International Journal of Obesity (1998) 22, 537±543
ß 1998 Stockton Press All rights reserved 0307±0565/98 $12.00
http://www.stockton-press.co.uk/ijo
Bioelectrical impedance analysis measurements
of total body water and extracellular water in
healthy elderly subjects
C VacheÂ1, P Rousset1, P Gachon1, AM Gachon2, B Morio1, A Boulier4, J Coudert3, B BeaufreÁre1 and P Ritz1
Laboratoire de 1Nutrition Humaine, 2Biochimie and 3Biologie et de Physiologie du Sport, Centre de Recherche en Nutrition
Humaine-Auvergne, BP 321, 63009 Clermont-Ferrand CEDEX 1 and 4Service des Explorations Fonctionnelles Multidisciplinaires,
HoÃpital X Bichat, 75018 Paris, France
OBJECTIVE: To address whether: (1) bioelectrical impedance analysis (BIA) can provide precise and accurate
estimates of total body water (TBW) and extracellular water (ECW) in healthy elderly subjects, that display ageinduced changes in body composition, (2) BIA models are improved by introducing variables related to geometrical
body-shape and osmolarity.
DESIGN: Cross-validation of available BIA models and models developed in the study.
SUBJECTS: 58 healthy elderly subjects (31 women, 27 men, 66.8 4.7 y, mean s.d.)
MEASUREMENTS: BIA at 5, 50 and 100 kHz, 18O labelled water measurements of TBW, Br measurements of ECW,
anthropometric variables, plasma osmolarity.
RESULTS: Published BIA models for estimating TBW, entail various degrees of bias. Precise models (SEE of the
models 0.8 L at 100 kHz, 1.0 L at 50 kHz) involving height2=resistance, weight, gender, circumferences and plasma
osmolarity were established with data from 30 subjects chosen at random. Cross-validation of an independent group
(n ˆ 28) showed no bias (71.5 3.2 L at 100 kHz, 71.4 3.2 L at 50 kHz, P ˆ NS).
CONCLUSION: We conclude that BIA models with increased accuracy and precision for predicting ECW and TBW can
be derived in healthy elderly subjects. Repeated measures had a mean difference of 0.2 1.2 L.
Keywords: body composition; bromide; fat mass; lean body mass; oxygen-18
Introduction
Despite recent advances in measuring techniques,1 ± 4
there is a need for non-invasive, but reliable and
simple, methods for measuring body composition in
the elderly,5 with knowledge of body water compartments. At its simplest, body composition can be
described with a two-compartment model, whereby
the body is separated into fat-free mass (FFM) and fat
mass (FM). FFM can be estimated from total body
water (TBW) provided its hydration coef®cient is
known.6 Well recognised changes in body composition occur with advancing age.1; 7; 8 FM increases and
FFM decreases, the latter mostly due to muscle loss.1
Although hydration of FFM appears to be similar in
elderly and younger adults,9,10 it might vary between
individuals. Finally, the ratio of extracellular water
(ECW) to TBW can be altered with age.11
Accurate and precise estimates of TBW can be
obtained by isotope dilution of labelled water.12,13
However, it requires access to mass spectrometers
and expertise in measurements of either 2H or 18O.
By contrast, bioelectrical impedance analysis (BIA) is
Correspondence: Patrick Ritz MD, PhD, Laboratoire de Nutrition
Humaine, BP 321, 63009 Clermont-Ferrand CEDEX 1, France.
Received 27 May 1997; revised 7 January 1998; accepted
23 January 1998
a quick, portable and easy way to assess body water
compartments. It merely requires that the subject be
quiet for a few minutes. The cost of the analyser is
moderate and the running cost is virtually zero. BIA
measurements using low current frequencies
( 5 kHz) can be used to estimate ECW, whereas
frequencies of > 50 kHz predict TBW.14,15 However,
BIA measures the body's resistance (R) to an electrical current, and R then needs to be transformed into
TBW or ECW.16 Models relating resistance to TBW
or ECW have been validated using a so-called reference method (such as TBW measured by isotope
dilution, dual X-ray absorptiometry or densitometry)
and are considered to be population-, age- and disease-speci®c.17 ± 22 With reference to TBW in elderly
subjects, only three such models have been published
as full papers.17,20,22 One model17 has come under
controversy and tends to underestimate TBW and
FFM.20,23 Another model20 applies to a very limited
age range. Both models were established from impedance measured at 50 kHz and ECW was not estimated. More recently, a model based on 2H dilution as
the reference method was proposed.22
With reference to ECW (the assessment of which
requires multiple frequency measurements), three
models are available for the 5 kHz frequency,15,22,24
only one of which having been established for elderly
subjects.22
BIA in elderly subjects
C Vache et al
538
None of the models (for TBW and ECW) described
above was cross-validated, that is tested for bias when
applied to an independent group of subjects. Since
impedance models are considered populationspeci®c21 they should be tested against proper reference methods in study populations.
