ALTERED BODY-WATER DISTRIBUTION IN SUBJECTS WITH JUVENILE RHEUMATOID-ARTHRITIS AND ITS EFFECTS ON THE MEASUREMENT OF WATER COMPARTMENTS FROM BIOELECTRIC IMPEDANCE

Objective: To assess the reliability of bioelectric impedance analysis (BIA) for predicting total body water (TBW) and extracellular water ( ECW) in children affected by juvenile rheumatoid arthritis (JRA). Subj ects: Thirty-nine children affected by JRA and 23 healthy children of similar age (11.0 +/- 3.6, range 3.0-19.0 y) were recruited for the st udy. Methods: TBW and ECW were measured by deuterium oxide and bromide dilution, respectively. Bioelectric impedance (Z) was measured at fre quencies of 5, 50 and 100 kHz. The prediction of TBW and ECW from BIA was based on the impedance index (ZI = height(2)/Z, cm(2)/Omega). Resu lts: TBW standardized per kg of body weight and ECW standardized per l itre of TBW were significantly higher in JRA as compared to control pa tients (59.7 +/- 2.4 vs 57.7 +/- 2.7% and 44.5 +/- 4.6 vs 38.1 +/- 7.9 %, with P < 0.005 and P < 0.0001, respectively). Moreover, intracellul ar water standardized per litre of TBW was significantly lower in JRA than in control subjects (55.5 +/- 4.6 vs 62.5 +/- 8.1, with P < 0.000 1). In both controls and patients, the use of ZI at 5 kHz offered the more accurate prediction of ECW. However, the use of ZI at 100 kHz did not offer a better prediction of TBW as compared to its value of 50 k Hz. Control-generated formulae for predicting water compartments from BIA [TBW = 0.716 x ZI at 100kHz-1.504, r = 0.934, s.e.e. = 2.2 l; ECW = 0.430 x ZI(5)-3.652, r = 0.869, s.e.e. = 1.7 l] underestimated TBW a nd ECW in JRA patients. However, population-specific formulae [TBW (1) = 0.766 x ZI at 100 kHz-0.053, r = 0.939, s.e.e. = 2.8 l; ECW (1) = 0 .399 x ZI at 5 kHz-0.283, r = 0.886, s.e.e. 1.7 l] allowed an accurate prediction of TBW and ECW in JRA patients, taking into account their altered body water distribution. Conclusions: Altered water distributi on impedes the use of formulae developed on healthy children to predic t TBW and ECW from BIA and JRA patients. It is hypothesized that chron ic inflammation and subclinical malnutrition may be responsible for th e altered body water distribution of JRA patients. Traditional body co mposition models may require adjustments for use in JRA children due t o their altered body hydration and water distribution. Sponsorship: Th e study was supported by MURST (Ministero Universita Ricerca Scientifi ca e Technologica) '60%' grants. Descriptors: juvenile rheumatoid arth ritis, body composition, total body water, extracellular water, bioele ctric impedance analysis