Changes in fat-free mass during weight loss measured by bioelectrical impedance and by densitometry

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Am JC/in Nutr l989;49:33-6. Printed in USA. © 1989 American Society for Clinical Nutrition 33
Changes in fat-free mass during weight loss measured by
bioelectrical impedance and by den ry’2
Paul Deurenberg, Jan A Weststrate, and Joseph GAJHaut vast
ABSTRACT A group of 13 apparently healthy, premenopausal obese women (134-196%
ideal weight) volunteered in a weight reduction study for 8 wk on a 4200 Id (1000 kcal) diet.
Before and after the weight reduction period body composition was measured by densitometry
and by the bioelectrical impedance method. Changes in fat mass and fat-free mass were calcu-
lated. Mean weight loss was 10.0 ± 2.8 kg and loss of fat-free mass was measured to be 2.3
± 1.7 kg (23%) by densitometry and 0.6 ± 1.9 kg (6%) by impedance measurements. The
underestimation of the change in fat-free mass measured by the impedance method could be
due to losses ofwater bound to glycogen after the weight-reduction period. For this reason the
impedance method may be not applicable in studies in which changes in glycogen stores can
be expected. Am J Clin Nutr 1989;49:33-6.
KEY WORDS Weight loss, bioelectrical impedance, densitometry, glycogen
Introduction
Obesity is an important health hazard (1-7) and the
prevalence of obesity is high in most Western societies
(8). Body weight increases in most people after adult life
is reached (9, 10). Furthermore it is difficult to maintain
a lower body weight after weight loss and most people
regain the lost weight quickly after a period of successful
slimming (1 1). One of the reasons of this bad prognosis
oflong-term weight loss could be the decrease in fat-free
mass(FFM) during weight loss associated with a decrease
in (resting) metabolic rate, causing lower energy require-
ment after weight loss.
In the past decade a number of new methods for the
assessment of body composition in man have been de-
veloped, eg, neutron-activation analysis (12), computed
tomography and NMR imaging (13), total-body electri-
cal conductivity (TOBEC) (14, 15), and bioelectrical im-
pedance. The latter method is regarded to be a valid
method to measure body composition (16, 17); it is rela-
tively inexpensive and can be applied in larger epidemio-
logical studies.
The aim ofthis investigation was to study the applica-
bility of the bioelectrical impedance method for deter-
mining changes in body composition (ie, in fat mass
[FM] and FFM) during weight loss as compared with the
densitometric method (underwater weighing).
Methods
Population and study design
Thirteen apparently healthy, premenopausal obese women,
134-196% ideal body weight (18), participated in a weight-
reduction study. They were recruited by advertisement in a re-
gional newspaper. After information was obtained on habitual
food consumption and eating behavior from a dietary history
and a questionnaire, a trained dietician prescribed an individu-
ally adapted 4200 Id (1000 kcal) diet (20% protein, 30% fat,
and 50% carbohydrate) for the women. This diet was followed
by the subjects for 8 wk. During this period the dietician con-
tacted the participants by phone weekly and every 2 wk a 24-h
dietary recall was obtained during a home visit.
The protocol ofthe study was approved by the Ethical Com-
mittee ofthe Department ofHuman Nutrition and all women
gave their informed consent. Characteristics ofthe participants
at the start ofthe study are given in Table 1.
Body composition
Body composition measurements were made in duplicate 2
d before and 2 d after the 8-wk weight-reduction period. Mea-
surements were made in the morning at least 4 h after a light
breakfast(l.7 mJ, 400 kcal)after the subject voided and dressed
in a swimming suit. Body weight was measured to the nearest
0.05 kg with a digital scale (ED-60T Berkel, Rotterdam, The
Netherlands). Body height was measured with a microtoise to
the nearest 1 mm. Bioelectrical impedance was measured with
the subject supine as described by Lukaski et al (16, 17) with a
body composition analyzer (RJL-Systems, BIA-lOl, Detroit,
MI). FFM was calculated with the equation
1 From the Department of Human Nutrition, Agricultural Univer-
sity, Wageningen, The Netherlands.
2 Address reprint requests to P Deurenberg, Department of Human
Nutrition, De Dreijen 12, NL-6703 BC Wageningen, The Netherlands.
Received November 25, 1987.
Accepted for publication February 24, 1988.
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TABLE 1
Age, some anthropometric characteristics, and habitual energy intake
ofthe obese women at the start ofthe study*
34 DEURENBERG ET AL
Age(y) 37±5
Body weight (kg) 90.7 ± 10.5
Body height (m) 1 .665 ± 0.064
Body mass index (kg/m2) 32.8 ± 3.6
Body fatt (%) 45.4 ± 3.5
Habitual energy intaket
MJ 9.6±2.8
kcal 2296 ± 670
*1± SD
t By densitometry.
t From a dietary history.
