European Geriatric Medicine
https://doi.org/10.1007/s41999-022-00652-2

RESEARCH PAPER

Extracellular water‑to‑total body water ratio is an essential
confounding factor in bioelectrical impedance analysis for sarcopenia
diagnosis in women
Akemi Hioka1 · Naoki Akazawa1 · Naomi Okawa2 · Shinji Nagahiro2
Received: 5 February 2022 / Accepted: 15 April 2022
© The Author(s), under exclusive licence to European Geriatric Medicine Society 2022

Key summary points
Aim To examine the relationships of extracellular water-to-total body water ratio (ECW/TBW) with handgrip strength, gait
speed, and skeletal muscle mass index (SMI) in older women.
Findings This study reveals an association between SMI and handgrip strength in community-dwelling older women with
ECW/TBW values less than 0.40, although there was no such association in women with ECW/TBW values of 0.40 or higher.
Message The ECW/TBW ratio represents a confounding factor, which should be considered when using bioelectrical impedance analysis for sarcopenia diagnosis.
Abstract
Purpose The extracellular water-to-total body water ratio (ECW/TBW) is used in bioelectrical impedance analysis (BIA)
for measuring muscle mass; however, ECW/TBW may be affected by several factors common in older individuals. Here,
we assessed the relationships of ECW/TBW with handgrip strength, gait speed, and skeletal muscle mass index (SMI) in
older women.
Methods In this cross-sectional study, 73 community-dwelling women aged ≥ 65 years who could independently perform
activities of daily living were included. ECW/TBW was measured using direct segmental multifrequency BIA. The participants were divided into ECW/TBW < 0.40 (n = 54) and ECW/TBW ≥ 0.40 (n = 19) groups, with the latter indicating
overhydration. SMI was calculated as appendicular skeletal muscle mass/(height)2 (kg/m2). The relationships of SMI withhandgrip strength and gait speed were assessed using partial correlation coefficients. Age, number of medications, pain, andmedical history were treated as control variables.
Results The average age of participants was 77.6 ± 6.0 years. The SMI was significantly related to handgrip strength (partial
correlation coefficient = 0.293, P = 0.016) but not to gait speed (partial correlation coefficient = − 0.183, P = 0.138). Similarly,
the SMI of the ECW/TBW < 0.40 group was significantly related to handgrip strength (partial correlation coefficient = 0.372,
P = 0.009) but not gait speed (partial correlation coefficient = − 0.267, P = 0.066); however, the SMI of the ECW/TBW ≥ 0.40
group was not related to either variable.
Conclusion ECW/TBW represents a confounding factor, which should be considered when using BIA for sarcopenia
diagnosis.
Keywords Sarcopenia · Bioelectrical impedance analysis · Handgrip strength · Gait speed · Skeletal muscle mass index

Introduction* Naoki Akazawa
akazawa@tks.bunri-u.ac.jp
1

Department of Physical Therapy, Faculty of Healthand Welfare, Tokushima Bunri University, 180 Hoji,
Yamashiro‑cho, Tokushima City 770‑8514, Japan

2

Yoshinogawa Hospital, Itano‑gun, Japan

Sarcopenia is a condition characterized by progressive lossof skeletal mass and is associated with decreased activities
of daily living and increased mortality; this condition isrecognized as a serious health problem in older individuals[1]. To date, several diagnostic criteria for sarcopenia have

