The Absorption of Iron

General agreement exists concerning the salient features of human iron absorption, namely, (1) iron absorption can occur at any level of the gastrointestinal tract from the stomach distally, although absorption is greatest in the duodenum and progressively less in a descending gradient; (2) divalent iron is better absorbed than the trivalent form; (3) iron uptake by the intestinal cell is unidirectional and is transported from the intestine via the bloodstream rather than lymphatics; (4) iron absorption increases under conditions of iron deficiency, advanced pregnancy, hemochromatosis and accelerated erythropoiesis; (5) iron absorption is often reduced in association with increased tissue iron stores, decreased erythropoiesis, diffuse malabsorption syndromes and increased dietary intake of phosphates and phytates. There is little agreement about the precise mechanism of iron absorption and the factors that regulate it. The classic "mucosal block" theory, in which ferritin plays a central role in regulating iron absorption, appears to have little factual support on close examination. An alternative hypothesis of iron absorption that reconciles the available data has not yet been formulated, although evidence from recent experiments concerning the mechanism and regulation of iron absorption is reviewed.