FFM (kg) = 0.698 X l0� X � + 3.55 + 9.4 (1)
where L is body height in meters, R is resistance in ohms, and
S is sex (women 0, men 1). For this formula SEE is 2.67 kg
and r2 is 0.92. This equation was derived from a study in our
laboratory (unpublished observations) in 203 subjects that
compared body composition measured by underwater weigh-
ing and the bioelectrical impedance method.
Body density was determined by underwater weighing (to
the nearest 0.05 kg; 3826 MP 8 1 Sartorius, G#{246}ttingen, FRG)
with simultaneous assessment of the residual lung volume by
helium dilution. Siri’s formula (19) was used to calculate body
fat and FFM from total body density. The equipment has a
precision of 1% (0.002 kg/L).
St atistical analysis
Pearson product-moment correlation coefficients were used
to evaluate linear relationships between variables and paired
(two-tailed) t statistics to assess significance in differences be-
fore and after treatment and between methods (20).
Results
From 24-h dietary recalls energy intake during the 8-
wk of weight reduction was determined to be 3.9 ± 0.7
mJ (933 ± 167 kcal, 1± SD). Body weight reduction was
10.0 ± 2.8 kg. Data on body composition before and af-
ter weight loss are given in Table 2. Mean body weight,
FM, FFM, and body-fat percentage measured by densi-
tometry decreased significantly during the diet period.
However, no significant changes in FFM as measured by
bioelectrical impedance could be observed.
FFM and FM measured by densitometry or bioelectri-
cal impedance did not differ significantly before weight
reduction. After weight reduction, however, the FFM
measured by bioelectrical impedance was higher (p
< 0.05) than the FFM measured by densitometry. In 6
of 1 3 subjects, FFM measured by bioelectrical imped-
ance was increased after weight reduction (Table 2).
Changes in body composition measured by densitom-
etry and impedance are listed in Table 3. The methods
differed significantly in assessing changes in body-fat per-
centage, FFM, and composition ofweight loss.
Discussion
The measurement ofbody composition by densitome-
try is generally advocated as the method of reference. As
reported by several authors (2 1, 22), the precision in
measuring body density is ‘��0.002-0.003 kg/L, corre-
sponding to a body-fat percentage of 1-1.5%. We found
a precision for the estimate ofbody-fat percentage of 0.5-
1 .5%, depending on the familiarity of the subjects with
the procedure (unpublished observations). Despite the
high technical precision ofthe densitometric method, er-
rors can be made in calculating body fat and FFM be-
cause it is not known whether Siri’s constant for the den-
sity ofthe FFM can be applied to all adults. In Siri’s for-
mula densities ofthe FM and the FFM are assumed to be
0.900 kg/L and 1 . 100 kg/L, respectively (1 9). For edema,
pregnancy, or an enlarged muscle mass, the density of
the FFM may not be 1 . 100 kg/L, leading to errors in the
estimation of body-fat percentage (23). In extremely
obese people in whom not only FM but also FFM (mus-
cle mass) are increased, Siri’s formula will slightly overes-
timatebody-fat percentage. Assuming a technical preci-
sion of the densitometric method of 1% of body fat for
subjects not familiar with the equipment, the SD of an
estimate ofthe FFM in our subjects (average FFM 50 kg)
would be 0.5 kg.
The SEE and r2 values for equation 1 are in good
agreement with data from other investigators, who found
SEEs of2.5 1 kg (1 7) and 3.2 kg(24). This implies that the
impedance method is less precise than the densitometric
method and is comparable with the accuracy of normal
anthropometnc measurements, such as skinfold mea-
surements (25). However, a major advantage of the im-
pedance method over the densitometric method is the
possibility ofusing it for field work in larger epidemiolog-
ical studies. Furthermore, the method is relatively inex-
pensive and the equipment is easy to operate and main-
tam. The advantages of the impedance method over
skinfold measurements are the independence of the ob-
server variance and the fact that body composition of
obese people is easily assessed.
In this study a mean weight loss of 10.0 kg was found.
Theoretically 70-80% ofthis weight loss must be due to
a loss of FM (3). From densitometry an average of 78
± 1 5% ofthe weight loss was found to be body fat. How-
ever the impedance method showed a much higher loss
of body fat (94 ± 17%), which seems to be an unrealistic
high percentage. FFM measured by bioelectrical imped-
ance even increased in 6 of 1 3 subjects (Table 2).
