Clinical Science (1991)81,249-256
249
Muscle composition in relation to age and sex
A. M. FORSBERG, E. NILSSON, J. WERNEMAN", J. BERGSTROM?
AND
E. HULTMAN
Departments of Clinical Chemistry 11, *Anaesthesiologyand $Renal Medicine, Huddinge University Hospital, Karolinska Institute,
Huddinge, Sweden
(Received 7 December 1990/4 March 1991; accepted 11 March 1991)
SUMMARY
1. A method is described enabling the determination
of fat, water, electrolytes, protein, DNA, RNA and total
creatine in a single sample of human muscle obtained by
the percutaneous needle-biopsy technique. The amino
acid content can also be analysed in the same muscle
sample.
2. Fifty healthy subjects were studied: 29 between 19
and 40 years of age, 11 between 4 1 and 60 years of age,
and 10 between 61 and 85 years of age. The two groups
aged less than 60 years showed only marginal differences
in muscle composition, whereas the highest age group
showed increases in muscle fat content in relation to
tissue weight and decreases in alkali-soluble protein
content in relation to both tissue weight and tissue DNA
content. Also, potassium, magnesium, total creatine and
RNA contents were decreased in this age group when
related to tissue DNA content. When alkali-soluble
protein was used as a reference base, only magnesium
content was decreased.
3. A comparison was also made between female
( n = 2 3 ) and male ( ? I = 18) subjects in the age groups
below 60 years. Differences observed included a higher
fat content in female muscle, and an increase in total
creatine content in relation to tissue weight. The alkalisoluble protein content was lower per muscle cell in the
females when calculated on the basis of DNA content.
4. The results show that in the assessment of muscle
constituents, age and sex must be taken into account.
Key words: alkali-soluble protein, cellular water, deoxyribonucleic acid, electrolytes, fat, human muscle, ribonucleic acid, total creatine.
Abbreviations: ASP, alkali-soluble protein; FFS, fat-free
solids; FT, fast-twitch; H,O,, extracellular water content;
H,Oi, intracellular water content; H,O,, total water
content; PCA, perchloric acid; SSA, sulphosalicylic acid;
ST, slow-twitch.
INTRODUCTION
In various clinical conditions imbalances occur between
intake and utilization of water, electrolytes, protein and
energy substrates, which interfere with the capacity of the
body to maintain normal cell composition. Skeletal
muscle comprises most of the lean body mass, amounting
to about 40% of the body weight in healthy individuals.
The largest stores of intracellular fluid, electrolytes,
energy-yielding metabolites, free amino acids and protein
are confined to the muscle tissue compartment. The
analysis of muscle tissue may therefore be of considerable
interest as a guide to whole-body metabolism.
In healthy subjects the content of muscle electrolytes,
free amino acids and energy-rich phosphagens is
influenced by sex and age [ 1,2]. These factors must therefore be taken into consideration when evaluating muscle
data from sick individuals.
In the present communication we describe a method
which enables multiple analyses to be performed on
portions from a single muscle specimen obtained by the
percutaneous needle biopsy technique. The components
analysed were fat, water, electrolytes, alkali-soluble
protein (ASP), RNA, DNA, total creatine and free amino
acids. These results were related to different reference
bases such as fat-free solids (FFS), intracellular water
content, DNA content and ASP content.
The methodology was applied in healthy male and
female subjects of various ages, one aim of the study being
to obtain control data in groups of healthy individuals
which can be used as age- and sex-matched controls in
subsequent clinical and physiological studies.
METHODS
Subjects
Correspondence: Professor Eric Hultman, Department of
Clinical Chemistry 11, Huddinge University Hospital, S-141 86
Huddinge, Sweden.
Fifty healthy subjects (26 women and 24 men) took
part in the investigation. The age range was from 19 to 85
A. M. Forsberg et al.
250
years. They all had normal body weight in relation to
height (body mass index range 19.0-25.1). The subjects in
the age groups from 19 to 58 years (40 subjects) all
belonged to the hospital staff, whereas the remaining 10
subjects aged 58-85 years were patients admitted to
hospital for minor surgery, i.e. inguinal hernia (nine
subjects) or varicose veins (one subject). All the subjects
were in normal physical condition, without excessive
training programmes or abnormal inactivity. The surgical
patients were admitted to hospital on the day of surgery
and were not physically disabled by the disorders. Blood
samples obtained before the investigation showed normal
ranges for haemoglobin, blood cell content and electrolyte concentrations, as well as for plasma proteins and
plasma enzyme activities. All subjects volunteered to
participate in the study, which was approved by the Ethics
Committee of the Karolinska Institute.
