LETTERS TO THE EDITOR
Letters to the Editor

	ABSTRACT

Top Abstract Letter References

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

Bracco, David, Daniel Thiébaud, René L. Chioléro, Michel Landry, Peter Burckhardt, and Yves Schutz. Segmental body composition assessed by bioelectrical impedance analysis and DEXA in humans. J. Appl. Physiol. 81(6): 2580-2587, 1996.The present study assessed the relative contribution of each body segment to whole body fat-free mass (FFM) and impedance and explored the use of segmental bioelectrical impedance analysis to estimate segmental tissue composition. Multiple frequencies of whole body and segmental impedances were measured in 51 normal and overweight women. Segmental tissue composition was independently assessed by dual-energy X-ray absorptiometry. The sum of the segmental impedance values corresponded to the whole body value (100.5 ± 1.9% at 50 kHz). The arms and legs contributed to 47.6 and 43.0%, respectively, of whole body impedance at 50 kHz, whereas they represented only 10.6 and 34.8% of total FFM, as determined by dual-energy X-ray absorptiometry. The trunk averaged 10.0% of total impedance but represented 48.2% of FFM. For each segment, there was an excellent correlation between the specific impedance index (length²/impedance) and FFM (r = 0.55, 0.62, and 0.64 for arm, trunk, and leg, respectively). The specific resistivity was in a similar range for the limbs (159 ± 23 cm for the arm and 193 ± 39 cm for the leg at 50 kHz) but was higher for the trunk (457 ± 71 cm). This study shows the potential interest of segmental body composition by bioelectrical impedance analysis and provides specific segmental body composition equations for use in normal and overweight women.

	LETTER

Top Abstract Letter References

Does Bioelectrical Impedance Analysis Predict Composition Independently of Anthropometry?

To the Editor: The recent report by Bracco et al. (1) evaluated bioelectrical impedance analysis (BIA) and dual-energy X-ray absorptiometry (DEXA) for measuring fat-free mass (FFM) of the total body and its segments in 51 women. As previously pointed out (2, 5, 6), BIA does not measure soft tissue composition or composition changes of and by itself but does so only because resistance, or impedance, is adjusted by a measure of body (or limb) length [FFM = (length²/resistance)]. BIA measures a pathway through the skin between the electrodes, not the current flow through extracellular fluid volume. Bracco et al. (1) showed moderate correlations (r ~ 0.4-0.7) between the BIA and the DEXA measurements of FFM. These correlations are similar in degree to those of FFM and stature or limb length; changes of resistance are poorly correlated (r < 0.1) with changes of fluid content (2). Alteration of hydration in meat samples, or in vivo, does not consistently change resistance by BIA. Kushner et al. (4) reported that total body water correlated highly (r = 0.995) with height²/resistance, but the correlation was almost the same for height² + weight (r = 0.986); the addition of resistance explained only a few percent of the variance. The body mass index (weight/height²) is often an equal or better predictor of fat-lean composition and of body water than is BIA (3, 6, 7, 9). The investigators should provide readers with the appropriate correlations between resistance by BIA and FFM (or length²/FFM) so that the influence of resistance alone can be evaluated. A multiple regression could be determined among resistance, FFM, and length to show the relative contributions of length and resistance.

The authors also indicate that DEXA software assumes that FFM contains 73.2% water. In fact, the attenuation coefficient for lean tissue is influenced to only a small degree by variation in water from 60 to 80% (7). It is because of this that DEXA can be used to measure fat and lean composition, even when fluid concentration is dramatically altered, for example, by dialysis or wasting diseases. Finally, the authors mistakenly cite me as the author of their reference no. 16 (4).

	REFERENCES

Top Abstract Letter References

1. Bracco, D., D. Thiébaud, R. L. Chioléro, M. Landry, P. Burckhardt, and Y. Schutz. Segmental body composition assessed by bioelectrical impedance analysis and DEXA in humans. J. Appl. Physiol. 81: 2580-2587, 1996[Abstract/Free Full Text].
2. Forbes, G. B., W. Simon, and J. M. Amatruda. Is bioimpedance a good predictor of body-composition change? Am. J. Clin. Nutr. 56: 4-6, 1992[Abstract/Free Full Text].
3. Jackson, A. S., M. L. Pollock, J. E. Graves, and M. T. Mahar. Reliability and validity of bioelectrical impedance in determining body composition. J. Appl. Physiol. 64: 529-534, 1988[Medline].
4. Kushner, R. F., D. A. Schoeller, C. R. Fjeld, and L. Danford. Is the impedance index (ht²/R) significant in predicting total body water? Am. J. Clin. Nutr. 58: 589-591, 1992.
5. Mazess, R. B. Bioelectrical impedance: does resistance contribute to composition measurement? Clin. Phys. Physiol. Meas. 12: 178, 1991[Medline].
6. Pennock, B. E. Sensitivity of bioelectrical impedance to detect changes in human body composition (Letter to the Editor). J. Appl. Physiol. 68: 2246-2247, 1990[Medline].
7. Pietrobelli, A., C. Formica, Z. Wang, and S. B. Heymsfield. Dual-energy X-ray absorptiometry body composition model: review of physical concepts. Am. J. Physiol. 271 (Endocrinol. Metab. 34): E941-E951, 1996.
8. Scheltinga, M. R., D. O. Jacobs, T. D. Kimbrough, and D. W. Wilmore. Alterations in body fluid content can be detected by bio-electrical impedance analysis. J. Surg. Res. 50: 461-468, 1991[Abstract/Free Full Text].
9. Svendsen, O. L., J. Haarbo, B. L. Heitmann, A. Gotfredsen, and C. Christiansen. Measurement of body fat in elderly subjects by dual-energy x-ray absorptiometry, bioelectrical impedance, and anthropometry. Am. J. Clin. Nutr. 53: 1117-1123, 1991[Abstract/Free Full Text].