Nitrogen metabolism and bone metabolism markers in healthy adults during 16 weeks of bed rest

Background: The associations between nitrogen metabolism and bone turnover during bed rest are still not completely understood. Methods: We measured nitrogen balance (nitrogen intake minus urinary nitrog en excretion) and biochemical metabolic markers of calcium and bone turnove r in six males before head-down tilt bed rest (baseline), during 2, 10, and 14 weeks of immobilization, and after reambulation. Results: The changes in nitrogen balance were highest between baseline and week 2 (net change, -5.05 +/- 1.30 g/day; 3.6 +/- 0.6 g/day at baseline vs -1.45 +/- 1.3 g/day at week 2; P < 0.05). In parallel, serum intact osteoca lcin (a marker of bone formation) was already reduced and renal calcium and phosphorus excretions were increased at week 2 (P < 0.05). Fasting serum c alcium and phosphorus values and renal excretion of N-telopeptide (a bone r esorption marker) were enhanced at weeks 10 and 14 (P < 0.05-0.001), wherea s serum concentrations of parathyroid hormone, calcitriol, and type I colla gen propeptide (a marker of bone collagen formation) were decreased at week 14 (P < 0.05-0.01). Significant associations were present between changes of serum intact osteocalcin and 24-h calcium excretion (P < 0.001), nitroge n balance and 24-h phosphorus excretion (P < 0.001), nitrogen balance and r enal N-telopeptide excretion (P < 0.05), and between serum osteocalcin and nitrogen balance (P < 0.025). Conclusions: Bone formation decreases rapidly during immobilization in para llel with a higher renal excretion of intestinally absorbed calcium. These changes appear in association with the onset of a negative nitrogen balance , but decreased bone collagen synthesis and enhanced collagen breakdown occ ur after a time lag of several weeks. (C) 2001 American Association for Cli nical Chemistry.