Muscle sampling
Needle-biopsy samples weighing 50-100 mg were
obtained from the lateral portion of the quadriceps
femoris muscle by needle biopsy as described by Bergstrom [3]. The samples were dissected free from blood
and visible connective tissue, weighed repeatedly for
extrapolation of the wet weight to time zero, frozen in
liquid nitrogen and freeze-dried after storage for a period
of maximum 3 days at - 70°C. The freeze-dried samples
were weighed, fat-extracted in petroleum ether for 60
min, dried at room temperature and re-weighed. The
weight is referred to as FFS. The sample was powdered in
an agate mortar and freed from flakes of visible connective tissue. The powder was divided into two portions:
-2.5 mg for the analyses of electrolytes and total
creatine, and 3 mg for the determination of DNA, RNA
and ASP or DNA, amino acids and ASP. The electrolyte
sample was dried at 80°C for 30 min. This procedure
reduced the weight by 4-6%. The true dry weight of the
remaining portion of powder was therefore calculated at
95% of the observed weight after powdering at room temperature and humidity. All glassware and utensils used in
contact with the muscle biopsy were rinsed in nitric acid
(1 mol/l) to remove traces of sodium and other electrolytes.
-
Total creatine
The same supernatant (2 X 25 pl) was boiled with 100
p1 of HNO, (1 mol/l) for 40 min in a water-bath. After
cooling and centrifugation, alkaline picric acid was added
(0.75 ml of 11.5 mmol/l picric acid and 100 pl containing
1.55 mol/l NaOH and 60 mmol/l Na,HPO,). The colour
development was stable after 1 h and the absorbance was
determined at 513 nm in LKB spectrophotometer 2074
(LKB, Stockholm, Sweden). In the boiling acid all the free
and phosphorylated creatine was transformed into creatinine. Creatine and creatinine from Sigma (C-3630 and
C-4255) were used as standards.
ASP, RNA and DNA
Three milligrams of the freeze-dried powder was
placed in a pre-weighed glass tube. The powder was
precipitated with 0.5 ml of perchloric acid (PCA;
0.2 mol/l) in an ice-bath for 10 min, centrifuged and the
precipitate was washed twice with 0.5 ml of PCA (0.2
mol/l). The supernatants were discarded and the washed
precipitate was dissolved in 1 ml of KOH (0.3 mol/l) by
incubation for 1 h at 37°C. The tube was re-weighed to
obtain the true dilution volume. Portions of the solution
(2 X 20 p l ) were used for protein determination using the
method of Lowry et 01. [5] with Seronorm protein
(Nycomed)as the standard.
DNA was precipitated with 1 ml of PCA (1.2 mol/l)
and left in an ice-bath for 30 min. The precipitate was
washed twice with PCA (0.2 mol/l, 0.5 ml each time). The
collected supernatants containing RNA were assayed by
the direct spectrophotometric method of Fleck & Begg
[6]. Standards from yeast (Calbiochem no. 55712) were
run through the whole precipitation and washing procedures.
For DNA extraction the precipitate was hydrolysed by
adding 0.25 ml of PCA (1 mol/l) and incubating for 1 h at
70°C. The tube was weighed again to obtain a dilution
volume for DNA. DNA was estimated by the diphenylamine reaction [7, 81. Calf thymus DNA (Sigma
D-1501) was used as the standard. The extraction
procedure has been described by Munro & Fleck [9] as a
modification of the technique used by Schmidt &
Tannhauser [lo].
Electrolytes
Sodium, potassium and magnesium were extracted
with nitric acid ( I mol/l; 100 pl/mg of muscle powder),
ultrasonicated for 15 min, left overnight, and thereafter
mixed in a vortex mixer and centrifuged. The supernatant
(100 pl) was diluted with 1.0 ml of HNO, (1 mol/l) and 9
ml of an ionization and compensation solution [12.4
pmol/l Fe(NO,),, 0.5 mmol/l H,SO,, 5.9 mmol/l H,PO,,
1.25 mmol/l CaCO,] and analysed in an atomic absorption photometer (IL 75 1)against standard solutions [4].
Chloride content was analysed against standard solutions of NaCl (Merck) in the same supernatant (2 x 25
p1) with electrometric titration against silver nitrate, using
a Radiometer pH-meter 62 [4].
