SPECIAL COMMUNICATION
Unreliable use of standard muscle hydration value in
obesity
G.
Mingrone1,
A.
Bertuzzi2,
E.
Capristo1,
A. V.
Greco1,
M.
Manco1,
A.
Pietrobelli4,
S.
Salinari3, and
S. B.
Heymsfield4
1 Istituto di Medicina Interna e Geriatria, Università
Cattolica del Sacro Cuore, 00168 Rome; 2 Istituto di Analisi
dei Sistemi ed Informatica del CNR 00185 Rome; 3 Dipartimento
di Informatica e Sistemistica, Università di Roma "La
Sapienza", 00184 Rome, Italy; and 4 Obesity
Research Center, St. Luke's-Roosevelt Hospital, Columbia University
College of Physicians and Surgeons, New York, New York
10025
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ABSTRACT |
Intramuscular water content is assumed to be constant in humans
independent of their anthropometric characteristics. To verify whether
this assumption is correct, intramuscular water, proteins, glycogen,
and both total and intramyocytic triglycerides were measured in 51 samples of rectus abdominis muscle obtained from 16 lean and 35 overweight and obese subjects (body mass index cutoff 24.9 kg/m2). Data (referred to as wet tissue) were analyzed by
means of a composition model at the cellular level of the skeletal
muscle (SM). The average SM water content was 76.3 ± 3.3% in
normal-weight individuals and 65.7 ± 5.8% in obese subjects
(P < 0.0001). Total triglycerides were 5.5 ± 2.3% in controls and 19.0 ± 7.0% in obese subjects
(P < 0.0001). The intramyocytic triglyceride fraction was also increased in obese subjects. The composition model provides an
explanation for the negative correlation between total triglycerides and intramuscular water, and some of the model parameters were determined from the experimental data. In conclusion, although the
hydration of fat-free SM mass may be unchanged in obese subjects, the
hydration of in toto muscle mass decreases as its lipid content increases.
muscle triglycerydes; mathematical model
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INTRODUCTION |
SKELETAL MUSCLE
(SM) accounts for up to 40% of body weight (36) and is a
major determinant of whole body energy expenditure, even at rest,
because of its large mass. Quantification of muscle mass is important
for cross-sectional studies and when normalizing various physiological
parameters such as muscle force production, metabolic rate of oxygen
uptake, blood flow, and protein turnover. Furthermore, investigations
into the causes of skeletal muscle loss with advancing age have
stimulated a renewed interest in the quantification of this important
component of body composition (3, 5, 10, 16).
Although it is well established that myocytes represent the largest
body reservoir of carbohydrates, in the form of glycogen, their role in
lipid storage and metabolism is not completely understood. Skeletal
muscle is thought to be the major site for removal of both
nonesterified fatty acids (NEFA) and triglycerides (TG) from the
circulatory stream, although the exact role and function of the
intramyocytic TG pool have not been thoroughly ascertained. According
to the few studies which have addressed this topic, the TG pool appears
to be in dynamic and rapid equilibrium with substrate utilization and
supply (7, 24). In addition, high levels of either
circulating NEFA (25, 26, 29-32) or TG (14, 27) along with supranormal levels of intramyocytic TG
(TGm) (28) may play a pivotal role in the
pathogenesis of insulin resistance in humans.
Electron micrographs reveal that the storage of TGm in
lipid droplets is less homogeneous than the storage of glycogen
(1, 9, 18, 19, 40). However, attempts to quantify the
TGm content in skeletal muscle biopsies have so far been
technically inadequate due to contamination with adipose tissue lipids
(11, 33, 47). On the other hand, the small specimens
(75-100 mg) of needle biopsy muscle tissue available for testing
make it difficult to perform the analysis of TGm in
duplicate or in triplicate, thus explaining the large coefficients of
variation (20 to 50%) observed (47). Recently, the use of
1H magnetic resonance spectroscopy has been validated to
quantitatively differentiate between adipocyte and intracellular
TGm stores in both animals and humans (38).
However, despite the excellent results obtained, this technique is
still too advanced and expensive to be used for large population studies.
