title: Serum osmolarity and haematocrit do not modify the association between the impedance index (Ht(2)/Z) and total body water in the very old: the Newcastle 85+ study.
abstract: OBJECTIVE
  Bioelectrical impedance is a non-invasive technique for the assessment of body composition; however, information on its accuracy in the very old (80+years) is limited. We investigated whether the association between the impedance index and total body water (TBW) was modified by hydration status as assessed by haematocrit and serum osmolarity.
  
  
  METHODS
  This was a cross-sectional analysis of baseline data from the Newcastle 85+Cohort Study. Anthropometric measurements [weight, height (Ht)] were taken and body mass index (BMI) calculated. Leg-to-leg bioimpedance was used to measure the impedance value (Z) and to estimate fat mass, fat free mass and TBW. The impedance index (Ht2/Z) was calculated. Blood haematocrit, haemoglobin, glucose, sodium, potassium, urea and creatinine concentrations were measured. Serum osmolarity was calculated using a validated prediction equation.
  
  
  RESULTS
  677 men and women aged 85 years were included. The average BMI of the population was 24.3±4.2kg/m2 and the prevalence of overweight and obesity was 32.6% and 9.5%, respectively. The impedance index was significantly associated with TBW in both men (n=274, r=0.76, p<0.001) and women (n=403, r=0.96, p<0.001); in regression models, the impedance index remained associated with TBW after adjustment for height, weight and gender, and further adjustment for serum osmolarity and haematocrit. The impedance index values increased with BMI and the relationship was not modified by hydration status in women (p=0.69) and only marginally in men (p=0.02).
  
  
  CONCLUSIONS
  The association between the impedance index and TBW was not modified by hydration status, which may support the utilisation of leg-to-leg bioimpedance for the assessment of body composition in the very old.
authors.affiliation: Institute of Cellular Medicine, Newcastle University, Campus for Ageing and Vitality, Newcastle on Tyne NE4 5PL, UK. Electronic address: mario.siervo@ncl.ac.uk.
authors.forename: Mario
authors.initials: M
authors.lastname: Siervo
authors.affiliation: Department of Nutrition, Food and Exercise Sciences, Florida State University, Tallahassee, FL 32311, USA.
authors.forename: Carla
authors.initials: C
authors.lastname: Prado
authors.affiliation: Norwich Medical School, University of East Anglia, Norwich, UK.
authors.forename: Lee
authors.initials: L
authors.lastname: Hooper
authors.affiliation: Institute of Cellular Medicine, Newcastle University, Campus for Ageing and Vitality, Newcastle on Tyne NE4 5PL, UK.
authors.forename: Alex
authors.initials: A
authors.lastname: Munro
authors.affiliation: Institute of Cellular Medicine, Newcastle University, Campus for Ageing and Vitality, Newcastle on Tyne NE4 5PL, UK.
authors.forename: Joanna
authors.initials: J
authors.lastname: Collerton
authors.affiliation: Institute of Cellular Medicine, Newcastle University, Campus for Ageing and Vitality, Newcastle on Tyne NE4 5PL, UK.
authors.forename: Karen
authors.initials: K
authors.lastname: Davies
authors.affiliation: Institute of Cellular Medicine, Newcastle University, Campus for Ageing and Vitality, Newcastle on Tyne NE4 5PL, UK.
authors.forename: Andrew
authors.initials: A
authors.lastname: Kingston
authors.affiliation: Institute of Cellular Medicine, Newcastle University, Campus for Ageing and Vitality, Newcastle on Tyne NE4 5PL, UK.
authors.forename: John C
authors.initials: JC
authors.lastname: Mathers
authors.affiliation: Institute of Cellular Medicine, Newcastle University, Campus for Ageing and Vitality, Newcastle on Tyne NE4 5PL, UK.
authors.forename: Thomas B L
authors.initials: TB
authors.lastname: Kirkwood
authors.affiliation: Institute of Cellular Medicine, Newcastle University, Campus for Ageing and Vitality, Newcastle on Tyne NE4 5PL, UK.
authors.forename: Carol
authors.initials: C
authors.lastname: Jagger
mesh_terms: D000369:Aged, 80 and over; D000375:Aging / Q000502:physiology*; D001823:Body Composition / Q000502:physiology*; D015992:Body Mass Index; D001834:Body Water; D001835:Body Weight; D015331:Cohort Studies; D003430:Cross-Sectional Studies; D017097:Electric Impedance; D005260:Female; D006400:Hematocrit; D006801:Humans; D008297:Male; D009765:Obesity / Q000503:physiopathology*; D009994:Osmolar Concentration
keywords: Bioimpedance; Body composition; Osmolarity; Total body water; Very old