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Hydration of fat-free body mass: new physiological modeling approach

2000; American Physiological Society; Volume: 278; Issue: 4 Linguagem: Inglês

10.1152/ajpendo.2000.278.4.e752

1522-1555

W Watson,

Thermoregulation and physiological responses

LETTERS TO THE EDITORHydration of fat-free body mass: new physiological modeling approachWalter WatsonWalter WatsonPublished Online:01 Apr 2000https://doi.org/10.1152/ajpendo.2000.278.4.E752MoreSectionsPDF (38 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat Hydration of fat-free body mass: new physiological modeling approach To the Editor: I read with great interest the recent article on the hydration of fat-free body mass (FFM) (9). The authors are to be congratulated on their attempt to formalize our understanding of the major determinants of FFM hydration in health and disease in humans and across species.One small detail did, however, worry me, and that was the suggestion that the ratio of extracellular to intracellular (E/I) water in humans could be obtained from the measurement of total body potassium (TBK) and total body water (TBW) as followsE/I=(155×TBW−TBK)/(TBK−5×TBW)This formula is appropriate for an intracellular potassium concentration (ICKC) of 155 mmol/l and an extracellular potassium concentration of 5 mmol/l; both these values are acceptable for normal healthy humans. However, it is known that in both humans (3, 5, 7) and animals (2, 4,6) ICKC can vary transiently in response to exercise and, over a longer term, in response to potassium depletion and disease. Therefore, when assessing E/I in nonhealthy subjects, in addition to measuring TBW it would be advisable to measure extracellular water (ECW) by either bioelectrical impedance (1) or bromide dilution (8) to obtain E/I, i.e.E/I=ECW/(TBW−ECW) REFERENCES1 Ellis KJ, Wong WW.Human hydrometry: comparison of multifrequency bioelectrical impedance with 2H2O and bromine dilution.J Appl Physiol85199810561062Link | ISI | Google Scholar2 Fitts RH, Balog EM.Effect of intracellular and extracellular ion changes on E-C coupling and skeletal muscle fatigue.Acta Physiol Scand1561996169181Crossref | PubMed | Google Scholar3 Hill AG, Teo W, Still A, Parry BR, Plank LD, Hill GL.Cellular potassium depletion predisposes to hypokalemia after oral sodium phosphate.Aust NZ J Surg681998856858Crossref | PubMed | Google Scholar4 Kjeldsen K, Norgaard A, Clausen T.Effect of K-depletion on 3H-ouabain binding and Na-K contents in mammalian skeletal muscle.Acta Physiol Scand1221984103117Crossref | PubMed | Google Scholar5 Monk DN, Plank LD, Franch-Arcas G, Finn PJ, Streat SJ, Hill GL.Sequential changes in the metabolic response in critically injured pateints during the first 25 days after blunt trauma.Ann Surg2231996395405Crossref | PubMed | ISI | Google Scholar6 Pichard C, Hoshino E, Allard JP, Charlton MP, Atwood HL, Jeejeebhoy KN.Intracellular potassium and membrane potential in rat muscles during malnutrition and subsequent refeeding.Am J Clin Nutr541991489498Crossref | PubMed | ISI | Google Scholar7 Sjogaard G.Water and electrolyte fluxes during exercise and their relation to muscle fatigue.Acta Physiol Scand556, Suppl.1986129136Google Scholar8 Vaisman N, Pencharz PB, Koren GJohnson JK. Comparison of oral and intravenous administration of sodium bromide for extracellular water measurements.Am J Clin Nutr46198714Crossref | PubMed | ISI | Google Scholar9 Wang Z, Deurenberg P, Wang W, Pietrobelli A, Baumgartner RN, Heymsfield SB.Hydration of fat-free body mass: new physiological modeling approach.Am J Physiol Endocrinol Metab2761999E995E1003Link | ISI | Google ScholarajpendoajpendoAJPENDOAmerican Journal of Physiology-Endocrinology and MetabolismAm J Physiol Endocrinol Metab1522-15550193-1849American Physiological SocietyBethesda, MDajpendoajpendoAJPENDOAmerican Journal of Physiology-Endocrinology and MetabolismAm J Physiol Endocrinol Metab1522-15550193-1849American Physiological SocietyBethesda, MDLETTERS TO THE EDITOR 1420002784E752E753LETTERS TO THE EDITOR ZiMian Wang, and Steven B. Heymsfield 1 Obesity Research Center 2 St. Lukes-Roosevelt Hospital Center 3 New York, New York 100251420002784E752E753Copyright © 2000 the American Physiological Society2000legacy-keton-price5.00The following is the abstract of the article discussed in the subsequent letter: Wang, ZiMian, Paul Deurenberg, Wei Wang, Angelo Pietrobelli, Richard Baumgartner, and Steven B. Heymsfield.Hydration of fat-free body mass: new physiological modeling approach. Am. J. Physiol. 276 (Endocrinol. Metab. 39): E995–E1003, 1999.–Water is an essential component of living organisms, and in adult mammals the fraction of fat-free body mass (FFM) as water is remarkably stable at ∼0.73. The stability of FFM hydration is a cornerstone of the widely used water isotope dilution method of estimating total body fat. At present, the only suggested means of studying FFM hydration is by experimental total body water (TBW) and FFM measurements. Although deviations from the classical hydration constant are recognized, it is unknown if these are explainable physiological aberrations and/or methological errors. Moreover, many questions related to hydration stability prevail, including body mass and age effects. These unresolved questions and the importance of the TBW-fat estimation method led us to develop a cellular level FFM hydration model. This physiological model reveals that four water-related ratios combine to produce the observed TBW-to-FFM ratio over the human life span. An extension of the model to the tissue-organ body composition level confirms on a theoretical basis a small but systematic decrease in hydration observed in mammals ranging in body mass by a factor of 105. The present study, the first to advance a physiological hydration model, provides a conceptual framework for the TBW-fat estimation method and identifies important areas that remain to be studied. REPLY To the Editor: We appreciate the suggestion of Dr. Watson regarding our use of total body potassium and total body water as a means of estimating water distribution (i.e., ratio of extracellular water to intracellular water). We did, indeed, formulate our model based on healthy adults with relatively stable intracellular and extracellular potassium concentrations of 155 mmol/l and 5 mmol/l, respectively. We agree with Watson's good suggestion that these concentrations might be influenced by exercise and disease processes. The suggestion to use bioimpedance analysis or bromide dilution as alternative extracellular fluid markers is a good one, although here, too, there are limitations. Bioimpedance analysis, at least up to now, is also based on assumptions related to stable measurement conditions. Although bromide dilution in our hands is a good extracellular fluid marker, this method is also based on some assumptions, such as 0.95 for Gibbs-Donnan effect correction and 0.90 for intracellular space correction for healthy subjects (1-1). It is unclear whether these correction values are stable in response to exercise and disease; hence, we need to continue to refine and improve our fluid distribution methods in accord with Watson's good suggestions.REFERENCES1-1. Schoeller DA.Hydrometry.Human Body Composition, Roche AF, Heymsfield SB, Lohman TG.19962544Human KineticsChampaign, IL Google Scholar Download PDF Previous Back to Top Next FiguresReferencesRelatedInformation More from this issue > Volume 278Issue 4April 2000Pages E752-E753 Copyright & PermissionsCopyright © 2000 the American Physiological Societyhttps://doi.org/10.1152/ajpendo.2000.278.4.E752PubMed10798881History Published online 1 April 2000 Published in print 1 April 2000 Metrics