1. Iron absorption from 10 mg Fe (as ferrous sulphate), labelled with 1.3 mg 58Fe, was measured in fasting, non-anaemic adult subjects by the faecal-balance technique. The measurement was performed twice, each subject being given, in random order, either 50 mg Fe or a placebo 18 h before the 58Fe-labelled FeSO,.
2.The 50 mg Fe load significantly reduced Fe absorption the following day (P < 0.01), from a mean of 35.4(SEM 4.6)% to 29.0 (SEM 5.1)%. This points to the importance of strict dietary control during Fe-absorption studies to eliminate bias in results.3. In a separate study, the feasibility of using S6Fe-enrichment of erythrocytes, measured by neutron activation analysis (NAA), 10 d after a meal labelled with 0.69 mg 58Fe as an index of Fe absorption was examined. The levels of 5R Fe in the blood were detectable by NAA. Regression analysis showed a significant relation between 5"Fe-enrichment of blood and 58Fe absorption, calculated as the difference between intake and faecal excretion ( R 0.59, P < 0.05).Isotopes are generally regarded as the most accurate and versatile method of studying mineral absorption and metabolism. The radioisotope 59Fe was first used in animal studies in 1939 (Hahn et al. 1939) and in humans in 1941 (Ross & Chapin, 1941). However, the hazards associated with ionizing radiation preclude its use in most human studies. This has led to the development of methods for utilizing the completely safe alternative of stable isotopes which have been successfully employed in a number of experiments investigating Fe availability.The traditional approach to such studies is to use the faecal monitoring technique (Janghorbani & Young, 1980). The test substance is labelled extrinsically with, for example, a small quantity of 58Fe and given to subjects after an overnight fast. The quantity of 58Fe which has been absorbed is calculated as the difference between the dose given and the amount excreted (unabsorbed) in the faeces, after allowing for the naturally occurring isotope present. Since it is presumed that complete isotopic exchange has occurred between the 58Fe label and the endogenous Fe in the test foodstuff, the absorption of 58Fe represents absorption of endogenous Fe from the food.Two points arise from the previous statements. First, it is firmly established in rats that the level of Fe in the diet consumed during the previous 1-3 d profoundly influences Fe absorption from a subsequent test meal (Fairweather-Tait & Wright, 1984; Fairweather-Tait e f al. 1985). If this is also true in man, then rigorous dietary control is required in human studies. Second, an alternative approach to measuring Fe absorption, which obviates the necessity for faecal collection and analysis, with all the accompanying drawbacks, is that of isotopic incorporation into the erythrocytes.The following experiments utilize the stable isotope 58Fe, measured by neutron activation analysis (NAA), to test whether in man previous Fe intake affects subsequent absorption, a3 already demonstrated in the rat (Expt l), and to...
