title: Calcium kinetics during bed rest with artificial gravity and exercise countermeasures.
abstract: We assessed the potential for countermeasures to lessen the loss of bone calcium during bed rest. Subjects ingested less calcium during bed rest, and with artificial gravity, they also absorbed less calcium. With exercise, they excreted less calcium. To retain bone during bed rest, calcium intake needs to be maintained.
  
  
  BACKGROUND
  This study aims to assess the potential for artificial gravity (AG) and exercise (EX) to mitigate loss of bone calcium during space flight.
  
  
  METHODS
  We performed two studies: (1) a 21-day bed rest (BR) study with subjects receiving 1 h/day AG (n = 8) or no AG (n = 7) and (2) a 28-day BR study with 1 h/day resistance EX (n = 10) or no EX (n = 3). In both studies, stable isotopes of Ca were administered orally and intravenously, at baseline and after 10 days of BR, and blood, urine, and feces were sampled for up to 14 days post dosing. Tracers were measured using thermal ionization mass spectrometry. Data were analyzed by compartmental modeling.
  
  
  RESULTS
  Less Ca was absorbed during BR, resulting in lower Ca balance in BR+AG (-6.04 ± 3.38 mmol/day, P = 0.023). However, Ca balance did not change with BR+EX, even though absorbed Ca decreased and urinary Ca excretion increased, because endogenous excretion decreased, and there was a trend for increased bone deposition (P = 0.06). Urinary N-telopeptide excretion increased in controls during BR, but not in the EX group. Markers of bone formation were not different between treatment groups for either study. Ca intake decreased during BR (by 5.4 mmol/day in the AG study and 2.8 mmol/day in the EX study), resulting in lower absorbed Ca.
  
  
  CONCLUSIONS
  During BR (or space flight), Ca intake needs to be maintained or even increased with countermeasures such as exercise, to enable maintenance of bone Ca.
authors.affiliation: NASA Lyndon B. Johnson Space Center, Attn: Mail Code SK3, 2101 NASA Parkway, Houston, TX, 77058, USA, scott.m.smith@nasa.gov.
authors.forename: S M
authors.initials: SM
authors.lastname: Smith
authors.forename: C
authors.initials: C
authors.lastname: Castaneda-Sceppa
authors.forename: K O
authors.initials: KO
authors.lastname: O'Brien
authors.forename: S A
authors.initials: SA
authors.lastname: Abrams
authors.forename: P
authors.initials: P
authors.lastname: Gillman
authors.forename: N E
authors.initials: NE
authors.lastname: Brooks
authors.forename: G J
authors.initials: GJ
authors.lastname: Cloutier
authors.forename: M
authors.initials: M
authors.lastname: Heer
authors.forename: S R
authors.initials: SR
authors.lastname: Zwart
authors.forename: M E
authors.initials: ME
authors.lastname: Wastney
mesh_terms: D000328:Adult; D001510:Bed Rest; D015415:Biomarkers / Q000378:metabolism; D001842:Bone and Bones / Q000378:metabolism*; D002118:Calcium / Q000493:pharmacokinetics*; D002136:Calcium, Dietary; D002149:Energy Intake / Q000502:physiology; D015444:Exercise / Q000502:physiology*; D018470:Gravity, Altered; D006801:Humans; D008297:Male; D008954:Models, Biological; D013026:Space Flight
chemical_list: D015415:Biomarkers; D002136:Calcium, Dietary; D002118:Calcium