From the SD of the mean difference from the FFM
measured by the impedance and densitometric methods,
it is possible to calculate the number ofsubjects required
for detection ofa change in FFM with a given power (20)
with the equation
n = (tfl + ta)2 . 5D�2/(�f’f�4)2 (2)
The number of subjects required to find a 2.0 kg
change in FFM with a power of0.9 (t�312 = 1.36) that is
significant (p < 0.05, two-tailed, ta12 = 2. 1 8) is 12 for the
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BODY IMPEDANCE AND WEIGHT LOSS 35
*1± SD
TABLE 2
Body composition measurements before and after weight loss
Body fat
Fat-free mass by Fat-free mass by Fat mass by Fat mass by percentage by
Body weight densitometry impedance densitometry impedance densitometry
Subjects Before After Before After Before After Before After Before After Before After
kg kg kg kg kg kg
1 88.6 79.3 44.3 42.8 49.5 47.8 44.3 36.5 39.1 31.5 50.0 46.0
2 80.0 69.4 46.2 40.9 46.0 45.2 33.8 28.5 34.0 24.2 42.2 41.0
3 88.3 78.8 47.1 45.7 45.7 47.8 41.2 33.1 42.6 31.0 46.7 42.0
4 85.0 70.8 44.3 43.0 48.4 50.2 40.7 27.8 36.6 20.6 47.9 39.3
5 90.3 80.4 53.2 48.6 52.2 53.0 37.1 31.8 38.1 27.4 41.1 39.0
6 86.4 78.7 49.5 47.6 49.6 48.2 36.9 31.1 36.8 30.5 42.8 39.5
7 84.6 78.6 49.7 49.5 50.6 51.2 34.9 29.1 34.0 27.4 41.3 37.0
8 100.3 91.1 52.4 49.5 55.6 56.6 47.9 41.6 44.7 34.5 47.8 45.7
9 107.3 92.5 58.7 54.3 56.3 53.1 48.6 38.2 51.0 39.4 45.3 41.3
10 78.0 72.4 46.4 46.1 46.6 45.8 31.6 26.3 31.4 26.6 40.5 36.3
1 1 109.6 98.0 53.9 50.5 51.9 50.3 55.7 47.5 57.7 47.7 50.8 48.5
12 79.8 71.0 43.7 43.0 47.1 47.4 36.1 28.0 32.7 23.6 45.3 39.5
13 101.1 88.6 52.2 49.8 55.4 51.1 48.9 38.8 45.7 37.5 48.4 43.8
j: 90.7 80.7* 49.4 470* 50.4 49.8t 41.5 337* 40.3 30.9t 45.4 41.5*
SD 10.5 9.2 4.5 3.9 3.7 3.2 7.2 6.4 7.7 7.4 3.5 3.6
* Significantly different (p < 0.001) compared with the before-treatment value.
t Significantly different (p < 0.005) compared with the after-treatment value ofthe densitometnc method.
impedance method and 10 for the densitometric
method. Thus, the number of subjects who participated
in the study was great enough to detect a change in FFM
of 1 .9 kg, which could be expected to coincide with a
weight loss of ‘�-6.5 kg.
The fact that the mean weight loss of FFM found by
the impedance method is much lower than that found
by the densitometric method (0.6 ± 1 .9 vs 2.3 ± 1 .6 kg
FFM) might be explained by a decrease in glycogen
stores and corresponding water during weight loss. The
current used in the impedance method (800 �A, 50 kHz)
does not totally penetrate the cell membranes (26) and
thus the intracellular water is not fully measured by the
method. The regression equation to calculate FFM by
the impedance method is based upon a population with
normal glycogen and associated water stores. It is evident
that using this equation in a population with exhausted
glycogen stores (like our subjects after weight loss) may
TABLE 3
Changes (�) in body composition before and after weight reduction
measured by densitometry and impedance*
Densitometry Impedance p
Bodyfat(%) 3.9±2.1 6.3±3.1 <0.02
�\FFM(kg) 2.3±1.7 0.6±1.9 <0.02
�FM(kg) 7.6±2.4 9.4±2.9 <0.1
�FM/M:�odywt 78±15 94±17 <0.01
4� FFM/�� body wt 22 ± 15 6 ± 17 <0.01
overestimate FFM. As a consequence the weight loss of
FFM during slimming is underestimated by 1-2 kg.
The influence ofthe loss ofglycogen and attached wa-
ter on the density of the FFM can be expected to be of
minor importance for the calculation of the FM and
FFM with Siri’s formula. The result would be only a
slight underestimation ofthe FM and an overestimation
ofthe FFM.
In conclusion, the bioelectrical impedance method is
not able to assess small changes in FFM especially if these
changes are due to changes in the glycogen and associ-
ated water stores. Losses ofglycogen stores with losses of
associated water during weight reduction may cause an
underestimation of the FFM of �- 1-2 kg depending on
the size of the glycogen stores. Because the size of glyco-
gen stores is quite constant in most Western individuals
(athletes are exceptions), a correction factor could be in-
troduced when the impedance method is used to follow
people during slimming exercise. #{163}3
We are grateful to the subjects who participated with enthusiasm
and to Greet Vansant and Cathaline den Besten for their help in con-
ducting the experiment.
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