allowance (60 mg) was evaluated by examining published literature (24 studies, 1412 subjects) in which ascorbic acid was part of a test meal given to determine effects on iron absorption. Three parameters associated with iron absorption were identified: (1) a relatively shallow slope for the dose-response curve relating ascorbic acid dosage (1-1000 mg) and percent iron absorption; (2) no significant effect of ascorbic acid on the absorption of high (60 mg) iron doses; and (3) an inverse relationship between iron absorption and plasma transferrin saturation. Ascorbic acid did not increase the incidence of 'high' iron absorbers (greater than 2 SD from population mean) above control levels; limited data for ascorbic acid doses greater than 100 mg/d indicated no change in the distribution of iron absorption values.