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Intergenerational
comparison of total and regional bone
mineral density and soft
tissue composition in Japanese women without vertebral
fractures
Shinjiro Takata, Hiroshi Yonezu,
Natsuo Yasui
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Department of Orthopedics, The University of
Tokushima School of Medicine, Tokushima, Japan
Abstract: We
measured total and regional bone mineral density (BMD) and soft
tissue mass in 115 healthy Japanese women without vertebral
fractures. The subjects, aged 20 to 75 years, were divided into four
age groups:20's to 40's group (n=33), 50's group (n=26), 60's group
(n=26), and 70's group (n=30). BMD was measured by dual energy X-ray
absorptiometry (DXA). The evaluated regions were the head, arms,
legs, ribs, thoracic vertebrae, lumbar vertebrae and pelvis. The
total and regional BMDs were the highest in the 20's to 40's group,
and they decreased with age, and reached their respective lowest
values in the 70's group. The decrease in BMDs of the spine and
pelvis was the most prominent of all regional BMDs. Total and
regional lean mass and fat mass were the highest in the 20's to 40's
group, and they decreased to their respective lowest values in the
70's group. The results showed that the decrease in BMD of the spine
and pelvis was the most prominent of all regional BMDs associated
with a decrease in the total and regional lean mass and total and
regional fat mass. J. Med. Invest. 49:142-146,
2002
Keywords:bone mineral density, soft tissue, aging, dual
energy X-ray absorptiometry
INTRODUCTION
Aging is
associated with estrogen deficiency (1, 2), low calcium absorption
in the small intestines (3), low production of vitamin D3 (4), and
low mechanical stress on bone as a result of a decrease in physical
activity, and activation of interleukin (IL)-1, IL-6 and tumor
necrosis factor (TNF) (5-7), all of which are risk factors for the
development of osteoporosis. In women, the bone mineral density
(BMD) of the lumbar spine reaches its maximum at between 20 and 30
years of age (8), and remains almost constant until the 40's.
Thereafter, the BMD decreases rapidly with the onset of menopause,
and thereafter continues to decline slowly with age (9, 10). Bone
resorption is markedly accelerated in the first five or ten years
following menopause. Over their lifetime, women lose approximately
50% of their peak trabecular bone and approximately 35% of their
peak cortical bone (11).
Einhorn (12) reported that the relative
content of trabecular bone varied among the different parts of the
skeleton. The trabecular bone is approximately eight-fold as
metabolically active as cortical bone, and the response to metabolic
changes is faster in trabecular bone than in cortical bone (13).
Based on these findings, the rate of bone loss should vary according
to the region examined and the age of each individual.
The
purpose of this study was to investigate the differences in the
rates of regional BMD decrease and soft tissue mass variations using
dual energy X-ray absorptiometry (DXA). In the present study, the
subjects were divided according to age into four age-brackets:20's
to 40's group, 50's group, 60's group, and 70's
group.
MATERIALS AND METHODS
Subjects (Table1)
This study comprised 115 Japanese women aged 21 to 79 years.
These 115 subjects were out-patients undergoing examinations for
osteoporosis. These healthy women were divided into four
age-brackets:20's to 40's group (n=33), 50's group (n=26), 60's
group (n=26), and 70's (n=30) group. All subjects gave their
informed written consent before DXA examination. None had received
any treatment affecting bone metabolism or had fractures of the
thoracic or lumbar vertebrae. Their physical characteristics
(height, weight, and body mass index (BMI)) are shown in Table 1.
The mean height and weight decreased with age. The height of the
20's to 40's group was significantly higher than that of the 60's
group (p<0.05) and that of the 70's group (p<0.05). The body
weight of the 70's group was significantly lower than that of the
20's to 40's group (p<0.05) and that of the 50's group
(p<0.05). However, BMI did not vary markedly among the four
groups.
Measurement of BMD and soft tissue composition
The
mean BMD of the 2nd to 4th lumbar vertebrae (L2-4BMD), total body
BMD and soft tissue mass were measured by DXA using a Hologic QDR
2000 (Waltham, MA, USA). The BMD (g/cm2) of the head, arms, legs,
ribs, thoracic vertebrae, lumbar vertebrae and pelvis were measured.
The lean mass (g) and the fat mass (g) of head, arms, legs, and
trunk were measured using a tissue bar (Figure 1)
(14).
