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

The following is the abstract of the article discussed in the subsequent letter:

	ABSTRACT

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.

	LETTER

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 follows
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.

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9.   Wang, Z, Deurenberg P, Wang W, Pietrobelli A, Baumgartner RN, and Heymsfield SB. Hydration of fat-free body mass: new physiological modeling approach. Am J Physiol Endocrinol Metab 276: E995-E1003, 1999[Abstract/Free Full Text].

Walter Watson, Nuclear Medicine Department Southern General NHS Trust Hospital Glasgow G51 4TF, Scotland

	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). 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.

	REFERENCES

1.   Schoeller, DA. Hydrometry. In: Human Body Composition, edited by Roche AF, Heymsfield SB, and Lohman TG.. Champaign, IL: Human Kinetics, 1996, p. 25-44.

ZiMian Wang, Steven B. Heymsfield, Obesity Research Center St. Lukes-Roosevelt Hospital Center New York, New York 10025