During Iron Supplementation, Endogenous Iron Losses Are Increased and Are a Function of the Amount of Iron Absorbed Not of the Dose Given: Stable Iron Isotope Studies

Introduction: Increased iron losses may reduce the effectiveness of iron supplementation. The objectives of our study were (i) to determine if daily oral doses of iron increase iron losses in Kenyan infants, measured using long-term labelling with a stable isotope of iron (⁵⁸Fe), (ii) to detect increases in gastrointestinal bleeding, and (iii) to compare the results in Kenyan infants with iron dose, absorption, and loss from other long-term isotopic studies. Long-term studies of iron absorption and loss require an equilibration period of one year or more to allow the isotopic tracer to uniformly label body iron. Thereafter, an increase in body iron from absorption is proportional to the rate of decrease in the concentration of the isotopic tracer in circulating blood. A loss of endogenous body iron is proportional to the rate of decrease in the amount of the isotopic tracer. Conventional use of biomarkers to evaluate the effect of iron supplementation programs do not separately determine iron absorption and loss and can estimate only net changes in body iron. Single meal or short-term studies using stable iron isotopes do not assess loss and evaluate only acute iron absorption.

Methods: We enrolled 24 iron-depleted Kenyan children, 24-27 months of age, whose body iron had equilibrated with ⁵⁸Fe tracer given at least one year earlier. The toddlers received 6 mg elemental iron/kg body weight (BW) daily for 91 days as an oral ferric ammonium citrate iron syrup. We estimated iron absorption and loss from the shift in stable iron isotopic ratios in blood over the study period. We measured occult blood in fecal samples with an HemoQuant fluorometric assay by determining hemoglobin and porphyrin concentrations. We measured fecal calprotectin to assess enterocyte damage and fecal pH to evaluate gut inflammation. We then examined the relationship of iron dose, absorption, and loss in Kenyan infants with the results from all published investigations using the long-term stable isotope methodology. We compared the 6 mg/kg BW dose in the Kenyan toddlers with doses of 0.8-1.2 mg/kg BW in Gambian toddlers (Br J Haematol. 2021;192:179-89) and women in Benin and Switzerland (Am J Clin Nutr. 2021;113:1657-1669), and with dietary iron alone in Malawian children (Am J Clin Nutr. 2021;114:986-996) and U.S. toddlers (J Nutr. 2005;135:771-777).

Results: In Kenyan infants, after 3 months of daily iron supplementation with 6 mg iron/kg BW, the median (interquartile range) iron absorption was 1.07 (0.98; 1.27) mg/day. During supplementation, 0.61 (0.26, 0.79) mg/day, or 57% of the amount of absorbed iron, was lost (Figure 1). For comparison, WHO estimates basal iron loss for 1-3-year-old children as 0.19 mg/day. The net gain in iron was 0.65 (0.21; 0.68) mg/day, and was associated with an increase in hemoglobin from 10.7 (10.2; 11.2) to 11.5 (11.1; 12.1) g/dL and in plasma ferritin from 12.9 (8.3; 16.1) to 26.2 (20.5; 29.9) μg/L, and a decrease in soluble transferrin receptor from 13.2 (9.6; 15.7) to 7.4 (6.4; 8.6) mg/L (for all, P < 0.001). Supplementation did not increase heme/porphyrin fecal losses or calprotectin and did not decrease fecal pH. The comparison of the iron dose, absorption, and loss in Kenyan infants with those of published studies using the long-term stable isotope methodology (Figure 2) found that daily iron loss was strongly correlated with iron absorption (Pearson r=0.95, p<0.005) but had no significant relationship with iron dose (p=0.24).

Conclusions: The source of the iron losses with supplementation remains to be determined. Because humans have no regulated means to eliminate iron, increased iron excretion cannot account for the greater iron loss observed during supplementation. Our results provide evidence that the amounts of iron lost increase with the amount of iron absorbed rather than the dose of iron given during iron supplementation. In general, limited net iron gains with iron supplementation have been attributed to internal influences, such as inflammatory increases in hepcidin, or to external effects, such as poor adherence or inhibition from dietary phytates, polyphenols or other components. The results of these long-term stable iron isotopic measurements suggest a potential alternative explanation, that increased iron losses during iron supplementation reduce the effectiveness of iron interventions.

No relevant conflicts of interest to declare.