Theoretically, introduction in the models of parameters related to geometrical body-shape and to
speci®c resistivity of body water, should improve
accuracy and precision. This is even more likely to
be true in elderly subjects where these parameters
vary between subjects.
Therefore, the primary aim of this study was to
estimate the validity of currently available models on
a group of healthy elderly subjects, using 18O dilution
as the reference method for TBW since it is the most
accurate probe for body water13 and bromide dilution
for ECW. The systematic biases observed for TBW
led us to establish and cross-validate different models
where variables related to the geometrical body-shape
and speci®c resistivity of body water were included.
Materials and methods
Volunteers
Fifty-eight healthy elderly volunteers (31 women, 27
men) participated in the study. Their physical characteristics are given in Table 1. None was taking any
medication known to in¯uence body composition and
hydration status. Informed written consent was
obtained and the protocol was accepted by the local
medical school ethical committee.
Study protocol
After an overnight fast, volunteers reported to the
laboratory for anthropometric measurements, BIA and
Table 1 Physical characteristics of the volunteers
Women (n ˆ 31)
Age (y)
Height (m)*
Weight (kg)*
BMI (kg/m2)
TBW (L)a
ECW (L)*,b,c
ECW/TBW (%)
FFM (kg)*,d
% Fat*,d
Men (n ˆ 27)
Mean
s.d.
Mean
s.d.
66.6
1.58
64.9
26.2
29.4
12.8
47.5
40.2
37.7
4.5
0.05
8.2
3.5
3.5
4.5
9.3
4.8
7.0
67.0
1.69
75.2
26.4
39.9
17.4
44.6
54.6
27.0
5.1
0.07
10.3
3.3
4.8
2.5
7.5
6.6
6.7
BMI ˆ body mass index; TBW ˆ total body water; ECW ˆ extracellular water; FFM ˆ fat-free mass.
a
TBW was calculated from 18O dilution space. b Data for ECW
were limited to 15 men and 15 women (see method section).
c
ECW was calculated from bromide dilution space. d FFM and
fat mass were calculated from TBW using a 73.2% hydration
coef®cient.
* P < 0.001, men compared to women.
determination of ECW and=or TBW using tracer
techniques.
Anthropometric measurements. Body weight was
measured to the nearest 0.1 kg with a SECA-709
scale (SECA, Les Mureaux, France). Height was
measured to the nearest 0.2 cm with the subjects
standing using a SECA microtoise. Skinfold thickness
was assessed by the same investigator with a Harpenden calliper on a seated and relaxed subject, according
to procedures described by Durnin and Womersley.25
Four skinfold thicknesses (biceps, triceps, subscapular, and supra-iliac) were measured. The mean of three
measurements per site was used in subsequent calculations. Waist circumference was taken as the smallest
circumference between the lower rib margin and the
iliac crest. Hip circumference was measured at the
trochanter level. Biceps circumference was measured
at the midpoint of the arm (halfway between the tip of
the shoulder and the tip of the elbow), with muscles
relaxed. Calf circumference was measured at the
maximum diameter of the calf. Wrist circumference
was measured at the lower end of the radius and ulna
bones. All circumferences were measured to the
nearest 0.1 cm with a plastic tape, with the volunteer
resting in a relaxed position.
