Bone Abstracts

Preterm infants develop reduced bone mass and fragility fractures. Nevertheless, normal ranges in preterm infants are poorly defined for concentrations of vitamin D; its isomers (C3α-epimer of 25(OH)D3) and metabolites (24, 25(OH)2D3, 1, 24(OH)2D); and other mineral-regulating hormones, including FGF23 in both intact (iFRG23) and inactive (C- terminal) forms.

Objective: To clarify normal concentrations of minerals and hormones in healthy, preterm infants (AGA).

Methods: We conducted a longitudinal, pilot, cohort study on infants born at 28–32 weeks gestation, monitoring blood and urine at 1, 3, and 5 weeks and at term. Analyses included morning plasma and urine calcium, phosphorus, creatinine (Beckman Coulter, USA), whole blood ionized calcium (Radiometer, Denmark), PTH, cFGF23 (C-terminal and intact; Immunotopics, USA), iFGF23 (Kainos, Japan), 1, 25(OH)2D (IDS, UK) and 25(OH)D3 and its C3α-epimer and metabolites by LC–MS/MS (Warnex, Canada). Data are mean±S.D.

Results: Eleven infants (mean GA 30.9±1.5 weeks, wt 1866±21.6 g, male=5) were recruited. At weeks 1, 3, 5, and term, the % achieving 50 nM concentrations of 25(OH)D3 were 40, 60, 100, and 67%, with good intakes of vitamin D. Levels of iFGF23 were mildly elevated (double adult norms) with a marked increase in cFGF23 (ten times adult norms). Nevertheless, tubular resorption of phosphorus (TRP) was normal (88±8%) for those with urine samples. Preterm infants had substantial concentrations of C3α-epimer, much higher than in adults. Calcitriol levels were robust at all ages.

Conclusions: Many infants achieved acceptable 25(OH)D3 concentrations. Despite elevated concentrations of FGF23, there was not excessive phoshaturia.