title: Time course of supine and standing shifts in total body, intracellular and extracellular water for a sample of healthy adults.
abstract: OBJECTIVE
  Traditional tetrapolar bioimpedance spectroscopy (BIS) is performed with the participant supine for 10 min. New vertical analyzers are penetrating clinical, home and fitness markets, but have body water values that differ from supine reference measures. The minimum time standing prior to assessment does not appear in the literature. We investigated the time course of body water shifts in healthy adults undergoing 30-min assessments in supine and vertical positions.
  
  
  METHODS
  While seated, participants were prepped for standard tetrapolar electrode placement. Starting position was counterbalanced and body water measurements were taken every 5 min for 30 min in both positions. Participants sat for 2 min prior to switching positions. Of the 64 participants, three were unable to stand for 30 min; their data were excluded. Body size differences were minimized via computation of relative (%) change between time intervals for total body water (TBW), extracellular water (ECW) and intracellular water (ICW).
  
  
  RESULTS
  ECW and ICW shifted in opposite directions while participants were supine; as ECW decreased at each time point, ICW increased (P<0.0125). Likewise, when participants stood, ECW increased incrementally (P<0.0125), but the decreases in ICW were not significant. At each time interval, the changes in supine ECW and ICW differed from the standing values (P<0.05). No postural or time differences were found for %change TBW.
  
  
  CONCLUSIONS
  For TBW, 5 min appears sufficient for fluid stabilization in either position. Supine ECW and ICW stabilization require more than 30 min as does standing ECW.
authors.affiliation: University of New Mexico Albuquerque, NM, USA.
authors.forename: A L
authors.initials: AL
authors.lastname: Gibson
authors.affiliation: University of New Mexico Albuquerque, NM, USA.
authors.forename: J R
authors.initials: JR
authors.lastname: Beam
authors.affiliation: University of New Mexico/California State University-San Bernadino, San Bernadino, CA, USA.
authors.forename: M K
authors.initials: MK
authors.lastname: Alencar
authors.affiliation: University of New Mexico/Central Michigan University, Mount Pleasant, MI, USA.
authors.forename: M N
authors.initials: MN
authors.lastname: Zuhl
authors.affiliation: University of New Mexico Albuquerque, NM, USA.
authors.forename: C M
authors.initials: CM
authors.lastname: Mermier
mesh_terms: D000328:Adult; D001834:Body Water; D045604:Extracellular Fluid / Q000502:physiology*; D005260:Female; D006801:Humans; D007424:Intracellular Fluid / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D011187:Posture / Q000502:physiology*; D016683:Supine Position / Q000502:physiology; D013997:Time Factors