Bioelectrical impedance analysis (BIA). BIA measurements were performed with an ANALYCOR-3
analyser (Eugedia, Chambly, France). Volunteers had
been resting in the supine position for at least 15 min
in a temperature controlled room and measurements
were performed with four surface electrodes (Sentry
silver EKG electrodes) placed on clean and degreased
skin at the limb ends. The current-injector electrodes
were located in the lower end of the third metacarpal
bone and of the second metatarsal bone. The currentdetector electrodes were located between the distal
preminences of the right radius and ulna, and between
the two maleoli of the ankle. Three frequencies were
used: 5, 50 and 100 kHz, at a current of 400 mAmp.
Electronic precision of the instrument is better than
1 O, and the response is linear between 100 and
2500 O.
Reproducibility with Sentry electrodes is better
than 2 O. Both resistance and reactance were
recorded. Measurements were performed at 08.00 h
and 8 h later, at the end of the sampling time for the
isotopic determination of TBW and ECW.
TBW and ECW measurements. TBW was measured
with 18O enriched water 26 and ECW was measured
with the bromide dilution technique.27 Brie¯y, after
collection of a baseline plasma sample, accurately
weighed amounts of potassium bromide syrup (®xed
dose of 1 g potassium bromide) and of 2% 18O
enriched water (1 g=kg body weight, Enritech Ltd,
Rehovot, Israel) were taken orally by the volunteers.
BIA in elderly subjects
C Vache et al
Blood was again taken hourly between 4 ± 8 h postdose. Volunteers remained fasted during these 8 h, but
were permitted light activity within the laboratory
(reading, watching television, etc). Plasma samples
were kept at 720 C until analysis. 18O enrichments
were measured with the CO2-H2O equilibration technique12 adapted for use with Vacutainers1 on a
continuous ¯ow gas chromatography-isotope ratio
mass spectrometer (mgas, VG Isotech, UK). Plasma
bromide concentrations were measured by means of
HPLC as described by Miller and Cappon27 using a
diode array detector (Partisil 10 SAX column, Whatman International Ltd, Maidstone, UK). Protein-free
plasma samples were obtained after centrifugation
using a MPS1 micropartition system (Amicon, Epernon, France). Plasma concentrations of sodium, potassium, urea, glucose, chloride, bicarbonate and total
protein were measured on a HITACHI 911 automatic
analyser.
men) were randomly selected to calculate the regression models. TBW (calculated from the 18O dilution
space) was the dependent variable; height2=resistance,
gender (as a dummy variable; 0 for women, 1 for
men), circumferences and plasma ion concentrations
were offered as variables. The remaining 28 subjects
were used for a cross-validation, that is, a comparison
between TBW predicted from the model and TBW
measured from the 18O dilution space. The two groups
(selected for the models and selected for the crossvalidation) did not differ in any physical characteristic. No cross-validation was performed for ECW
since currently available equations for estimating
ECW were accurate (see results section).
Results
Body composition
Calculations and statistical methods
18
O dilution spaces were calculated from increases
between mean plasma enrichments in 18O (4, 5, 6, 7
and 8 h post-dose) and baseline values. TBW was
considered 1% smaller than 18O dilution space to
account for exchange with non-aqueous compounds.13
FFM was calculated from TBW using a 73.2% hydration coef®cient.6 ECW was calculated from mean
concentrations of plasma bromide (4, 5, 6, 7 and 8 h
post-dose), according to Miller and Cappon.27 The
equation that gives ECW is:
ECW ˆ 0:90*0:95*
Br dose
Br plasma
where Br dose is the dose given, Br plasma is the
difference between mean plasma concentration after
the dose and the baseline concentration. Correction
factor 0.95 is for the Donnan equilibrium and 0.90
corrects for the distribution of Br in the nonextracellular sites. The precision of TBW and ECW measurements was estimated in 10 weight-stable subjects
having had body water compartments measured
twice, a week apart. Mean CV was 0.7% for TBW
and 5.4% for ECW (Ritz, unpublished data).
Plasma osmolarity was calculated as:
o…mosm=L† ˆ 2…‰Na‡ Š ‡ ‰K‡ Š† ‡ ‰ureaŠ ‡ ‰glucoseŠ
where [Na‡], [K‡], [urea], and [glucose] are the
corresponding plasma concentrations in mmol=L.
