NDT Advance Access published online on January 21, 2008
Nephrology Dialysis Transplantation, doi:10.1093/ndt/gfm909
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Total body water in health and disease: Have anthropometric equations any meaning?
1 Nephrology and Dialysis Units, Miulli General Hospital, Acquaviva delle Fonti, Italy 2 Nephrology and Dialysis Units, Giannuzzi Hospital, Manduria, Italy 3 Nephrology and Dialysis Units, Landolfi Hospital, Solofra, Italy 4 Nephrology and Dialysis Units, Gambro Health Care, Acquaviva delle Fonti, Italy
Correspondence and offprint requests to: Carlo Basile, Via C. Battisti 192, 74100 Taranto, Italy. Tel: +39-80-3054205; Fax +39-80-762165; E-mail: basile.miulli{at}libero.it
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Abstract |
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Background. The accurate measurement of total body water (TBW) requires isotopic dilution techniques that are not easily applicable to the clinical setting. Therefore, indirect methods of estimating TBW are commonly employed, such as bioelectrical impedance analysis (BIA) and anthropometry. In the human body, >90% of the measured impedance is composed of resistance (R).
Methods. The aim of the present study was to compare TBW estimated by means of two anthropometric equations (by Watson and Hume) with TBW obtained by BIA (equations proposed by Sun et al.) in a group of white disease-free individuals (n = 3625, 1860 men and 1765 women) and white haemodialysis (HD) patients (n = 688, 443 men and 245 women). They underwent one single-frequency BIA measurement, on the nondominant side of the body, injecting an 800-µA and 50-kHz alternating sinusoidal current with a standard tetrapolar technique. The BIA variable measured was R.
Results. Among them, a selection of disease-free individuals (n = 481) and HD patients (n = 270), pair-matched by age, body weight and height, after stratification by gender, was made. When comparing the four pair-matched groups, it was found that (1) TBW was not different (disease-free men versus HD men; disease-free women versus HD women) when using anthropometric equations, which utilize quite identical parameters (age, body weight and height); (2) R was statistically significantly different in the four groups (511 ± 58 SD 
in disease-free men versus 558 ± 80 in HD men, P < 0.0001; 593 ± 70 in disease-free women versus 615 ± 100 in HD women, P < 0.02) and (3) therefore, TBW was statistically significantly different only when applying BIA equations (P < 0.0001 and 0.05, respectively).
Conclusions. The present study demonstrates that anthropometric equations for the estimation of TBW can be used only within a specific population in order to assess individual differences; they cannot be used in order to compare two different populations.
Keywords: anthropometry; bioelectrical impedance analysis; body mass index; haemodialysis; total body water
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Introduction |
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Water is the most abundant compound in the body and an essential regulator in its internal environment. Total body water (TBW) is constantly maintained in normal individuals [1]. Approximately 65% of TBW is intracellular with 35% extracellular water in a 70-kg person. However, TBW is largely altered by disease, especially end-stage renal disease [2]. The accurate measurement of TBW is difficult, requiring isotopic dilution techniques that are not easily applicable to the clinical setting. Therefore, several indirect methods of estimating TBW are commonly employed by researchers and clinicians. Bioelectrical impedance analysis (BIA) is a noninvasive method of body composition analysis. Impedance is represented with a complex number (a point) in the real-imaginary plane (Z vector) that is a combination of resistance (R) (i.e. the opposition to flow of an alternating current through intra- and extracellular ionic solutions, representing the real part of Z) and reactance Xc (i.e. the capacitative component of cell membranes and organelles and tissue interfaces, representing the imaginary part of Z) [3–6]. Thus, R is low for blood, urine and muscle but high for adipose tissue, bone and air, which contain little or no fluid or electrolyte ions [6]. For a constant signal frequency (at 50 kHz), the electrical impedance of a conductor is proportional to the specific impeditivity (