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been proposed. According to the European Working Groupof Sarcopenia in Older People 2 (EWGSOP2) and Asian
Working Group of Sarcopenia (AWGS) 2019, sarcopenia isdefined by a certain decrease in muscle strength or physicalfunction in addition to a decrease in muscle mass [2, 3].
Typically, dual-energy X-ray absorptiometry (DEXA) andbioelectrical impedance analysis (BIA) are used for muscle
mass measurement, which is then used to calculate the skeletal muscle mass index (SMI) [2, 3].
The EWGSOP2 reported the importance of assessing
muscle mass and muscle quality as a diagnostic tool for sarcopenia [2]. In BIA measurements, not only muscle mass
but also intracellular water (ICW) and extracellular water(ECW) can be measured to determine the state of muscle
quality [4]. The skeletal muscles retain large amount ofwater, which accounts for approximately 75% of the muscle
mass [5]. Water in the skeletal muscles is classified as either
ICW or ECW according to its placement within the muscle
cell membranes, and the sum of ICW and ECW constitutes
the total body water (TBW) [6]. The ICW reflects the muscle
cell mass, whereas the ECW represents the sum of interstitial fluid and blood plasma in the extracellular space [6]. The
ECW/TBW ratio is used in clinical settings to predict the
prognosis of patients with heart failure, dialysis, and cirrhosis [7–9], and is also a standard edema index [10]. The presence of edema increases both ICW and ECW, with a higher
increase in latter than in the former, resulting in an increased
ECW/TBW ratio [11]. When excess water accumulation in
the muscle tissue due to edema results in overhydration, themuscle mass value measured using BIA increases (i.e., overestimation of muscle mass) [11]. However, a high ECW/TBW ratio does not necessarily indicate edema. For example, the ICW can decrease owing to a decrease in muscle
cell mass related to aging and nutritional deterioration, also
resulting in high ECW/TBW [12] and decreased muscle
quality.
In a previous study, participants with high ECW/TBW
had lower physical function and more severe sarcopenia
than those with low ECW/TBW [13]. These results wereconsidered to be related to muscle overhydration owing
to edema; thus, the muscle mass value is overestimatedor ICW is decreased owing to decreased muscle cell mass
related to aging and nutritional deterioration. According tothis hypothesis, the relationships between SMI and muscle
strength or physical function may be studied in participants
with a reference ECW/TBW value, although these relationships may not be observed in participants with an ECW/TBW value above the reference value. However, the differences between individuals with an ECW/TBW ratio close
to or above the reference value in the associations between
handgrip strength, gait speed, and SMI are unclear. Clarifying such differences would highlight the importance ofconsidering the ECW/TBW when determining the muscle

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mass value using BIA. This is particularly important wheninvestigating the prevalence of sarcopenia. Thus, the aim of
this study was to investigate the associations between the
ECW/TBW and handgrip strength, gait speed, and SMI incommunity-dwelling older women.

Methods
Study design and participantsThis study adopted a cross-sectional design. We recruited
73 older women (aged ≥ 65 years) who could independently
perform activities of daily living and resided in Yoshinogawa City, Tokushima, Japan. To recruit participants fromamong community-dwelling older women, an advertisement
was placed in the public magazine of the Yoshinogawa City
office. Individuals with a cardiac pacemaker were excluded
as BIA measurements could not be conducted in them. Eachparticipant provided written informed consent prior to participation. The study was approved by the Ethics Committeeof our institution, and was conducted in accordance with the
Declaration of Helsinki of 1996.

Bioelectrical impedance analysis
The ECW/TBW and SMI were measured while participants
were in the standing position using a direct segmental multifrequency bioelectrical impedance analysis (DSM-BIA)(S10, InBody, Tokyo, Japan). The DSM-BIA analysis was
employed using an 8-point tactile electrode system, with
30 impedance measurements taken using 6 frequencies (1,5, 50, 250, 500, and 1000 kHz). The tool is not statisticallycorrected based on age, sex, and race for measuring body
water and skeletal muscle mass and calculates ECW, ECW/
TBW, and ICW using a formula in the software based on themeasured height, weight, and impedance. Next, fat-free mass
was calculated from the body water information, and skeletal muscle mass was calculated from the fat-free mass [14,15]. The water content and muscle mass determined using
DSM-BIA have been compared and verified by deuterium(D2O) dilution and DEXA, and they have been reported tobe highly reliable [16, 17].
Touch-type electrodes were attached between the heel
and the ankle of the participants’ right and left feet and on
their right and left middle fingers and thumbs. The participants held their arms and legs out to avoid contact withany other body segment during the measurement. SMI was
calculated using the appendicular skeletal muscle mass/(height)2 (kg/m2). The reference range of ECW/TBW waspreviously established to be 0.36–0.40 [18–20], and ECW/TBW ≥ 0.40 is considered to indicate overhydration [21].