Another related problem is the intramuscular water content, which is
assumed to be constant in humans independent of their anthropometric
characteristics and is derived from the data of the so-called
"reference man" (36). For instance, the ratio of
tissue water to lean tissue mass was assumed to be constant (and = 0.81 in SM) in the analysis of data of 1H magnetic
resonance spectroscopy (38). This point is a relevant one
in the assessment of body composition, because it is on the aforementioned data as well as on animal studies (8, 15) that the mean whole body cellular hydration is assumed to be equal to
0.70 (43). This factor is used in the fat-free body mass hydration model recently proposed (43) as the most
reliable method for the calculation of the ratio of total body water to fat-free mass and also serves as the basis of the dual-energy X-ray
absorptiometry body composition model.
To clarify the hydration level of SM across subjects differing in body
weight, a large sample of SM tissue, taken during abdominal surgical
operations in subjects with body mass index (BMI) ranging between 17.1 and 66.7 kg/m2, was chemically analyzed. An improved
technique was used to avoid contamination of muscle fibers by adipocyte
lipids and employing a large amount of skeletal muscle tissue to
obtain a low within-biopsy variability. Data were analyzed by
means of a composition model at the SM cellular level, and estimates of
model parameters were obtained.
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MATERIALS AND METHODS |
Subjects.
Subjects were 51 nondiabetic patients, 26 males and 25 females, who
underwent abdominal surgery for colecystectomy due to the presence of
cholesterol gallstones (21 patients) or biliopancreatic diversion for
morbid obesity (30 patients). The subjects were divided into a lean
(control) group and a group including overweight and obese subjects,
denoted in the present paper as the obese group. Subjects with BMI
>24.9 kg/m2 were included in the obese group, so some of
the gallstone patients were classified as obese. The average age was
52.2 ± 11.2 yr in lean subjects (n = 16) and
45.1 ± 15.6 yr in obese subjects (n = 35); weight
was 59.9 ± 11.0 vs. 117.7 ± 35.2 kg and BMI was 21.8 ± 2.4 vs. 43.0 ± 11.3 kg/m2 (P < 0.001), respectively, in the two groups. None of the subjects had
abnormalities in renal function or serum electrolytes.
The study was approved by the Institutional Review Board as designated
by the Helsinki Declaration. Patients gave their informed consent
before their enrollment in the present study.
Body composition.
The day preceding the surgical procedure, body weight was measured to
the nearest 0.1 kg with a beam scale and height to the nearest 0.5 cm
with a stadiometer (Holatin, Crosswell, Wales, UK). Subjects were
measured in the morning after an overnight fast, in a hospital gown,
and after emptying of the bladder.
Muscle biopsy.
During surgery, a rectus abdominis muscle biopsy (
6 g) was obtained
after opening of its investing fascia. Each sample was free from any
visible contamination from subcutaneous adipose depot store and was
divided into three parts to determine chemical components. All of the
analyses were performed in triplicate.
Skeletal muscle water and protein analysis.
SM water (~1 g of tissue) was determined by freeze drying with
continuous pumping. The difference between wet weight and dried weight
was used to calculate the water content of the sample examined. The
coefficient of variation for SM water determination on 3 aliquots of
muscle for the same biopsy (within-biopsy variability) was 2.3 ± 0.9% (mean ± SD). The dried samples were stored at
80°C for
no more than 1 mo until the analyses were performed.
To measure the protein content, the dried tissue was suspended in 200 µl of a lysis buffer and homogenized by use of a homogenizer (Ultra-Turrax TP 18-10; Janke and Kunkel, Ikawerk, Breisgau,
Germany). The lysis buffer was composed of a 100-ml solution of
TRIS · HCL (1 M, 2 ml, at pH 7.4), NaCl (5 M, 2 ml),
MgCl2 (1 M, 0.5 ml), and Tween-20 (0.1%, 10 ml). The
homogenate was centrifuged for 20 min at 6,000 rpm at 4°C. Protein
content was determined from the clear supernatant by spectrophotometry
at a wavelength of 565 nm by use of a colorimetric test kit (Bio-Rad
protein assay; Bio-Rad Laboratories, Hercules, CA).
Skeletal muscle TG analysis.