Results are expressed as mean s.d. unless stated
otherwise. Comparisons of means were performed
with a paired Student t-test or ANOVA where applicable. Multiple regression models were calculated with
stepwise forward regressions (F to enter ˆ 4, F to
exit ˆ 3.96). Agreement between measurements
obtained with different methods was assessed with
the technique described by Bland and Altmann.28
Among the 58 volunteers, 30 (15 women and 15
Table 1 displays body composition data for the 58
volunteers. TBW (18O dilution), ECW (bromide dilution) and FFM were signi®cantly lower in women than
in men, while % fat mass was higher in women. Percent
fat mass did not differ whether calculated from skinfold
thicknesses (30.5 7.7%) or from TBW (32.7 7.8%,
P ˆ NS). ECW represented 46.1 8.2% of TBW.
Bioelectrical impedance analysis ± TBW
Published BIA equations derived speci®cally for
elderly subjects are presented in Table 2. It is noteworthy that Deurenberg et al's model17 is designed to
calculate FFM, whereas Svendsen et al's model20
calculates % FM. Calculations of TBW were therefore
performed assuming a 73.2% hydration coef®cient.6
Figure 1 (panels A, B, C) displays the comparison of
TBW measured by 18O dilution with TBW predicted
from the equations mentioned in Table 2, in the 58
volunteers. On average, all models gave signi®cantly
different results. In the present group of elderly
subjects, Deurenberg's model underestimated TBW
by a mean value of 5.0 2.8 L (P < 0.001) while
Svendsen's model overestimated TBW by a mean
value of 6.8 2.7 L (P < 0.001). Visser's model22
slightly overestimated TBW by a mean value of
1.3 2.6 L (P < 0.001).
A model was therefore established from 30 subjects
chosen at random. Height2=resistance (100 kHz), on
its own, explains 95% of the variance in TBW with a
residual s.d. of 1.8 L (Table 3). Other variables
(weight, gender and wrist, mid-arm, hip and waist
circumferences, plus osmolarity) contributed independently to a ®nal model with an R2 of 0.992 and a
residual s.d. of 0.8 L. Coef®cients for these variables
are given in Table 3. Plasma ion concentrations were
in the normal range for all subjects. Height2=resistance (5 kHz), reactance (5 and 100 kHz) and calf
circumference were not signi®cant variables in this
TBW model. This model was applied to the 28
539
BIA in elderly subjects
C Vache et al
540
Table 2 Published bioelectrical impedance analysis (BIA) equations used for
comparison of BIA estimates of total body water (TBW) or extracellular water (ECW)
with measured TBW or ECW
TBWa (L)
Deurenberg et al 17
Visser et al 22 for women
Visser et al 22 for men
Svendsen et al 20 for both genders
ECWa (L)
Deurenberg et al 24
Visser et al 22 for women
Visser et al 22 for men
Segal et al 15
3.9 ‡ 0.671 Ht2/R ‡ 3.1 Gender
11.9 ‡ 0.272 Ht2/R ‡ 0.109 Wt
8.3 ‡ 0.323 Ht2/R ‡ 0.165 Wt
774.23 ‡ 2.14 BMI ‡ 0.04 R ‡ 0.34 SS
73.24ST ‡ 0.17Ht70.14 waist
2.5 ‡ 0.189
1.7 ‡ 0.200
4.8 ‡ 0.225
76.1 ‡ 0.284
Ht2/R ‡ 0.067 Wt 7 0.02 Age
Ht2/R ‡ 0.057 Wt
Ht2/R
Ht2/R ‡ 0.112 Wt
Deurenberg's model 17 is designed to calculate fat-free mass (FFM) while Svendsen's
model 20 is designed to calculate % fat mass. Therefore, TBW was calculated for values
derived in these two models using a 73.2% hydration coef®cient for FFM.
a
Height 2 /resistance (Ht2/R) measured at 50 kHz for TBW and 5 kHz for ECW. Weight (Wt)
in kg and age in y. Gender is a dummy variable (0 for women and 1 for men); BMI ˆ body
mass index; SS ˆ subscapular skinfold; ST ˆ ratio of subscapular to triceps skinfold;
Waist ˆ waist circumference.
