Blocking of iron absorption by a preliminary oral dose of iron.

Using a small-dose iron tolerance test (ITT) in mildly iron-deficient healthy adults, we have examined the phenomenon of mucosal block. First proposed and demonstrated by Hahn et al with radioiron, "mucosal block" describes diminished avidity for iron of the intestinal mucosa following an orally administered blocking dose. In the ITT, the index of absorption is an increase in plasma iron concentration, rather than retained radioactivity. The absorption of a blocked 10-mg test dose of iron was compared with that of a 10-mg control dose given to the same subject. Both 60- and 30-mg blocking doses of iron resulted in diminished absorption of the test dose. The refractory state of the intestine lasted as long as 24 hours.

The effect of antacids on the absorption of simultaneously ingested iron.

Most discussions of iron therapy include a statement about the ineffectiveness of iron ingested simultaneously with antacids. This study was designed to determine whether or not antacids inhibit iron absorption. A small-dose iron tolerance test was used to compare absorption of iron with and without various antacids. Liquid antacid containing aluminum hydroxide and magnesium hydroxide did not significantly decrease iron absorption. Sodium bicarbonate and calcium carbonate caused the plasma iron increase to be 50% and 67% less than the control values, respectively. However, when calcium carbonate was present in a multivitamin-plus-minerals tablet, the plasma iron change was not significantly different from control trials. Presumably the competitive binding of iron by ascorbic acid in the vitamin pill allowed uninhibited absorption of the iron. Our results suggest that certain antacids may be combined with iron therapy without reducing the efficacy of the iron.

A small-dose iron tolerance test as an indicator of mild iron deficiency.

Small doses of iron (5 to 20 mg) were used to study absorption in normal and mildly iron-deficient men. Plasma iron concentration was measured before and at frequent intervals after ingestion of iron. Men with normal iron stores showed little change in plasma iron levels, while mildly iron-deficient men showed large increases. This consistent observation of increases in plasma iron levels indicates that the iron tolerance test using small doses of iron is a sensitive indicator of iron deficiency. It provides a means of studying iron absorption without the use of radioactive tracers.

Effect of excess dietary zinc on tissue storage of iron in rats.

Earlier investigators observed that addition of large amounts of zinc to the diet or rats can retard growth, lower their hemoglobin levels and reduce storage of iron. In the present studies, addition of 0.75% zinc to a synthetic diet confirmed the reduced storage of iron in the livers and spleens of growing rats, but failed to show an effect on growth rate or hemoglobin levels. The adverse effects of zinc excess on growth and hemoglobin level could, however be reproduced by replacing the Rogers-Harper salt mixture used in the present studies with the Wesson salt mixture use in the earlier studies. Rats fed excess zinc along with the Wesson salt mixture grew less well, had anemia and also had low level in copper in their livers. It is suggested that addition of zinc to the low copper Wesson salt mixture reduced copper absorption sufficiently it deplete liver copper to a level at which mobilization of liver iron stores by a copper-dependent mechanism became impaired, thus depriving red cell production and tissue enzymes of stored iron. The mechanism by which iron stores are depleted by addition of zinc to the better balanced Rogers-Harper salt mixture remains unexplained It is not due to interference by zinc with iron adsorption from the diet nor with cellular uptake of iron from circulating transferrin, and the capacity of tissues to store iron as ferritin is not impaired.