Urinary Calcium and Phosphorus Excretion in Vitamin D-Sufficient Preterm Infants: Establishing Age-Specific Normative Data.

Abstract

Aim: This study evaluated urinary calcium (Ca) and phosphorus (P) excretion in very low birth weight infants ( < 1500 g, VLBW) without severe morbidity, treated with methylxanthines.

Methods: Eighty-one VLBW infants were analysed. Urinary Ca and P and their creatinine ratios were measured biweekly until day 56. Serum parathyroid hormone (PTH), 25-hydroxyvitamin D [25-(OH)D], total Ca, P and alkaline phosphatase (ALP) were assessed. Bone densitometry was performed before discharge.

Results: In 254 urine samples, urinary Ca and P excretion differed significantly from their creatinine ratios. Vitamin D status influenced urinary mineral excretion, likely via PTH regulation, but did not affect serum Ca, P, or ALP. Normative data were established only for infants with serum 25-(OH)D > 50 nmol/L. Due to age-related variability, a single reference range was not possible.

Conclusions: We provide age-specific normative data for urinary Ca and P excretion in healthy VLBW infants with sufficient vitamin D. Urinary mineral excretion varies significantly with postnatal age, emphasising the need for individualised monitoring to prevent metabolic bone disease of prematurity. Vitamin D status is crucial for calcium-phosphorus homeostasis, with sufficient levels linked to increased Ca excretion and relative P depletion from the 2 month of life.