ASP, amino acids and DNA
When amino acid analysis was included, sulphosalicylic
acid (SSA) instead of PCA was employed for the extraction procedure. Three milligrams of the freeze-dried
powder was extracted with 0.35 ml of 4% (w/v) SSA in an
ice-bath for 1 h and the supernatant was used for amino
acid analyses. The precipitate was treated as described
above for protein and DNA analyses. RNA could not be
analysed because of interference of SSA with the photometric determination. Amino acid analysis was not performed in whole material in this investigation and is
therefore not included in this paper.
Muscle composition in relation to age and sex
Table 1. Muscle constituents
Mean values and the calculated imprecision of each
determination are shown. Total creatine, Mg2+,K+,Na+
and CI- contents are expressed as mmol/kg of FFS, and
DNA, RNA and ASP contents are expressed as g/kg of
FFS.
Constituent
n
Mean
(mmol or g/kg of FFS)
Total creatine
Mg2+
K+
Na+
c1-
DNA
RNA
ASP
34
53
53
53
41
52
41
52
Coefficient of
variation (%)
134.8
40.93
423.8
98.7
79.3
1.83
3.64
714.5
0.96
1.61
1.20
3.17
3.87
3.22
2.47
1.58
Error of the method
The error of the analytical method was calculated from
duplicate analyses of muscle powder from each tissue
sample (Table 1).The relative SD as a percentage (coefficient of variation) was calculated according to the
following formula:
Coefficient of variation= 100-./~[2.(a-b)/(a + b)P/2n
where a and b are the two samples from the same powder
and n is the number of duplicates [ 111.
Reference bases and calculations
The muscle constituents are referred to a series of
reference bases: FFS, DNA and ASP.
The water distribution in the muscle sample was calculated from the chloride and water contents in the muscle
sample. Since chloride is freely diffusable across the
muscle cell membrane and follows Nernst’s equation [12],
the ratio between extra- and intra-cellular chloride
concentrations will be 26: 1, assuming a resting membrane
potential of - 87.2 mV.
The extracellular chloride concentration was calculated from the plasma concentration, which was corrected
for the protein content and Donnan’s equilibrium (0.96)
[13]. The estimates of total water (H,O,), extracellular
water (H,O,) and intracellular water (H,Oi) contents of
the sample were based on these analyses and were calculated as described previously [14]. The concentrations of
intracellular electrolytes (sodium, potassium and magnesium) were calculated by subtracting extracellularly
distributed electrolytes from total muscle contents,
assuming that interstitial electrolyte concentrations were
equal to those in plasma.
Grouping of the subjects and statistical methods
The results were evaluated by using Student’s f-test
(significance level P < 0.05) and by calculating the correlation coefficients. Data are expressed as m e a n s k s ~ ,and
are calculated for all the subjects or for the subjects
divided into groups according to age. Group I comprised
25 1
29 subjects aged 19-40 years with a mean age of 28.1
years, group I1 had 11 subjects between 41 and 60 years
with a mean age of 47.2 years, and group 111 contained 10
subjects aged between 6 1 and 85 years with a mean age of
72.6 years. T h e subjects were also divided into females
and males in age groups I and 11, i.e. 2 3 females with a
mean age of 34 years, and 1 7 males with a mean age of 32
years. The group with the highest age (group 111) was not
included in the comparison between the sexes, since it
contained only three females against seven males.
RESULTS
Fat content, water content and distribution, electrolyte
contents, total creatine, ASP and DNA contents in relation to FFS in the three age groups and in separate sex
groups are presented in Table 2. For none of the muscle
constituents was there any significant difference between
age groups I and 11. However, in age group 111 (61-85
years) the fat and DNA contents were higher than in
groups I and 11, and the extracellular water content was
significantly higher than in group I, the youngest age
group. T h e magnesium content was significantly lower in
group I11 than in groups I and 11. Potassium and ASP
contents were also slightly lower in group I11 than in the
youngest age group.
In the comparison between sexes only the first two age
groups were utilized because of the uneven distribution of
female and male subjects in group 111. Using FFS as a
reference base we found that the females had a higher
content of fat and DNA and a slightly lower total water
content than the males. The total creatine content in
relation to FFS was much lower in the male subjects than
in the female subjects (132 f 10 versus 145 k 10 mmol/
kg, mean k SD).
