Effects of graded phosphate deficiency and vitamin D intervention on growth, bone metabolism, and mineralization in a rat model of neonatal-onset metabolic bone disease

To investigate the dose-dependent effects of neonatal-onset phosphate deficiency on bone growth and mineralization and assess whether supraphysiological vitamin D₃ or calcitriol can rescue skeletal defects. Newborn Sprague-Dawley rats were randomly assigned to 7 diets: phosphate-free (0P), low phosphate (1/2P), normal phosphate (NP), calcium-free (0Ca), phosphate/vitamin D-free (0P/D), and 0P/D supplemented with either supraphysiological vitamin D₃ (0P/D + D₃) or calcitriol (0P/D + calcitriol). Longitudinal radiographic assessments were performed before euthanasia at 6-8 wk. Serum analyses measured phosphate (sP), calcium (sCa), phosphotropic hormones, and bone turnover markers. Tibial growth plates were examined by H&E staining, micro-CT, and histomorphometry. The 0P group developed severe hypophosphatemia, rickets-like growth plate widening, osteopenia, and growth retardation. The 1/2P group showed similar hypophosphatemia but no growth impairment and non-significant reductions in bone mass. The 0Ca group exhibited hypocalcemia, secondary hyperparathyroidism, and high bone resorption, yet maintained normal growth and intermediate mineralization. Vitamin D interventions normalized sP but worsened bone loss and growth impairment compared to the 0P group. Biochemically, sP correlated positively with bone formation markers and negatively with fibroblast growth factor-23 (FGF23); vitamin D showed dual effects on bone turnover. Phosphate sufficiency during the early postnatal period is critical for bone mineralization and growth in neonatal rats. Isolated calcium deficiency caused a distinct osteomalacic phenotype with preserved growth. Supraphysiological-dose vitamin D metabolites corrected hypophosphatemia but failed to rescue-and may have exacerbated-skeletal defects, cautioning against vitamin D monotherapy without concurrent phosphate supplementation.