The Role of the Intestinal Mucosa in Iron Absorption.

External iron exchange is normally very limited in proportion to the body iron content of between 3 and 4 g (2). In the adult male the total loss from exfoliated cells and secretions is between 0.5 and 1.0 mg daily. These losses are matched by the absorption of iron from food, so that the total iron content of the body tends to remain fixed within relatively narrow limits. Although there is some evidence that losses are influenced by the total body content, such variations in excretion seem to be fixed within fairly narrow limits, so that normal iron balance is primarily dependent on the behavior of the absorbing cells of the upper gastrointestinal tract. Thus the absorption rate tends to rise when the body is depleted of iron and to fall when stores are larger than normal. The manner in which this control is effected is, however, less certain. Originally Granick suggested that the amount of iron absorbed at any one time was influenced by the ferritin content of the absorbing cells (3). However, some doubt was subsequently cast on this theory since ferritin, although demonstrable in gut cells after the feeding of very large doses of iron (3-5), could not be shown to be present after small quantities (6-8). Recently Crosby and co-workers reinvestigated the problem and, on the basis of their findings, proposed that only a proportion of the iron taken up by the gut reaches the plasma (911). The remainder is held within mucosal cells and is subsequently lost into the lumen of the bowel when the cells exfoliate. Although these workers did not positively identify the nature of