Radioautographic studies provide evidence to support a concept of the mechanism whereby the small intestine controls absorption of iron. Three different states of the body’s iron stores have been considered in this regard: iron excess, iron deficiency and normal iron repletion. As the columnar epithelial cells of the duodenal villi are formed they incorporate a portion of intrinsic iron from the body’s iron store, the amount depending upon the body’s requirement for new iron. It is predicated that with iron excess the iron-receptor mechanism in these cells is saturated with intrinsic iron; this then prevents the cell from accepting dietary iron. In the normal state of iron repletion the receptor mechanism remains partly unsaturated, allowing small amounts of dietary iron to enter the cell. Part of this proceeds into the body to satisfy any metabolic requirement for iron. Part is retained in the mucosal epithelial cells to complete the saturation of the iron-receptor mechanism. This bound iron is subsequently lost when the epithelial cells are sloughed at the end of their life cycle. In iron deficiency it is postulated that the receptor system is inactive or diminished so that entry of dietary iron into the body is relatively uninhibited.

1. Using Fe59 and whole body counting, various aspects of gastrointestinal absorption of iron salts were studied in normal, iron-loaded, and iron-deficient rats. 2. Following a single intragastric dose of iron, peak small intestinal uptake was observed by 30 to 60 minutes with subsequent gradual loss over 24-hour period. 3. Iron absorption showed two phases, an initial period of rapid absorption lasting up to 2 hours during which 60 to 80 per cent of total absorption into carcass took place. The remainder took place at a slow rate over the subsequent 12 to 20 hours. 4. Depending on the relationship between body iron stores and dose of administered iron, the intestine may take up more iron than is ultimately transferred to plasma. A variable amount of this iron is lost when the epithelial cell is sloughed into the gastrointestinal lumen. 5. Regulation of iron absorption appears to involve two steps, mucosal uptake and transfer to the blood.

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