m) multiplied by the length and divided by the cross-sectional area of the conductor [7,8]. Because the current tends to follow the path of least resistance, measured R correlates most strongly with TBW, and correlations decrease for other body composition components, depending on the amount of water in these components. In the human body, <90% of the measured impedance is composed of R. For this reason, most BIA applications use R, rather than impedance, to predict body composition [9,10]. TBW measured by BIA is highly correlated with TBW measured by the isotopic dilution techniques [11]. An alternative method for body composition assessment is anthropometry. The widely used equations by Watson et al. [12] were based on a meta-analysis of previous studies where TBW was estimated using a variety of dilution techniques and markers of water volume in 458 adult males and 265 adult females. The equations by Hume and Weyers were derived using tritiated water as a marker [13]. The aim of the present study was to compare TBW estimated by means of anthropometric equations [12,13] with TBW obtained by BIA equations by Sun et al. [10] in a very large group of white disease-free individuals and in a large group of white haemodialysis (HD) patients. These gender-specific, regression equations have been recently validated (multicomponent model) in a large healthy population (734 men and 1095 women, of whom 1474 whites and 355 blacks aged 12–94 years) with a standard error of estimate of 3.8 (bias 0.5) and 2.6l (bias 0.3) for healthy males and females, respectively [10]. As outlined by the authors, these equations have excellent precision and are recommended for use in epidemiologic studies to describe normal levels of body composition [10]; furthermore, it must be said that they were validated against the gold standard for TBW measurement, i.e. deuterium [10].
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Subjects and methods |
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Design of the study
The study protocol was designed according to the Declaration of Helsinki of 1975, as revised in 2000, and approved by the local ethical committees. Moreover, all the individuals gave their informed consent to the study. The design of the present study included three steps:
- Step 1: enrolment of white disease-free adult individuals and white prevalent HD adult patients of both genders. Body weight was measured to the nearest 0.1 kg and height to the nearest 0.5 cm. Body mass index (BMI) was subsequently calculated as the ratio body weight/height2 (kg/m2). All individuals underwent at least one single-frequency BIA measurement (average of two measurements). It was determined on the nondominant side of the body, injecting an 800-µA and 50-kHz alternating sinusoidal current with a standard tetrapolar technique (BIA 101 Impedance Analyzer; Akern, Florence, Italy). BIA was performed in standardized conditions: a quiet environment, ambient temperature of 22–24°C and after being 20 min at rest in the supine position [14]. Specifically, it was performed 30 min after the end of the dialysis session in HD patients. Actually, BIA variables remain constant and highly reproducible over the 120 min after the end of HD [8]. The BIA variable measured was R.
- Step 2: (A) selection of disease-free individuals and HD patients pair-matched as far as BMI and age are concerned, after stratification by gender; (B) being aware that selection by BMI could not necessarily imply that individuals having a pair-matched BMI necessarily have the same body weight and height, a further selection was done in which disease-free individuals and HD patients were pair-matched according to their age, body weight and height. Pair-matching means, for example in the instance B, that as many disease-free individuals as possible could be selected only if each of the variables (age, body weight and height) ranged from –20 to +20% with respect to the corresponding values of a given HD patient.
- Step 3: calculation of the derived variable TBW by means of the BIA equations proposed by Sun et al. [10] and two anthropometric equations [12,13] in these two specific classes (one pair-matched according to BMI and age—as described in Step 2A and the other pair-matched according to age, body weight and height-–as described in Step 2B).
Equations by Sun et al. [10]:
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Equations by Watson et al. [12]:
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Equations by Hume et al. [13]:
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Study population
The study population consisted of white adult individuals who were subdivided into two different categories:
- Disease-free individuals (n = 3625, 1860 men and 1765 women): they were present in a unique database under the responsibility of one single unit (D.I.B.). It is noteworthy that all the BIA measurements of these individuals were performed by the same operator using the same device.