To measure total TG, a skeletal muscle sample of ~500 mg was
homogenized (Ultra-Turrax TP 18-10) in a 2-ml solution of
chloroform-methanol (2:1 vol/vol) acidified with 5-10 mg of
trichloroacetic acid (TCA) to precipitate proteins. Lipids were
extracted twice with 8 volumes, the solution being stirred at 6°C for
30 min.
To measure TGm, another specimen of ~500 mg was taken and
immediately placed into a calcium-free Hanks' solution added with EDTA
and bubbled with 95% O2-5% CO2. The sample
was washed and then immersed in a fresh Hanks' solution added with
collagenase type IV (50 mg) and calcium ions and agitated in a Dubnoff
water bath maintained at 37°C until the tissue appeared soft. At this point the specimen was gently removed, and cells were brushed with a
blunted spatula, filtered, suspended in PBS, and centrifuged twice at
50 g for 2 min. The supernatant was discarded, and the muscle cells were dried under a nitrogen stream. After protein precipitation with 5-10 mg of TCA, lipids were extracted twice with 8 volumes of chloroform-methanol (2:1 vol/vol) with the solutions being stirred at 60°C for 15 min. The combined extracts were dried in
a GyroVap apparatus (GV1, Gio.DeVita, Rome, Italy) operating at 60°C,
coupled with a vacuum pump and a gas trap (FTS System, Stone Ridge,
NY). The dry weight of lipid extracts was obtained by weighing the
sample tube before and after drying the extracts. The low within-biopsy
variability (~5%) suggested that the TGm measurement
technique was reliable and that the between-biopsy variability was not
a result of measurement error.
Skeletal muscle glycogen analysis.
After a 10-min incubation with 0.1 M NaOH at 80°C of the fat-free
material to destroy background hexose monophosphates and glucose,
glycogen was degraded to glucose by amyloglucosidase, and glucose was
measured by a Beckman Glucose Analyzer II (Beckman Instruments,
Fullerton, CA) as described by Bergmeyer (4). Glycogen was
corrected for the highest total creatine content in the skeletal muscle
specimens of a given subject.
Skeletal muscle composition model at the cellular level.
Taking as a reference the model of Wang and Pearson (42)
and the model of the fat-free body mass at the cellular body
composition level developed by Wang et al. (43), we
propose the skeletal muscle model presented in Fig.
1. This model contains three components: the muscle cell mass (MCM), the extracellular space (ECS) containing fluids and the solid matrix, and the adipose tissue mass (ATM). Thus
muscle mass (MM) is equal to the sum of the three components
|
(1)
|
These three components consist of an aqueous and a solid
compartment. Following Wang et al., and denoting by ICW the
intramyocytic water, by ECW the extracellular water, and by E/I the
ratio ECW/ICW, we write MCM = ICW/a and ECS = ECW/b = ICW × (E/I)/b, where a and b are the fractions of water in the
myocytes and in the extracellular space, respectively. Furthermore,
denoting by ATW (adipose tissue water) the water content of adipose
tissue, we may write ATM = ATW/
,
being the fraction of
water in adipose tissue. The total muscle water (TMW) is thus expressed
as
|
(2)
|
and we note that this amount of water was actually measured as
skeletal muscle water in the SM samples examined.

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Fig. 1.
Model of skeletal muscle composition at the cellular
level. MM, muscle mass; MLM, muscle lean mass; MCM, muscle cell mass;
ECS, extracellular space; ATM, adipose tissue mass; ICW, intramyocytic
water; ECW, extracellular water; ATW, adipose tissue water. The
fraction of intramuscular water is represented by the hatched area.
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|
We can now consider the fraction of intramuscular water, TMW/MM, as the
(observed) hydration level of the in toto skeletal muscle mass. This
ratio differs from the hydration of fat-free mass defined by Wang et
al. (43), because it takes into account adipose tissue and the water
associated with this tissue. Following the approach of Wang et al.,
from Eqs. 1 and 2 we have
|
(3)
|
To rewrite Eq. 3 in a more suitable form, we
introduce as a new parameter the ratio of adipose tissue mass to muscle
lean mass (MLM), ATM/MLM, with MLM = MCM + ECS (MLM includes
the TGm). This parameter can be considered an "adiposity
index" of skeletal muscle. Thus we have
Because MLM = ICW/a + ICW × (E/I)/b, we obtain
|
(4)
|
For ATM/MLM = 0 the quantity TMW/MM in Eq. 4
represents the hydration of MLM, which is thus given in the present
model by the expression (1 + E/I)/[1/a + (E/I)/b].
