Sex and Iron Modify Fibroblast Growth Factor 23 Concentration in 1-Year-Old Children.

CONTEXT

Fibroblast growth factor 23 (FGF23) plays an important role in phosphate homeostasis, but its regulation is inadequately characterized.

OBJECTIVE

To examine FGF23 regulators, especially sex and iron status, in early childhood.

DESIGN

A cross-sectional study involving 1-year-old children.

SETTING AND PARTICIPANTS

Healthy term infants with a birth weight appropriate for gestational age were recruited to an ongoing vitamin D trial at Kätilöopisto Maternity Hospital, Helsinki, Finland. At 12-month follow-up visits, serum FGF23, 25-hydroxyvitamin D (25OHD), phosphate, ionized calcium, parathyroid hormone, and iron status were measured. All 721 children (51% girls) with complete data were included.

MAIN OUTCOME MEASURES

Intact and C-terminal FGF23 concentrations and iron status at 1 year of age.

RESULTS

Intact FGF23 was greater in girls than in boys [median, 44.4 pg/mL; interquartile range (IQR), 36.8 to 51.9; median, 40.9 pg/mL; IQR, 34.5 to 49.0, respectively; P < 0.001]. C-terminal FGF23 was similar in boys and girls (median, 2.8 pmol/L; IQR, 2.1 to 3.7; median, 2.9 pmol/L; IQR, 2.2 to 3.7, respectively; P = 0.393). The iron concentration was positively associated with intact FGF23 and was the strongest modifier of intact FGF23 (regression coefficient, 0.498; 95% confidence interval, 0.333 to 0.663; P < 0.001) with ferritin, season, ionized calcium, 25OHD, and sex as other covariates. The association between iron and C-terminal FGF23 was inversely related (regression coefficient, -0.072; 95% confidence interval, -0.092 to -0.051; P < 0.001).

CONCLUSIONS

At 1 year of age, FGF23 status was different in girls and boys, with intact FGF23 concentrations higher in girls. Iron modified FGF23 concentrations, with intact FGF23 higher and C-terminal lower, in those with greater iron concentrations.