The calculated concentration of intracellular electrolytes and the ratio between muscle potassium and magnesium contents are presented in Table 3. The
intracellular sodium and potassium concentrations were
similar in the three age groups and also between men and
women. However, the intracellular magnesium concentration was about 8% lower in age group 111 than in the two
younger age groups, and was also slightly lower in females
than in males. Correspondingly, the potassium/magnesium ratio was higher in group I11 and also slightly
higher in females than in males.
Muscle constituents were also related to the ASP
content and to the DNA content in muscle in the three
age groups and in females and males (Table 4). No
difference was observed for any muscle constituent
between the younger age groups in groups I and 11. In
group I11 (61-85 years), however, the magnesium content
was lower and the DNA content higher in relation to the
ASP content than in the younger age groups, and
potassium, magnesium, total creatine, RNA and ASP
contents in relation to the DNA content were markedly
reduced.
Comparing females with males, the latter had lower
potassium, total creatine and DNA contents in relation to
the ASP content. Using the DNA content as the reference
A. M. Forsberg et al.
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DISCUSSION
m e
t--
-r-
base, potassium, magnesium and ASP contents were
higher in males than in females.
The relationship between potassium and ASP contents,
both related to the DNA content, grouped on the basis of
age and sex, is shown in Fig. 1. There was a strong linear
correlation over a wide range of ASP/DNA values,
indicating that in normal muscle cells ASP content is a
convenient measure of cell mass, to which the intracellular concentration of potassium can be related.
However, when scrutinizing the data we note a slight difference between men and women, reflecting the slightly
lower potassium/ASP ratio in men than in women (Table
4).
A strong correlation was also observed between total
creatine and ASP contents in relation to the DNA
content, although it was not as perfect as the relation
between potassium and ASP contents (Fig. 2 ) . A more
marked difference between males and females than for
potassium content was observed, the females having a
higher total creatine content in relation to the ASP
content than the males (Table 4).
t
5
-I-0
?9
m
N
1
I
c
In earlier studies of muscle composition based on the
percutaneous needle-biopsy technique, the muscle
material had to be divided into several pieces which were
weighed and processed separately for each type of
analysis [3, 41. The new methodology for processing
muscle specimens presented here affords several advantages. The handling and weighing procedure is simplified
and is also less time-consuming, which means less risk of
degradation of components before the metabolic processes are inactivated by deep-freezing. The separation of
connective tissue from the freeze-dried and fat-extracted
specimen is easier and presumably more complete than
when wet tissue is dissected. The various tissue components are generally evaluated in relation to one another,
some of them serving as reference bases which represent
the cell mass or the number of cells in the sample. By performing all analyses on portions from the same muscle
specimen we can minimize sampling variability.
The fat content of muscle varies unpredictably, an
increase in muscle fat being a non-specific finding in
various groups of severely ill patients [3]. Ageing and
female gender are also factors associated with increased
muscle fat stores, as demonstrated in the present study.
Considerable variations in total water content and
extraccllular fluid volume are observed in diseases which
involve disturbances in water and electrolyte homoeostasis. By using FFS as the basis of reference for intracellular muscle components we minimize the influence of
variation in fat and water contents [15]. We should point
out that FFS represent not only muscle cell solids but also
connective tissue solids, which may be increased in the
muscle of patients with degenerative diseases and malnutrition. The extracellular space is mainly composed of
connective tissue, which is soaked in a plasma ultrafiltrate
and therefore has a high content of sodium and chloride
but a low content of the intracellular electrolytes, potas-
10.4 f 0.4
10.5 f 0 . 3
10.1 t 0 . 3 t i t
14.4k0.7
14.9 f 0.7t
151 f 6
150f5
11.3 f 1.8
11.1 f3.0
Females ( n= 23)
Males ( n= 17)
SD
Male ( n= 17)
Mean
SD
Female(n=23)
Mean
SD
AgegroupIII,61-85 years(n= 10)
Mean
SD
AgegroupII,41-60years(n= 11)
Mean
SD
Age group I, 19-40 years ( n= 29)
Mean
Constituent...
Referencebase.. .