European Geriatric Medicine

Therefore, the participants were divided into two groups:
ECW/TBW < 0.40 (n = 54) and ECW/TBW ≥ 0.40 (n = 19).

Handgrip strength, gait speed, and othermeasurements
Handgrip strength was measured using a Jamar dynamometer (563213, Patterson Medical, Nottinghamshire, UK). The
measurement position was 90° elbow flexion while sitting[3] and the measurement was performed on the right side.
Handgrip strength was assessed twice, and the maximum
score was recorded. To measure gait speed, we asked theparticipants to walk a distance of 6 m at a comfortable speed[2, 22]. The gait speed (m/s) was calculated based on thetime required to walk the middle 4 m of the route. Other
parameters including age, height, weight, body mass index,
number of medications, pain (presence or absence), andmedical history (presence or absence) were also considered.
The body mass index was calculated as weight/(height)2 (kg/m2). An increased number of medications and presence of
pain are reportedly associated with decreased motor functionin the community-dwelling older population [23–25]. Thus,
we measured the number of medications, pain, and medical
history using a self-reported questionnaire. Medical history
was investigated for the presence of the following conditions: hypertension, diabetes, dyslipidemia, heart disease,cerebrovascular disease, fractures, and osteoarthritis.
According to the recommendations of the AWGS 2019,
sarcopenia was assessed based on muscle mass, muscle
strength, and physical performance. Participants were considered to have sarcopenia if they presented a low handgrip strength (< 18 kg) and/or a low gait speed (< 1.0 m/s)
along with a low SMI (< 5.7 kg/m2) as per the AWGS 2019
criteria.

Statistical analysis
All statistical analyses were conducted using the SPSS software (version 24, IBM SPSS Japan, Tokyo, Japan). Variables were assessed for normality using the Shapiro–Wilk
test. Parametric data are reported as mean ± SD, whereas
non-parametric data are expressed as median (25th–75th
percentile).
The characteristics of both groups—ECW/TBW < 0.40
and ECW/TBW ≥ 0.40—were compared using Student’s
t-test, Mann−Whitney U test, and Chi-square test as appropriate. The relationship between SMI and handgrip strength
or gait speed was assessed in the participants of both groups
using partial correlation coefficients. Age, number of medications, pain, and medical history were considered as control
variables. Statistical significance was set at P < 0.05.

Results
Table 1 shows the characteristics of all participants andthe results of comparisons between characteristics of thetwo groups (ECW/TBW < 0.40 and ECW/TBW ≥ 0.40).
The mean ± SD of ECW/TBW of all participants, ECW/TBW < 0.40 group, and ECW/TBW ≥ 0.40 group were0.395 ± 0.006, 0.392 ± 0.004, and 0.404 ± 0.003, respectively. Age, rate of medical history of hypertension,
prevalence of sarcopenia, and ECW/TBW of the ECW/TBW ≥ 0.40 group were significantly higher than those of
the ECW/TBW < 0.40 group. The ICW, handgrip strength,
and gait speed of the ECW/TBW ≥ 0.40 group were significantly lower than those of the ECW/TBW < 0.40 group.
The results of the partial correlation analyses are shownin Table 2. The SMI of all participants was significantly
related to handgrip strength but not gait speed. The SMIof the ECW/TBW < 0.40 group was significantly related
to handgrip strength but not to gait speed. However, the
SMI of the ECW/TBW ≥ 0.40 group was not significantly
related to handgrip strength and gait speed.