Other important quantities that have been found to exhibit differences
in SM samples from nonobese and obese patients are the fractions of
TGm and of total TG in skeletal muscle mass, TGm/MM and TT/MM, respectively, where TGm
denotes the content of intramyocytic triglycerides and TT denotes the
total content of TG in the muscle mass. We will indicate by
TGat the content of TG in SM adipose tissue and by
the
fraction (referred to myocyte mass) of TG in the myocytes, and we will
assume that adipocytes are essentially composed of water, proteins (5%
according to Ref. 36), and lipids (75% according to Ref.
42). Neglecting the lipids in the ECS and following the
same procedure as above, we may write
|
(5)
|
Another quantity that can be derived from the model is the
fraction of proteins (P) in the SM mass. Denoting the fractions of
proteins in myocytes and extracellular space as
and
',
respectively, and because the fraction of proteins in adipocytes has
been assumed equal to 0.05, the fraction P/MM can be expressed as
|
(6)
|
Similarly, assuming that the glycogen in the SM sample is almost
exclusively present in myocytes with a fraction
(referred to
myocyte mass), an equation for the glycogen fraction in the SM sample,
G/MM = (
/a)/[1/a + (E/I)/b](1 + ATM/MLM), can be
easily obtained.
The preceding equations show that the quantities that have been
measured in the SM samples: TMW/MM, TGm/MM (given by the
first term on the right hand side of Eq. 5), TT/MM, P/MM,
and G/MM can be expressed in terms of the following factors: a, b, E/I,
, ATM/MLM,
,
,
', and
. Although these parameters will
exhibit variability from sample to sample, it is not possible to
estimate all their individual values on the basis of the present
measurements. However, some of these parameters can be assumed to be
constant in the population of samples. Parameters a and b are likely to have a reduced variability, because they appear to be maintained stable
by regulatory mechanisms (43). On the basis of data
reported in Wang et al. (43) for the SM cells, we assumed
a
0.72. Regarding parameter b, we note that the organic
extracellular solids like the fibers are included in the extracellular
space in the present model, whereas they were included in a compartment
of extracellular solids in Wang et al.; thus we took for b a smaller
value than in Wang et al., b
0.97. Adipose tissue hydration was
found by Wang and Pierson (42) to be relatively invariant,
so the value of
was set to 0.14, according to Wang and Pierson.
Moreover, because the term representing the extracellular proteins is
scarcely influent in Eq. 6, at least in the absence of
atrophy or fibrosis, when the content of extramyocytic proteins may
become large, we gave to
' the constant value of 0.02 (2,
36).
As far as the other parameters are concerned, we note that a relative
increase of extracellular with respect to intracellular fluid has been
found in obesity (42), so E/I was assumed to have a linear
increase with the adiposity index ATM/MLM. In particular, we took
E/I = 0.4 + 0.5(ATM/MLM), and thus larger than the
experimental value of 0.2 found for rats and humans (6,
13). With the previous assumptions, the parameters ATM/MLM
(and thus also E/I that is a given function of ATM/MLM),
,
, and
were reliably estimated for each sample from the experimental data
of TMW/MM, TGm/MM, TT/MM, P/MM, and G/MM.
Statistical analysis.
Each single value represents the average of triplicate
determinations. The results are presented as means ± SD unless
otherwise specified. The variability of the determinations from three
samples of the same biopsy was expressed as the coefficient of
variation (CV = 100 × SD/mean).
The Fisher's exact test was used to compare sex proportions between
the two groups. Between-group comparisons were performed using a
Student's t-test for the difference of means, with
significance at P < 0.05. The linear correlation
between anthropometric data and total lipids or intramyocytic
triglycerides was assessed by the correlation coefficient. In the
linear regression, the sum of absolute deviations was minimized
to reduce the effect of outliers. The estimates of the regression
parameters are given as estimate ± SE (34, 37).
