title: Nitrogen metabolism and bone metabolism markers in healthy adults during 16 weeks of bed rest.
abstract: 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 nitrogen excretion) and biochemical metabolic markers of calcium and bone turnover 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 osteocalcin (a marker of bone formation) was already reduced and renal calcium and phosphorus excretions were increased at week 2 (P <0.05). Fasting serum calcium and phosphorus values and renal excretion of N-telopeptide (a bone resorption marker) were enhanced at weeks 10 and 14 (P <0.05-0.001), whereas serum concentrations of parathyroid hormone, calcitriol, and type I collagen 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), nitrogen balance and 24-h phosphorus excretion (P <0.001), nitrogen balance and renal N-telopeptide excretion (P <0.05), and between serum osteocalcin and nitrogen balance (P <0.025).
  
  
  CONCLUSIONS
  Bone formation decreases rapidly during immobilization in parallel 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 occur after a time lag of several weeks.
authors.affiliation: Department of Nutrition Science, University of Bonn, Endenicher Allee 11-13, 53115 Bonn, Germany.
authors.forename: K
authors.initials: K
authors.lastname: Scheld
authors.forename: A
authors.initials: A
authors.lastname: Zittermann
authors.forename: M
authors.initials: M
authors.lastname: Heer
authors.forename: B
authors.initials: B
authors.lastname: Herzog
authors.forename: C
authors.initials: C
authors.lastname: Mika
authors.forename: C
authors.initials: C
authors.lastname: Drummer
authors.forename: P
authors.initials: P
authors.lastname: Stehle
mesh_terms: D000328:Adult; D015415:Biomarkers / Q000097:blood / Q000652:urine; D001823:Body Composition; D001835:Body Weight; D001842:Bone and Bones / Q000378:metabolism*; D002118:Calcium / Q000097:blood / Q000652:urine; D004734:Energy Metabolism; D018475:Head-Down Tilt; D006801:Humans; D007103:Immobilization; D008297:Male; D009584:Nitrogen / Q000378:metabolism*; D010758:Phosphorus / Q000097:blood / Q000652:urine; D011506:Proteins / Q000378:metabolism; D013997:Time Factors
chemical_list: D015415:Biomarkers; D011506:Proteins; D010758:Phosphorus; D009584:Nitrogen; D002118:Calcium