~
613t
20
630
27
634
45
622
26
623
25
60.9
2.7
60.2
2.9
57.4*
2.5
60.1
2.1
60.7
3.1
183itt
13
205
16
200
18
20 1
15
195
19
2.27~
0.26
2.63
0.31
3.53***
0.36
2.6 1
0.31
2.43
0.34
5.53
0.40
5.37
0.57
5.07
0.35
5.35
-
5.47
0.53
243
30
177***
14
24 1
23
260
39
27.ltt
4.0
23.2
3.1
16.4***
1.7
23.3
2.6
25.4
4.3
78.3
7.4
78.0
10.9
56.8***
4.9
77.6
5.4
78.4
11.1
2.3 1
0.22
2.19
0.23
1.45***
0.18
2.21
-
2.25
0.24
446t1-t
52
385
46
286***
30
389
46
420
59
Table 4. Muscle constituents with ASP and DNA as reference bases related to age and sex
Significant differences between age groups and between sexes: *difference between group I11 and groups I and 11; tdifference between males and females. One, two and
three symbols denote P< 0.05, P< 0.01 and P< 0.001, respectively. Values of n for total creatine: group I, 24; females, 22; males, 13. n Values for RNA: group I, 9;
group II,2; females, 6; males, 5 .
10.9fO.4**
14.5 f 0.5
13.5 f 0.6**
151 rt4
148 f 4
10.6 5 1.4
Age group III,61-85 years ( n= 10) 10.0 f 1.9
Agegroup II, 41-60 years ( n= 11)
10.3 ? 0.3
14.7f 0.8
151 f 6
11.5f2.7
AgegroupI, 19-40years(n=29)
K+/Mg2+
(mol/mol)
Mg2
(mmol/l of
i.c. water)
Na+
(mmol/l of
i.c. water)
Constituent
K+
(rnmol/l of
i.c. water)
Table 3. Intracellular electrolyte contents related to age and sex
Values are means k SD. Abbreviation: i.c., intracellular. Significant differences between age groups
and between sexes: *group I11 differs from groups I and 11; tmales differ from females. One, two
and three symbols denote P<O.O5, P<O.Ol and P< 0.001, respectively.
+
x
0
v)
a
5'
e
8.
g.
s
a
0
A. M. Forsberg et al.
254
400 i
19-40
41-60
61-85
0
.
A
0
A
I
300
500
400
600
ASP/DNA (kg/kg)
Fig. 1. Relationship between potassium content per cell,
i.e. potassium/DNA, and ASP content per cell, i.e. ASP/
DNA, in male (M)and female (F)subjects in different age
groups. Correlation curve: y = 2.83 + 0.6 lx, r = 0.98.
120 100 -
80 60 40 -
(years)
M F
19-40
41-60
61-85
A
o
A
0
I
O*’
300
400
500
600
ASP/DNA (kg/kg)
Fig. 2. Relationship between total creatine content per
cell, i.e. total creatine/DNA, and ASP content per cell, i.e.
ASP/DNA, in male (M) and female (F) subjects in
different age groups. Correlation curves: males,
y = 9.20 + 0.16x, r = 0.94; females, y = 5.72 + 0.19x,
r = 0.89.
sium and magnesium [16]. When the proportion of connective tissue is relatively high, low potassium and
magnesium concentrations may be recorded in relation to
FFS despite normal extracellular electrolyte concentrations, a condition commonly referred to as ‘pseudodepletion’. In patients with extracellular fluid retention or with
an increased proportion of connective tissue in muscle,
meaningful information about the intracellular content of
solutes present in appreciable amounts both in extra- and
intra-cellular fluid can be obtained only by subtracting the
extracellular fraction from the total content.
We used the chloride method as described previously
[4] to calculate the distribution of water and solutes
between the extra- and intra-cellular compartments,
thereby minimizing the influence of variations in extracellular space on the assessment of the muscle intracellular solute contents. Intracellular water may then be used
as a basis of reference for calculating the concentration of
intracellular electrolytes and amino acids and the intra- to
extra-cellular concentration gradients.
To solve the problem of increased connective tissue
solids, ASP, representing non-collagen protein or cell
protein, was introduced as a basis of reference for cell
mass [17]. In this normal material we found that ASP
constituted about 70% of the total FFS, being only slightly
reduced (by a mean of 2.5%) in the highest age group, in
which muscle extracellular water in relation to FFS
tended to be higher and potassium in relation to FFS was
slightly reduced, suggesting a minor increase in connective tissue content. However, the difference in extracellular water content (and in sodium and chloride)
between old and young subjects was less marked than in
reports of earlier studies where similar analytical methods
were used [2]. A reason for this may be that the method of
dissecting the muscle specimen after freeze-drying was
more efficient in removing increased amounts of connective tissue, conceivably present in the muscle of aged
individuals, than the dissection of wet muscle used in earlier studies. The intracellular concentrations of sodium
and potassium were quite stable and showed no significant variation with age and sex.