The values of ATM/MLM, E/I,
,
, and
were estimated for
each sample from the measurements of TMW/MM, TGm/MM, TT/MM,
P/MM, and G/MM by least squares solution of model equations, whereas the other parameters of the model were set to the values specified above. The solution was found with the constraint that the sum a +
+
+
of water, triglycerides, proteins, and
glycogen in the myocyte mass, a sum that represents all the main
cellular components excluding nucleic acids and miscellaneous
metabolites, has to be close to 97%. The standard errors were
evaluated from the inverse Hessian matrix of the least squares index at
the minimum (34).
 |
RESULTS |
Table 1 reports the data of tissue
composition in the two groups: mean values ± SD refer to 100 mg
of wet muscle tissue. The fractions of TT and TGm were
significantly smaller in the control group compared with the obese
group. The fraction of intramuscular water was significantly lower in
the obese group. A positive correlation was found between BMI and both
the fractions of TGm (r = 0.52, P < 0.0001) and of TT (r = 0.87, P < 0.0001). A negative correlation was found between
the fraction of intramuscular water TMW/MM and the fraction of total
triglycerides TT/MM (r =
0.67, P < 0.0001). The experimental data of TT/MM vs. TMW/MM are reported in Fig. 2, where it is noted that lean subjects
are represented in the right region of the plane (high fraction of
intramuscular water and low lipid content). The dotted line in the
figure is the regression line (slope
1.044 ± 0.032, intercept
0.874 ± 0.025).

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Fig. 2.
Experimental data of the fraction of total intramuscular
water (TMW) vs. the fraction of total triglycerides (TT) in the
skeletal muscle samples (controls, ; obese subjects,
). Theoretical lines predicted by the model of skeletal
muscle composition at the cellular level (Eqs. 4 and 5), with the parameter values indicated in the text [solid lines:
top, ratio of ECW to ICW (E/I) = 0.68 and fractions of
TG in the myocytes ( ) = 9.7; bottom, E/I = 0.40 and = 1.5); dotted line, regression line.
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The individual estimates of ATM/MLM, E/I,
,
, and
were found
with standard errors that were generally much smaller than the 10% of
the estimate. Table 2 gives the mean
values and standard deviations of the estimates over the control and
the obese group. As expected, the mean value of
(fraction of
TGm referred to myocyte mass) is larger than the mean value
of the TGm referring to the wet mass of the sample reported
in Table 1 and similarly for
and
compared with the fractions of
proteins and glycogen in Table 1. The predicted values of the ratio of
H2O to proteins are also reported in Table 2. All of the
parameters of the model except
are significantly different in the
two groups. The estimates of the adiposity index ATM/MLM were found to
range from 0.01 to 0.09 in controls and from 0.06 to 0.56 in the obese
group. The fraction of SM lean mass that can be computed from the
adiposity index as MLM/MM = 1/(1 + ATM/MLM) ranges thus from
0.91 to 0.99 in controls and from 0.64 to 0.94 in obese subjects.
The negative correlation observed between intramuscular water content
and total TG content can be explained on the basis of the skeletal
muscle composition model at the cellular level of Fig. 1. The solid
lines in Fig. 2 were plotted by computing abscissae and ordinates by
Eqs. 4 and 5, respectively, with the model
parameters a, b, and
fixed at the values given in MATERIALS
AND METHODS and with E/I and
set to the extreme values found
in the SM samples (see the legend of Fig. 2). The adiposity index
ATM/MLM was changed from 0 to 0.9 to cover the entire range of the
measured intramuscular water fractions. These lines, which are easily
shown to be straight lines, are thus parameterized by the adiposity
index, and the terminal point at the right corresponds to ATM/MLM = 0. If a subject were to change from lean to obese while maintaining
the SM parameters a, b,
,
, and E/I constant, its representative
point on the plane of Fig. 2 should move on a straight line from right
to left. The two lines of Fig. 2 show the effect of the variability of three parameters of the model, the adiposity index ATM/MLM, E/I, and
the fraction of intramyocytic triglycerides
.
