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.