Discussion
Our results indicate that the SMI was independently associated with handgrip strength in community-dwelling older
women with an ECW/TBW less than 0.40. Conversely, nosuch association was observed in women with an ECW/TBW of 0.40 or higher. The ECW/TBW cutoff value in thepresence or absence of overhydration is considered as 0.40
(normal reference range: 0.36–0.40) [18–21], which was
adopted in our study. A previous study reported that the
ECW/TBW cutoff value of 0.391 can be used for detecting locomotive syndrome [26]. Park et al. [13] reported
that people with low muscle strength and muscle mass
have a higher ECW/TBW value (i.e., 0.391 or higher) thanthat of healthier individuals. Although the cutoff value of
ECW/TBW in this study was different from that used in
the study by Park et al. [13], the results of both the studies
are comparable.
In previous studies [16, 17], high reliabilities wereobserved between values of the body water and muscle
mass measured with the DSM-BIA analysis and deuterium(D2O) dilution and DEXA. In addition, the DSM-BIA
device used in this study calculates body water and muscle mass based on height, weight, and impedance without
statistical corrections for age, sex, and race. Therefore,
body water and muscle mass can be accurately assessedwith DSM-BIA in older, obese, or athletic people [14, 15].
Taken together, values of body water and muscle mass

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European Geriatric Medicine
Table 1  Characteristics of all participants and comparisons between the ECW/TBW less than 0.40 group and ECW/TBW 0.40 or higher groupAge (years)
Height (cm)Weight (kg)BMIa (kg/m2)
Number of medications
Pain
Medical history
Hypertension
Diabetes
Dyslipidemia
Heart disease
Cerebrovascular disease
Fracture
Osteoarthritis
Prevalence of sarcopenia­SMIb (kg/m2)­ECWc (L)
­ICWd (L)ECW/TBWe
Handgrip strength (kg)
Gait speed (m/s)

All participants

ECW/TBW less than 0.40 group

P value(n = 54)

ECW/TBW 0.40 or higher
group(n = 19)(n = 73)
77.6 ± 6.0
150.0 (145.9–152.3)
51.2 ± 8.0
23.0 ± 3.1
2.0 (1.0–4.0)i29 (40.3)i

76.1 ± 5.5
150.8 (146.9–152.5)
51.7 ± 7.1
23.1 ± 2.8
2.0 (1.0–4.0)
20 (37.7)j

81.8 ± 5.3
147.2 (145.2–151.9)
49.5 ± 10.1
22.6 ± 3.7
3.0 (1.0–4.0)
9 (47.3)< 0.001f
0.113g0.302f0.594f0.604g0.463h28 (38.4)
4 (5.5)12 (16.4)
3 (4.1)6 (8.2)9 (12.3)
18 (24.7)
16 (21.9)
5.8 ± 0.6
9.9 ± 1.0
15.2 ± 1.7
0.395 ± 0.006
20.3 ± 5.0
1.2 ± 0.3

17 (31.5)
3 (5.6)
11 (20.4)
2 (3.7)4 (7.4)7 (13.0)
14 (25.9)
8 (14.8)
5.9 ± 0.5
10.0 ± 0.9
15.4 ± 1.5
0.392 ± 0.004
21.3 ± 5.0
1.2 ± 0.3

11 (57.9)
1 (5.3)1 (5.3)1 (5.3)2 (10.5)
2 (10.5)
4 (21.1)
8 (42.1)
5.7 ± 0.9
9.8 ± 1.3
14.5 ± 2.0
0.404 ± 0.003
17.6 ± 3.8
1.0 ± 0.3

0.042h
0.724h
0.118h0.601h
0.495h
0.570h
0.465h
0.019h
0.343f0.639f0.034f< 0.001f
0.004f0.002f

Data are presented as mean ± standard deviation, median (25th–75th percentile), or n (%)
a

b
c
d
e
f

BMI, Body mass index
SMI, Skeletal muscle mass index
ECW, Extracellular water
ICW, Intracellular water
ECW/TBW, Extracellular water-to-total body water ratio

Student’s t testgMann–Whitney U test

h 2
i
j

χ testn = 72n = 53measured with the DSM-BIA tool used in this study arevalid, and reveal the influence of ECW/TBW on the associations between muscle strength and muscle mass.
We found a different relationship between handgrip
strength and SMI between the ECW/TBW < 0.40 and ECW/TBW ≥ 0.40 groups. Thus, our study outcomes suggest that a
relationship between SMI and handgrip strength exists only
when the ECW/TBW is less than 0.40. We speculate thatthese different relationships may be attributable to overhydration of the muscle or a decrease in muscle cell mass due
to aging and nutritional deterioration. ECW/TBW has notbeen fully considered in previous studies wherein muscle
mass was measured using BIA to investigate the prevalenceof sarcopenia [1, 27]. Therefore, sarcopenia prevalence