- Prevalent patients on long-term HD for at least 6 months in a stable state of hydration at the time of assessment without overt oedema (n = 688, 443 men and 245 women): they were treated in two dialysis units (D.I.B. and D.N.). It is noteworthy that all the BIA measurements of these individuals were performed by only two operators (one for each unit).
Statistical analyses
The data distributions were studied by means of the Kolmogorov–Smirnov test. Data are reported as mean (SD) and ranges. The comparisons of the variables between disease-free individuals and HD patients were made by means of Student's t-test for unpaird data. The one-way ANOVA followed by Tukey's post hoc test was used in order to compare the TBW values obtained by means of three different equations both in disease-free individuals and in HD patients. The 
2 test was utilized for the distributions between groups of the categorical variables. All the statistical inferences were made by means of the SPSS statistical package (SPSS Chicago, IL, USA) and an 
value <5% was considered as statistically significant.
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Results |
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- Step 1: enrolment of white disease-free adult individuals and white prevalent HD adult patients of both genders.
Demographic, anthropometric and BIA characteristics of 3625 disease-free individuals and 688 HD patients are described in Table 1. The mean dialysis vintage was 60.4 ± 10.1 months (range 12–110 months). The dialysis treatment consisted of a three times a week 4-h bicarbonate dialysis with a delivered Kt/V dose of at least 1.2 in the last 3 months. When considering the major parameters of nutritional status, 19% of patients had serum albumin concentrations <3.5 g/dl. Systolic arterial pressure ranged from 170 to 110 mmHg and diastolic arterial pressure from 100 to 65 mmHg.
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- Step 2: selection of disease-free individuals and HD patients pair-matched by BMI, age, body weight and height, after stratification by gender.
Demographic, anthropometric and BIA characteristics of 650 disease-free individuals and of 331 HD patients pair-matched by BMI and age are described in Table 2A. When comparing disease-free men with HD men, R, body weight and height were found to be statistically significantly different (P < 0.0001); when comparing disease-free women with HD women, only R was found to be statistically significantly different (P < 0.02) (Table 2A).
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Demographic, anthropometric and BIA characteristics of 481 disease-free individuals and of 270 HD patients pair-matched by age, body weight and height are described in Table 2B. When comparing disease-free men and women with HD men and women, respectively, only R was found to be statistically significantly different (P < 0.0001 and P < 0.02, respectively) (Table 2B).
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- Step 3: calculation of TBW by means of equations by Sun et al. [10] and two anthropometric equations [12,13].
The comparison of TBW calculated by means of equations by Sun et al. [10] and two anthropometric equations [12,13] in the two groups of disease-free individuals (n = 650) and HD patients (n = 331) pair-matched by BMI and age is shown in Table 3A. When comparing disease-free men with HD men, TBW was found to be statistically significantly different with all the three equations (P < 0.0001); when comparing disease-free women with HD women, only TBW calculated by means of equations by Sun et al. [10] was found to be statistically significantly different (P < 0.05) (Table 3A).
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The comparison of TBW calculated by means of three different equations in the two groups of disease-free individuals (n = 481) and HD patients (n = 270) pair-matched by age, body weight and height is shown in Table 3B. When comparing disease-free men and women with HD men and women, respectively, only TBW calculated by means of equations by Sun et al. [10] was found to be statistically significantly different (P < 0.0001 and P < 0.05, respectively). Furthermore, when comparing the 3 equations for each group of individuals, they were found to be statistically significantly different in the groups of disease-free men and HD men (P < 0.0001), whereas there was no difference when performing the same comparison in the groups of disease-free women and HD women (Table 3B).
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The BIA equations unequivocally show that both disease-free women and HD women had a much lower value of the ratio TBW/body weight when compared with the pair-matched disease-free men and HD men, respectively (50.44 versus 57.58% and 49.29 versus 54.80%, P < 0.001 in both cases) (Table 4). Furthermore, HD women had a lower mean value compared with the disease-free women (49.29 versus 50.44%, P < 0.05) and HD men had a much lower mean value compared with the disease-free men (54.80 versus 57.58%, P < 0.0001) (Table 4). Finally, the percent decrease of the ratio TBW/body weight was smaller in HD women than in HD men, when compared with the pair-matched disease-free women and men, respectively (2.28 versus 4.83%, P < 0.0001).