Statistics
Results were expressed as
means±standard deviation (S.D.). One-way analysis of
variance (ANOVA) was used to evaluate the significance of
differences among the four groups. When ANOVA indicated significant
differences among the four groups, the differences were evaluated
using Fisher's protected least significant difference (PLSD). A p
value of less than0.05was considered statistically
significant.
RESULTS
L2-4BMD, total BMD and
regional BMD (Table 2).
As shown in Table 2, L2-4BMD in the 20's
to 40's group was the highest of all groups. L2-4BMD of the 50's
group was approximately 80% of that of the 20's to 40's group.
L2-4BMD of the 70's group was approximately 69% of that of the 20's
to 40's group. The difference in L2-4BMD was the greatest between
the 70's group and the 20's to 40's group. The decrease in the
lumbar BMD as assessed by whole body-scanning was similar to that of
L2-4BMD.
The difference in the thoracic BMD between the 70's
group and the 20's to 40's group was the second greatest among the
regional BMDs, and that regarding pelvis BMD was the third greatest.
The tendency of pelvic BMD to decrease with age was similar to that
of lumbar and thoracic BMD. In contrast, the decrease in BMD of the
left and right arms, as non-weight-bearing bones, was small compared
with that of weight-bearing bones such as the thoracic and lumbar
spine, pelvis and legs.
Total and regional lean mass (Table
3).
Total lean mass of those in the 20's to 40's group was the
highest among all generations. Total lean mass was significantly
decreased in the 70's group (p<0.05), being 90.1% of the total
lean mass of those in the 20's to 40's group. The lean mass of all
regions, except for the lean mass of the left arm and head,
decreased with age.
Total and regional fat mass
(Table4).
Total fat mass, left and right arms fat mass, left and
right legs fat mass of those in the 20's to 40's group were
significantly greater than those in the 70's group (p<0.05). In
addition, total fat mass, left arm fat mass, and trunk fat mass of
subjects in the 50's group were significantly greater than those of
subjects in the 70's group (p<0.05). Total and regional fat mass
of subjects in the 70's group, except for head fat mass, showed the
lowest values. Total fat mass of women in the 70's group was 80.4%
of that of those in the 20's to 40's
group.
DISCUSSION
As shown in this study, the
magnitude of the decrease in lumbar and thoracic BMD was high
compared with other regional BMDs. Einhorn (12) showed that the
relative content of trabecular bone varied among the different parts
of the skeleton, and that the content of trabecular bone of
vertebrae was 66-90%, that of the hip at the intertrochanteric
region was 50%, that of the hip at the femoral neck was 25%, that of
the distal radius was 25%, that of the mid-radius was 1%, and that
of the femoral shaft was 5%. As for bone metabolism, the trabecular
bone is approximately eight times as metabolically active as
cortical bone, because the surface of trabecular bone is larger than
that of cortical bone, and the response to metabolic changes in
trabecular bone is faster than that of cortical bone (13).
Therefore, the marked rate of decrease in lumbar, thoracic and
pelvic BMD may be due to the high content of trabecular bone
compared with other regional bones.
In addition, physical
activity decreases with age, which may lead to trabecular bone
resorption in postmenopausal women. Changes in trabecular bone
resulting from low mechanical stress on bone are more prominent than
those observed in cortical bone. The differences in the patterns of
bone loss in different regions may be ascribable to the
site-specific cortical to trabecular bone ratio. The lumbar and
thoracic spines are rich in trabecular bone (12), and this may
explain why the decrease in BMD was more marked in the spine
compared with other regions, as shown in the present study.
In
the present study, we also demonstrated that total lean mass
declined with age to their respective minimal levels in women in
their 70's. The tendency of a decrease in total lean mass was
similar to that observed in L2-4BMD compared with total fat mass.
Based on this finding, lean mass may become a predictor of L2-4BMD.
This study was a cross-sectional one. The divergent levels of
dietary calcium intake and other environmental factors in addition
to aging must have effects on BMD, which may account for the
differences in BMD among these age-groups. Therefore, a longitudinal
study is required to assess the effects of aging on total and
regional BMD, and soft tissue
composition.
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Received for publication April 5, 2002;accepted May 14,
2002.
Address correspondence and reprint requests to Shinjiro
Takata, M.D., Department of Orthopedics, The University of Tokushima
School of Medicine, Kuramoto-cho, Tokushima 770-8503, Japan and
Fax:+81-88-633-0178.
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