Figure 1 Bland and Altman 28 plot of residual values (in L) between measured total body water (TBW) (with 18O) and TBW predicted
(from bioelectrical impedance analysis (BIA) equations speci®c for elderly subjects) vs mean values (in L). Panel A corresponds to
Deurenberg et al 13 (bias ÿ5.0 2.8 L, the correlation coef®cient between the residuals and the mean is 0.20, P ˆ NS). Panel B
corresponds to Svendsen et al 20 (bias ‡6.8 2.7 L, the correlation coef®cient between the residuals and the mean is 0.24, P ˆ NS).
Panel C corresponds to Visser et al 22 (bias ÿ1.3 2.6 L, the correlation coef®cient between the residuals and the mean is 0.40,
P ˆ 0.002). In panels A, B, and C residual values are calculated for the 58 volunteers. Panel D corresponds to the residuals with
predicted values from the present model, in the 28 subjects used for cross-validation. Predicted values were derived from resistance
measured at 50 kHz.
remaining subjects and was free of bias; compared to
measured TBW (18O) the mean difference was
71.5 3.2 L (not signi®cantly different from (0)
with 95% con®dence limits of 70.3 to 72.7 L
(Figure 1, panel D).
When the regression model was established with
resistance measured at 50 kHz, it incorporated the
same variables with an adjusted R2 of 0.984 and a
residual s.d. of 1.0 L. Applied to the 28 remaining
subjects this model is free of bias (71.4 3.2 L).
BIA in elderly subjects
C Vache et al
Table 3 Regression models developed in the group of 30 volunteers to predict total body
water (TBW) from bioelectrical impedance analysis (BIA). Statistical procedures were stepwise
forward multiple regressions (F to enter ˆ 4)
Step
Variable
0
1
2
3
4
5
6
7
8
Intercept
Ht2/R 100
Step 1‡Weight
Step 2‡Gender
Step 3‡Wrist
Step 4‡Waist
Step 5‡Mid-arm
Step 6‡Osmo
Step 7‡Hip
R2
P
Residual s.d.
0
0.950
0.957
0.971
0.979
0.982
0.985
0.990
0.992
< 0.001
< 0.001
< 0.001
< 0.001
< 0.001
< 0.001
< 0.001
< 0.001
7.5
1.8
1.6
1.3
1.2
1.1
1.0
0.87
0.79
Ht ˆ height; R ˆ resistance; Gender is a dummy variable 0 for women and 1 for men;
Osmo ˆ osmolarity; waist, hip, wrist and mid-arm are the corresponding circumferences in
cm. Similar values are obtained with R 50 (measured at 50 kHz).
Equations at step 3:
TBW …L† ˆ 2:896 ‡ 0:366* Ht2 =R 100 ‡ 0:137* Weight ‡ 2:485* Gender
TBW …L† ˆ 3:026 ‡ 0:358* Ht2 =R 50 ‡ 0:149* Weight ‡ 2:924* Gender
Equations at step 8:
TBW …L† ˆ 61:6 ‡ 0:423* Ht2 =R 100 ‡ 0:314* Weight ‡ 2:964* Gender ÿ 0:151*Osmo
ÿ 0:707* Wrist ÿ 0:389* Mid-arm ÿ 0:162* Waist ‡ 0:091* Hip
TBW …L† ˆ 63:3 ‡ 0:432* Ht2 =R 50 ‡ 0:338* Weight ‡ 3:33* Gender ÿ 0:147*Osmo
ÿ 0:923* Wrist ÿ 0:394* Mid-arm ÿ 0:157* Waist ‡ 0:081* Hip
Repeated estimates of TBW (calculated from the
model including circumferences and osmolarity) did
not differ signi®cantly when two BIA measurements
were performed 8 h apart; the mean difference was
70.2 1.2 L (not signi®cantly different from 0; 95%
con®dence limits of 70.6 to ‡ 0.2 L).
function of the speci®c resistivity (r), the length or
height (Ht) and the area (s) of the volume (V ˆ
Ht 6 s) containing the electrolyte solution:
Ht
Rˆr
s
Derivation of this equation leads to:
Bioelectrical impedance analysis ±ECW
Ht2
Eqn 1
R
Therefore, a higher degree of accuracy should be
achieved if height2=resistance is included in the
model, which has not always been the case.20 Accuracy of the model should be improved if measured
TBW (or ECW) is the dependent variable (`V' in Eqn
1). Models derived in the present study and in Visser
et al22 use isotope (2H or 18O) dilution techniques to
measure TBW in groups with very similar age and
body composition characteristics. The present data
show that the Visser et al model22 applied to our
subjects entails a moderate and quite acceptable,
although signi®cant, bias (1.3 L or 3.8%).