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reported in previous studies may have been calculated with
the inclusion of people with an ECW/TBW value of 0.40 or
higher [1, 27]. The results of this study provide evidence that
ECW/TBW must be treated as a confounding factor whenusing BIA for sarcopenia diagnosis to avoid the potential
influence of overhydration.
In this study, the majority of participants were in the
ECW/TBW < 0.40 group (54/73, 74%); thus, only 26% of
the total participants had an ECW/TBW of 0.40 or higher.
Although there are limits to generalizations, other populations may show similar results. We consider that paying
attention to ECW/TBW (i.e., whether or not the ratio is 0.40
or higher) is essential when diagnosing sarcopenia using
BIA. Moreover, attention to this aspect may be crucial inEuropean Geriatric Medicine
Table 2  Relationships between SMI and handgrip strength or gait
speed in all participants and ECW/TBW less than 0.40 and ECW/TBW 0.40 or higher groups
SMIa
Partial correla- P value
tion coefficient
All participants (n = 73)
Handgrip strength
Gait speed
ECW/TBWb less than 0.40 group (n = 54)
Handgrip strength
Gait speed
ECW/TBWb 0.40 or higher group (n = 19)
Handgrip strength
Gait speed
a
b
c

0.293
− 0.183

0.016c0.138c0.372− 0.267

0.009c0.066c

0.194
0.040

c

0.487
0.886c

SMI, Skeletal muscle mass index
ECW/TBW, Extracellular water-to-total body water ratio
Adjusted for age, number of medications, pain, and medical history

clinical settings because many patients in the at-risk population for sarcopenia have edema [9, 18–21].
Despite presenting some credible evidence, this study has
four main limitations. First, because of the cross-sectional
study design, the longitudinal associations between ECW/
TBW, handgrip strength, and gait speed are not clear; thus,
additional research is required to clarify the causal associations between these factors. Second, the participants in thisstudy were community-dwelling older women. Therefore,
whether similar associations exist in community-dwellingolder men remains unclear. Third, the prevalence rate ofsarcopenia in all the participants in this study was 21.9%.
This prevalence rate was higher than that in previous studies. In recent studies, the prevalence rate of sarcopenia wasreported to be influenced by aging as well as by the presenceof pain [28, 29]. The mean age of the participants in thisstudy was 77.6 ± 6.0 years, and 40.3% of all the participantspresented with pain. Taken together, the background forthe high prevalence rate of sarcopenia in this study may be
attributable to higher age and higher rate of presence of painin the participants. Further studies examining the influenceof ECW/TBW on the associations between muscle strengthand muscle mass, based on the participants categorized forage and nutrition and pain status, will be needed. Finally,
although this study targeted the participants who could independently perform activities of daily living, those who had
symptoms, such as hypertension, dyslipidemia, and osteoarthritis, were also included. These participants may have
had attended a hospital for the management of symptoms.
However, we were not able to examine whether the participants had gone to the hospital and the number of visits tothe hospital. In addition, we did not examine information

on heart failure, renal failure, liver cirrhosis, and the use ofdiuretics, which could increase ECW/TBW. In future studies, we would examine these factors.
This study revealed an association between SMI andhandgrip strength in community-dwelling older women with
ECW/TBW values less than 0.40, although there was nosuch association in women with ECW/TBW values of 0.40
or higher. Thus, when using BIA for sarcopenia diagnosis,
ECW/TBW must be considered as a confounding factor.
Acknowledgements We thank the participants and the Yoshinogawa
City office for assistance with participant recruitment.
Funding This research did not receive any specific grant from funding
agencies in the public, commercial, or not-for-profit sectors.

Declarations
Conflict of interest The authors declare that there is no conflict of interest.
Ethical approval The study was approved by the Ethics Committee of
our institution, and was conducted in accordance with the Declarationof Helsinki of 1996.
Informed consent Each participant provided written informed consent
prior to participation.

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