Adaptation in iron metabolism.

Humans can adapt successfully to a wide range of iron requirements and intakes. The lower limit of the adapted state is best defined by serum ferritin greater than or equal to 12 micrograms/L, because lower values indicate that iron stores are fully depleted. Successful adaptation is achieved by modifying the rate of gastrointestinal absorption according to body iron needs. Several dietary variables, including total intake, content of heme, and bioavailability of nonheme iron, determine the ceiling of the adaptive response when iron demands are high. With lesser demands the nature of the diet has a limited influence on body iron reserves, although there is some evidence that the adaptive response to variations in heme iron intake is less complete than the response to differences in nonheme bioavailability. How the intestinal mucosal cell achieves adaptation is one of the most important unsolved questions in iron metabolism.