Muscle magnesium content, on the other hand, was
significantly lower in the highest age group, irrespective of
which basis of reference was used. Magnesium in skeletal
muscle is to a large extent complex-bound to adenine
nucleotides. Up to 95% of ATP and 50% of ADP is
bound to magnesium [ 181. In elderly subjects the total
concentration of adenine nucleotides and the ADP
content are significantly lower than in young subjects [ 13
and muscle ATP and magnesium contents are significantly
correlated [2]. Hence, an age-related reduction in muscle
adenine nucleotides may be a factor contributing to the
low total magnesium content in muscle in healthy elderly
subjects. Moderately low muscle magnesium contents in
elderly patients should therefore be evaluated with
caution, unless the control subjects are age-matched.
The skeletal musculature comprises the largest protein
store in the body. It is involved in adaptation to starvation,
disease, injury and inactivity, when a net breakdown of
muscle protein takes place. Thus, the determination of
muscle cell protein (ASP) per ‘cell unit’ represented by the
DNA content should provide the best quantitative information for evaluating the protein status of an individual.
Tissue RNA content, which may reflect the activity of
protein synthesis, is also best expressed in relation to
DNA.
We found that the ASP content in relation to the DNA
content was significantly lower in the 61-85 year age
group than in the younger groups and also the females
had significantly lower values than the males. The lower
protein content per unit of DNA should correspond to a
lower amount of contractile protein per cell unit. This is
consistent with the decrease in muscle fibre area,
especially of type I1 fibres, reported previously in age
groups above 60 years [19], which also bears out earlier
findings [20, 2 11 that the cross-sectional areas, especially
that of fast-twitch (FT) fibres in skeletal muscle, are
Muscle composition in relation to age and sex
smaller in females than in males. The relatively large
variation of ASP in relation to DNA within each age and
sex group presumably reflects variations in muscle mass
and physical fitness.
In relation to DNA, the intracellular components
potassium and total creatine varied with the muscle
protein content (ASP). Fig. 1 shows an excellent correlation between the cell content of potassium (potassium/
DNA) and ASP (ASP/DNA) per cell unit over a wide age
range and also a minor difference between the sexes,
potassium content in relation to ASP content being
slightly higher in females than in males (see Table 4). The
cell content of total creatine (total creatine/DNA) was
also correlated to the cell protein content (ASP/DNA)
(Fig. 2) and the sex difference was more marked, females
having on average a total creatine content in relation to
ASP about 10% higher than that of males (Table 4).
Variations in the total creatine content in different
muscle groups of the rat and guinea-pig have been
described, showing that muscle with predominantly FT
fibres had a higher total creatine content than muscle with
a high content of slow-twitch (ST) fibres [22]. The same
tendency was observed in human skeletal muscle with a
significantly higher creatine content in the vastus lateralis
muscle which had 60% FT fibres as compared with the
soleus muscle, which had 33% FT fibres [22]. In this
series the same muscle (vastus lateralis) was analysed in
both males and females, and no difference in fibre-type
distribution between the sexes was observed [20,23-251.
Studies of cross-sections of FT and ST fibres showed a
lower volume fraction of FT fibres in females than in
males [20,24-271, a result contrary to that expected from
the difference in total creatinine contents.
To sum up, the differences in muscle water and electrolyte contents due to age were insignificant, except for a
small increase in extracellular fluid volume and a minor
decrease in potassium content in relation to FFS and a
more marked decrease in magnesium content in relation
to FFS, intracellular water content and ASP content in
the highest age group. No variation in muscle electrolyte
content with sex was discerned, except that potassium
content in relation to ASP content was slightly higher in
women than in men. The protein content per unit muscle
cell (ASP/DNA) was lower in the highest age group than
in the younger groups and was also lower in females than
in males. The total creatine content in relation to the ASP
content was higher in females than in males, but showed
no age-related variation. On the basis of these results, we
may conclude that a careful evaluation of muscle composition with regard to water, electrolyte, total creatine,
protein and DNA contents in clinical situations must
include age- and sex-matching of the control subjects.
ACKNOWLEDGMENTS
This work was supported by grants from the Swedish
Medical Research Council (02647 and 01002). We thank
the entire staff of the Department of Clinical Chemistry I1
for excellent collaboration in this investigation. Special
thanks are due to Mrs Anci Adestam for secretarial help.
255
We also express our gratitude to Mrs Elsy Digrkus of the
Nephrology Unit for help with muscle biopsy sampling in
some of the subjects.
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