The model in Fig. 1 shows that the hydration level of skeletal
muscle can be reasonably defined in three ways. The ratio TMW/MM is the
observed hydration of the in toto muscle mass; this quantity was seen
to change in the range from 0.5 to 0.8 in the SM samples examined (see
Fig. 2). We can also consider the hydration referred to the muscle lean
mass, TMW/MLM, and the hydration defined as (ICW + ECW)/MLM. This
latter quantity is given by Eq. 4 for ATM/MLM = 0, as
seen above, and depends on the obese or nonobese status of the subject
only through the parameters a, b, and E/I. In contrast, the quantity
TMW/MM decreases with the adiposity index, because it takes into
account the water contained in the adipose tissue that has a small
water content. For instance, with E/I = 0.6, the ratio TMW/MM
decreases continuously from a value of 0.797 for ATM/MLM = 0 to a
value of 0.492 for ATM/MLM = 0.9.
To assess the predictive capacity of the model, we computed the
theoretical values of the ratio of total triglycerides to proteins
(TT/P) and of the ratio of total muscle water to proteins (TMW/P), as
predicted by Eqs. 4-6, with the parameters estimated on
the individual samples as described above. A Bland-Altman plot of the
experimental and theoretical values of TT/P is shown in Fig.
3. It is seen that the experimental
ratios exhibit a remarkable variability from sample to sample
especially in the obese group, mainly because of the presence in this
group of five samples in which a very reduced protein content (from 5.5 to 3.6%) was measured and thus a high value of the ratio was obtained.
The correlation coefficient for experimental and predicted values of
TT/P is equal to 0.998. For the TMW/P ratio, whose experimental values
are in the range of 2-18, the predicted mean and SD are reported
in Table 2 for the two groups, and the correlation coefficient is equal to 0.995.

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Fig. 3.
Bland-Altman plot of measured values and of theoretical
predictions of TT/protein (P) in the SM samples (controls,
; obese subjects, ). The solid line,
mean difference; broken lines, mean difference ± 2 SD.
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DISCUSSION |
Only recently has the importance of maintaining the SM mass in
dynamic disease conditions stimulated new approaches to the in vivo
assessment of this variable (12, 16, 17, 45). A
fundamental limitation to the availability of a useful technique for
the measurement of SM mass is represented by the relative lack of
direct information on anatomic tissue composition. In fact, data on SM
mass by cadaver dissection have been reported in only 25 subjects
(23, 39, 41, 46). Total body water, fat, protein,
minerals, and carbohydrates were measured in two cadavers, one
underweight (BMI 19.35 kg/m2) and the other with cachexia
(BMI 10.92 kg/m2) (21). Indeed, the study in
Ref. 21 was addressed to demonstrate the usefulness of the
in vivo neutron activation analysis in severely wasted patients.
However, the extrapolation of data obtained by cadaver dissection to
the assessment of body composition, and of SM mass in particular, in
living individuals with a wide range of body weights appears at best inaccurate.
A few studies have been performed on the SM composition in humans but
without addressing the question of whether the lipid content of the SM
mass may influence the assessment of body composition. Landin et al.
(22) found in muscle biopsies that only obese men, but not
women, had a higher fat content than did lean men. Using computed
tomography of the thigh, Kelley et al. (20) reported similar findings in obese men, and these data were later confirmed in
women (35). However, these investigations failed to
demonstrate whether fat is accumulated in the connective tissue between
muscle fibers or is stored within muscle cells. By use of data obtained in the "reference man" studies (36), the water content
of human skeletal muscle as a percentage of its total weight has been
generalized for all humans to be a constant value of 81%. However, the
assumption that the water content of skeletal muscle is constant
throughout the population may turn out to be incorrect in light of
different amounts of fat mass among different subjects
(44).