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Discussion |
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The main aim of the present work was not that of validating any given method compared with the isotopic dilution techniques. As far as BIA is concerned, hundreds of excellent validation studies have established a solid relation between whole-body impedance at 50 kHz, through the impedance index height2/R, and body fluid volume through isotope dilution in non-uraemic subjects [10,15–18]. The main aim of the present work was that of comparing some indirect methods of estimating TBW, namely anthropometry versus BIA. Here, we have to underline the crucial role played by a perfect pair-matching (as far as gender, age, body weight and height are concerned) in allowing us to demonstrate an issue that, even though evidently logical, has never been clarified before; two populations, let us say HD patients and disease-free individuals (but the same could be true also for other populations, such as patients affected by cirrhosis, nephrotic syndrome, renal failure and heart failure), when pair-matched as far as age, height, gender and body weight are concerned, have exactly the same TBW if evaluated by means of purely anthropometric formulae. All of us know that this is not absolutely true [2,19–22]. Intuitively, one can easily realize that a biological parameter calculated by means of a formula utilizing variables that are independent of any pathological status (such as for example age, height and body weight in the case of the Watson et al. formula) should not be different in subjects with different clinical situations. Our study gives only a ‘numerical’ sense to this intuition. Thus, the conclusion that can be drawn is that anthropometric equations can be used only within a specific population in order to assess individual differences; they cannot be used in order to compare two different populations. If a difference does exist between the two populations as far as TBW is concerned, it means that the two populations are not pair-matched. Actually, Piccoli et al. have already demonstrated [22] that subjects with the same body weight or BMI, with or without oedema, had the same TBW estimated with a number of anthropometric equations, including equations by Watson et al. and Hume et al. [12,13]. They used vector BIA in order to discriminate among different conditions of hydration [22]. However, it must be said that vector BIA, even though able to discriminate effectively TBW content, allows only an individual qualitative evaluation and not a quantitative one, either individually or in general populations, as is the case for equations by Sun et al. [10].
Furthermore, our study shows that TBW was statistically significantly different only when applying BIA equations. Our choice about the best BIA equations, at least in our and other authors’ [23] opinion, fell on those by Sun et al. [10]. These equations utilize gender, body weight and height as the anthropometric equations do [12,13], with the addition of R. The significantly higher R both in HD men and women, when compared with R of both disease-free male and female individuals, determined a significantly lower TBW both in HD men and women when compared with TBW of both disease-free men and women.
Finally, the present work offers some interesting aspects of discussion in relation to the background so far described; firstly, it shows that BMI, when assumed as a surrogate of anthropometric characteristics of a given individual with the aim of pair-matching two different populations, may be misleading. This example will clarify the issue: the same BMI of 26.66 kg/m2 may be observed in an individual whose body weight is 60 kg and height 150 cm and in an individual whose body weight is 86.4 kg and height 180 cm, which indicates an identical BMI in two individuals with completely different anthropometric variables! When we pair-matched the two groups only by BMI and age, the result was that height and body weight remained statistically significantly different between disease-free men and HD male patients (Table 2A). Consequently, TBW was statistically significantly different when applying all the three equations (Table 3A). When restricting the pair-matching, by including body weight and height besides age (Table 2B), TBW was statistically significantly different when applying only equations by Sun et al. [10] (Table 3B).
In conclusion, the present study demonstrates that anthropometric equations for the estimation of TBW can be used only within a specific population in order to assess individual differences; they cannot be used in order to compare two different populations. Furthermore, our study cannot show how reliable the BIA equations are in estimating TBW in uraemics, because it was not aimed to validate any given method compared with the isotopic dilution techniques.
Conflict of interest statement. None declared.
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Accepted in revised form: 30.11.07
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