Resistivity (`r' in Eqn 1) depends on the ion
concentration of the solution. The volume is not a
cylinder but a series of cylinders (limbs and trunk) the
latter having a different resistivity. Changes in geometrical shape might affect speci®c resistivity. Therefore, those parameters should improve the BIA
models. This is particularly likely to be the case in
elderly subjects, because of the changes in body
composition and in the distribution of fat depots,
and possibly because of variation in osmolarity. The
present study shows that parameters related to geometrical body-shape (that is, wrist, mid-arm, waist
and hip circumferences) and plasma osmolarity,
ECW measured by bromide dilution was compared to
ECW predicted from models (Table 2) using resistance at 5 kHz, designed for adults15,24 or for elderly
subjects.22 No signi®cant difference between these
models and bromide dilution values could be demonstrated in the present group. Mean differences were:
Deurenberg et al's model24, bromide dilution:
0.4 2.4 L; Segal et al's model15, bromide dilution:
70.0 3.0 L; Visser et al's model22, bromide dilution: 0.0 2.5 L.
Discussion
The present study shows that in healthy elderly subjects, body water compartments can be measured precisely by means of BIA. Increased precision is obtained
provided variables of clinical importance and physiological relevance to BIA are added to the model,
that is, geometrical body-shape and osmolarity.
The quality of a BIA model requires that a true
value is obtained for the dependent variable and that
when equations are established, carefully chosen variables are selected. In the case of BIA, the measured
variable is the resistance (R) of the electrolyte solution
to which the electrical current is applied. R is a
Vˆr
541
BIA in elderly subjects
C Vache et al
542
contribute to the variance in TBW independent of
height2=resistance. They improve the precision of the
prediction since with resistance measured at 100 kHz,
residual s.d. drops from 1.4 L to 0.8 L. It is noteworthy
that these extra variables are easy to measure, especially in clinical settings. It merely requires a rubber
tape and the collection of a small blood sample.
18
O labelled water is probably the best tracer to
measure TBW, since analytical measurements of 18O
enrichments in body ¯uids are easy, accurate and precise.12,13 The 18O dilution technique also has advantages
over the 2H dilution technique for physiological reasons.
18
O dilution space only overestimates TBW by 1%
compared to 1±4% for 2H dilution space13 and there is
little fractionation of 18O between water and its
vapour.29 In our laboratory, repeated measurements of
TBW with 18O labelled water has a CV of 0.7% (Ritz,
unpublished data). Furthermore, in the present study, the
plateau in plasma 18O enrichments was used to calculate
TBW to obtain an optimum precision and accuracy.
Theoretical considerations predict that higher current
frequencies improve TBW estimates.14,15,24 The present
study of healthy elderly volunteers shows a slight
advantage of the 100 kHz over the 50 kHz frequency,
since the precision of the estimate is 0.8 L compared to
1.0 L. Therefore, we have provided models for both
frequencies.
Estimates of TBW with the BIA models established
here are therefore precise, reproducible (to 0.2 L)
and accurate as suggested by the absence of bias when
the equations are applied to an independent group.
The ratio of extra cellular to TBW varies with age and
disease.11 It is therefore important to develop `userfriendly' methods to estimate ECW. BIA with low
frequencies (< 5 kHz) can be used to estimate ECW.
The three models for calculation of ECW, with tetrapolar arrangement of surface electrodes, available in the
literature15,22,24 provide accurate estimates of ECW.
Conclusion
BIA can provide easy, precise and accurate estimates
of body water compartments (TBW and ECW) in
healthy elderly subjects. A high degree of precision
(better than 1 L TBW) is achieved with the use of
variables related to the geometrical body-shape and
the speci®c resistivity of the body. In clinical and ®eld
studies, where tracer techniques cannot be used as
routine, BIA could therefore provide useful information. Whether these models apply to diseased patients
or to different populations, remain to be established.
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