In the present paper, the composition of samples of skeletal muscle
from subjects with a wide range of BMI was analyzed. The fractions of
intramuscular water, proteins, glycogen, and both total and
intramyocytic triglycerides, which represent the typical lipids stored
for energy supply, were measured. Our data showed that obese subjects
have a significantly larger amount of SM total triglycerides, whose
fraction was found to be threefold in the obese subjects. Also the
content of intramyocytic triglycerides increased with the BMI, albeit
to a lesser extent. The increase of SM triglycerides in obese subjects
was thus mainly due to the accumulation of adipocytes between muscle
fibers. As a consequence, the presence in obese subjects of larger
amounts of lipids resulted in a lower fraction of intramuscular water,
as confirmed by the negative correlation found. In our study, the
measured intramuscular water percentage for normal-weight individuals
was 76.3 ± 3.3% and thus comparable to the reference man value
and to the value of 0.8034 obtained from Ref. 43, whereas
the water percentage in obese subjects was much lower (65.7 ± 5.8%), with a decrease of ~15% compared with the reference value.
Our data were interpreted on the basis of a model of SM composition at
the cellular level, which was developed starting from previously
described models of total body mass (42, 43). The present
model considers, in addition to the lean mass, the adipose tissue mass
(adipocytes between muscle fibers) and is thus suitable to represent a
sample of in toto muscle mass. The model describes the SM composition
in terms of factors whose variations can account for the
interindividual variability of SM composition. In particular, one of
these factors, denoted as adiposity index (ratio between adipose tissue
mass and lean mass in the skeletal muscle), takes specifically into
account the amount of triglycerides that is present in the adipocytes
between muscle fibers. This factor is likely to show a wide
intersubject variability (values from 0.01 to 0.56 were estimated on
the present samples). A certain degree of variability, both between
control and obese groups and within the two groups, has to be expected
also for the other parameters of the model, as seen in Table 2.
The model shows that the hydration level of SM (interpreted as the
ratio of total intramuscular water to muscle mass) decreases when the
adiposity index increases, whereas all other parameters of the model,
and, hence, the hydration of SM lean mass, are unchanged. In other
words, SM hydration may still be normal in obese subjects after
correction for intramuscular lipids. However, in the transition from
the lean to overweight and, eventually, to obese status, all of the
model parameters are likely to change to some degree. A complete study
of the physiological (or associated to obesity) variability of the
parameters of the model, in conjunction with the plausible effects of
the experimental errors, could account for the scatter observed in the
data. It has to be stressed that the measurement of the extracellular
water in the samples would provide a more accurate estimate of E/I,
whose between-sample variability was only partially considered in the
present study.
The model allowed us to reasonably reproduce the variability of the
data, as shown in Fig. 3. We note that a subgroup of samples from obese
subjects had a greatly increased value of TT/P, so a larger deviation
from model prediction was found. This increased TT/P is possibly
associated with a specific loss of muscle proteins. All of these
subjects, in fact, in addition to morbid obesity, also presented us
with associated complications, namely degenerative arthritis of the hip
and knee, which severely restricted their walking. For these cases,
some specific modification of the model, to take into account the
possible effects of the loss of proteins, should be considered.
Because skeletal muscle accounts for a major portion of body cell mass,
the finding that the water fraction of skeletal muscle is highly
variable with BMI implies that the estimation of the fat-free mass
should be revised. The assumption of a constant water fraction in
skeletal muscle should also be reconsidered. It is clear, therefore,
that knowledge of a more accurate estimate of the SM water content
might allow a more appropriate calculation of the intramyocytic
triglyceride amount in individuals with different anthropometric
characteristics, with particular reference to lean and obese subjects.
In conclusion, the classical 81% as a standard reference value for the
water content in skeletal muscle is not appropriate when used across
lean and obese subjects. However, the error in lean subjects is smaller
compared with that found in obese individuals, in whom the high lipid
content of SM causes a decrease of the percentage of water.
 |
ACKNOWLEDGEMENTS |
We are grateful to A. Caprodossi for technical support.
 |
FOOTNOTES |
Address for reprint requests and other correspondence: G. Mingrone, Istituto di Medicina Interna e Geriatria, Università Cattolica del Sacro Cuore, Largo A. Gemelli 8, 00168 Rome, Italy (E-mail: gmingrone{at}rm.unicatt.it).
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 2 March 2000; accepted in final form 20 October 2000.
 |
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