Body iron excretion in man: A collaborative study

Abstract

A collaborative study was undertaken in an attempt to document obligatory iron losses in adult male subjects, using a variety of isotopic and chemical methods. Total body excretion was measured in four groups of subjects by injecting Fe 55 intravenously and following the decline in red cell activity over several years. Calculated daily iron losses were as follows: Seattle white subjects (group I) 0.95 mg. (±0.30); Venezuelan Mestizos (group II) 0.90 mg. (±0.31); Johannesburg Bantu (group III) 2.42 mg. (±1.09); Durban Indians (group IV) 1.02 mg. (±0.22); and Durban Bantu (group V) 2.01 mg. (±0.94). The higher values in the Bantu subjects were ascribed to the greater than normal iron stores in this population group. That losses in the Durban Indian subjects, who were working in an extremely hot and humid environment, were not greater than those in the white subjects suggests that excessive sweating does not represent a major route for iron excretion. The results of isotopic experiments to determine the quantities of iron lost via the gastrointestinal tract suggested a daily loss of approximately 0.1 mg. within desquamated mucosal cells and 0.4 mg. in blood. Chemical analyses of bile indicated a mean daily content of 0.26 mg. However, it was not possible to establish what proportion of this iron is reabsorbed into the body. Direct chemical measurements of iron in urine revealed a mean daily content of approximately 0.1 mg.; this quantity did not seem to be influenced by the size of the body stores. The amount of iron taken up daily from the plasma by eccrine skin at normal transferrin saturations was between 0.2 and 0.3 mg. When the transferrin saturation was high this figure rose to between 0.6 and 0.7 mg. In a final analysis, the calculated iron losses from individual compartments were added together and compared with those obtained in the long-term excretion study. Agreement was close in all but the Bantu groups. Even when maximum figures for individual compartmental losses were used, the figures were still lower than those obtained for total excretion. These discrepancies may reflect methodologic errors but it is equally possible that subjects with overload lose iron in ways other than those examined in the present study, such as bile and/or iron-loaded reticuloendothelial cells shed into the lumen of the gastrointestinal tract.