Antenatal iron supplementation, FGF23, and bone metabolism in Kenyan women and their offspring: secondary analysis of a randomized controlled trial

Abstract

Background: Fibroblast growth factor-23 (FGF23) regulates body phosphate homeostasis primarily by increasing phosphaturia. It also acts as a vitamin D-regulating hormone. Maternal iron deficiency is associated with perturbed expression and/or regulation of FGF23 and hence might be implicated in the pathogenesis of hypophosphatemia-driven rickets in their offspring.

Objectives: We aimed to determine the effect of antenatal oral iron supplementation on FGF23 concentration and maternal and infant markers of bone-mineral regulation.

Methods: We performed a secondary analysis of a trial in which 470 rural Kenyan women with singleton pregnancies and hemoglobin concentrations ≥ 90 g/L were randomly allocated to daily, supervised supplementation with 60 mg elemental iron as ferrous fumarate or placebo from 13-23 weeks of gestation until 1 mo postpartum. As previously reported, iron supplementation improved iron status in mothers and neonates. For the present study, we reanalyzed all available plasma samples collected in mothers and neonates at birth, with primary outcomes being concentrations of FGF23, measured by 2 assays: 1 that detects intact hormone and C-terminal cleavage products (total-FGF23) and another that detects the intact hormone only (intact-FGF23).

Results: Analysis was performed on 433 women (n = 216, iron group; n = 217, placebo group) and 414 neonates (n = 207, iron group; n = 207, placebo group). Antenatal iron supplementation reduced geometric mean total-FGF23 concentrations in mothers and neonates by 62.6% (95% CI: 53.0%, 70.3%) and 15.2% (95% CI: -0.3%, 28.4%, P = 0.06), respectively. In addition, it increased geometric mean neonatal intact-FGF23 concentrations by 21.6% (95% CI: 1.2%, 46.1%), increased geometric mean maternal hepcidin concentrations by 136.4% (95% CI: 86.1%, 200.3%), and decreased mean maternal 25-hydroxyvitamin D concentrations by 6.1 nmol/L (95% CI: -11.0, -1.2 nmol/L).

Conclusions: Analysis of this randomized trial confirms that iron supplementation can reverse elevated FGF23 production caused by iron deficiency in iron-deficient mothers and their neonates. Further investigations are warranted to assess to what extent iron supplementation can prevent FGF23-mediated hypophosphatemic rickets or osteomalacia.

Keywords: Africa; bone; fibroblast growth factor (FGF23); iron deficiency anemia; phosphate; pregnancy; vitamin D.

FIGURE 1

Putative mechanisms whereby deficiencies of iron and calcium lead to hypophosphatemia and rickets. FGF23 is a hormone that is mainly secreted by osteocytes in response to elevated concentrations of 1,25(OH)₂D. FGF23 causes the internalization of sodium phosphate cotransporters in the proximal tubule of the kidney, inhibiting 1-α-hydroxylase [which catalyzes the hydroxylation of 25(OH)D to 1,25(OH)₂D] and stimulating expression of 24-hydroxylase [which initiates the degradation of 1,25(OH)₂D to an inactive form of vitamin D]. The net effect is an increased phosphate loss in the urine, an accompanying reduction in circulating phosphate concentration, and a decrease in 1,25(OH)₂D. Iron deficiency probably stimulates FGF23 gene expression by activating HIF-1 (7, 13), the master transcriptional regulator of cellular and developmental responses to hypoxia. Iron deficiency also increases renal EPO production. EPO mediates the relation between HIF-1 and FGF23 resulting in further increases in FGF23 gene expression (14). In osteocytes, a proportion of FGF23 is normally broken down by proteolytic cleavage before secretion, resulting in both intact, biologically active hormone and C-terminal and N-terminal fragments (with unclear biological activity) in the circulation (15). Some studies in healthy adults have indicated that low serum iron was associated with elevated total-FGF23 but not intact-FGF23, suggesting that cleavage maintains phosphate homeostasis despite increased FGF23 expression (15, 16), whereas others have shown that low serum iron is associated with elevated total- and intact-FGF23 (17, 18). EPO, erythropoietin; FGF23, fibroblast growth factor-23; HIF-1, hypoxia inducible factor 1; intact-FGF23, intact fibroblast growth factor-23; total-FGF23, intact and C-terminal fragments of fibroblast growth factor-23; 1,25(OH)₂D, 1,25-dihydroxyvitamin D; 25(OH)D, 25-hydroxyvitamin D.

FIGURE 2

Flow-through of participants in the study and blood samples available for analysis.

FIGURE 3

Effect of iron supplementation on selected outcomes at delivery, by iron status at baseline. Cumulative frequency distribution is represented by the gray line. The dependent variables are all maternal outcomes (iron, n = 216; placebo, n = 217) except for neonatal intact-FGF23 concentration (iron, n = 200; placebo, n = 205). Iron status is indicated by body iron index, i.e., the ln of the ratio of plasma concentrations of ferritin (µg/L) to plasma concentrations of soluble transferrin receptor (mg/L), both adjusted for plasma concentrations of C-reactive protein, α₁-acid glycoprotein, and Plasmodium infection. P values were obtained by multiple fractional polynomial regression analysis, with adjustment for potentially influential maternal characteristics assessed at randomization, i.e., hemoglobin concentration, BMI, gestational age, parity, HIV infection, and Plasmodium infection. Left panels: associations between outcomes and body iron index for women who received supplementation with iron (blue lines) or placebo (red lines). The difference between these lines is the treatment effect (i.e., the relative difference in outcome between the iron group and the placebo group, with the placebo group used as the reference) conditional to body iron index. Right panels: treatment effect as a function of body iron index, with corresponding 95% confidence bands and P values. Horizontal dashed lines indicate zero effect and the unadjusted effect as measured in a regression model without covariates other than the intervention (Table 2). eGFR, estimated glomerular filtration rate; intact-FGF23, intact fibroblast growth factor-23; total-FGF23, intact and C-terminal fragments of fibroblast growth factor-23.