Bone mineral density (aBMD) is equivalent to bone mineral content (BMC) divided by area. We rechecked the significance of aBMD changes in aging by examining BMC and area separately. Subjects were 1167 community-dwelling Japanese men and women, aged 40–79 years. ABMDs of femoral neck and lumbar spine were assessed by DXA twice, at 6-year intervals. The change rates of BMC and area, as well as aBMD, were calculated and described separately by the age stratum and by sex. In the femoral neck region, aBMDs were significantly decreased in all age strata by an increase in area as well as BMC loss in the same pattern in both sexes. In the lumbar spine region, aBMDs decreased until the age of 60 in women, caused by the significant BMC decrease accompanying the small area change. Very differently in men, aBMDs increased after their 50s due to BMC increase, accompanied by an area increase. Separate analyses of BMC and area change revealed that the significance of aBMD changes in aging was very divergent among sites and between sexes. This may explain in part the dissociation of aBMD change and bone strength, suggesting that we should be more cautious when interpreting the meaning of aBMD change.
Bone mineral density (aBMD) decreases with age [
The subjects were selected among people who participated in both the 1st and 4th waves of the National Institute for Longevity Sciences Longitudinal Study of Aging (NILS-LSA). Details of the NILS-LSA are presented elsewhere [
Areal bone mineral densities (aBMD) were measured using Hologic QDR4500, both at the 1st and 4th wave. Only one DXA scanner was used. Data on the right femoral neck (Figure
Femoral neck region of interest, derived from the Hologic QDR 4500 Operator’s Manual.
The statistical analyses were made to test for significance of change (versus no change) in each subgroup defined by age decade and sex, using paired
Characteristics of subjects were shown in Table
Characteristics of subjects.
Women | Men | |
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Age (years) |
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Height (cm) | ||
All |
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40s |
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50s |
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60s |
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70s |
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Weight (kg) | ||
All |
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40s |
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50s |
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60s |
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70s |
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BMI (kg/m2) | ||
All |
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40s |
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50s |
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60s |
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70s |
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BMD at 1st wave | ||
Femoral neck (g/cm2) |
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Trochanter (g/cm2) |
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Lumbar spine (L2–4) (g/cm2) |
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BMC at 1st wave | ||
Femoral neck (g) |
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Trochanter (g) |
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Lumbar spine (L2–4) (g) |
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Area at 1st wave | ||
Femoral neck (cm2) |
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Trochanter (cm2) |
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Lumbar spine (L2–4) (cm2) |
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Values are mean ± SD.
The change rates (CR) from the first to fourth what were expressed as an annual rate. Mean variation between the two DXA measurements was 6 years.
ABMDs significantly decreased in all age strata both in women (−
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Sex difference analysis | ||||||
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women | men | 40s | 50s | 60s | 70s | ||
Femoral neck | BMD | 0.0126 | 0.1682 | <0.0001 | <0.0001 | <0.0001 | 0.0982 |
BMC | 0.0027 | 0.2519 | <0.0001 | <0.0001 | 0.0298 | 0.7122 | |
Area | 0.2084 | 0.9947 | 0.9436 | 0.0434 | 0.0987 | 0.2391 | |
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Lumbar spine |
BMD | <0.0001 | 0.006 | <0.0001 | <0.0001 | <0.0001 | 0.815 |
BMC | <0.0001 | 0.027 | <0.0001 | <0.0001 | <0.0001 | 0.4277 | |
Area | 0.0115 | 0.3383 | <0.0001 | <0.0001 | 0.0052 | 0.0986 |
(a) Changes in the femoral neck region by age group in women. Results are the mean (±SD) CR of four different age strata. **
ABMDs significantly decreased in women in their 40s, 50s, and 60s (−
(a) Changes in the lumbar spine region by age group in women. Results are the mean (±SD) CR of four different age strata. *
The patterns of aBMD changes were much different in men. BMDs significantly increased in the 50s, 60s, and 70s (
ABMD is equivalent to BMC divided by an area, but when we encounter cases of BMD decline, we simply consider the decline of the BMC at the measured sites without incorporating the change of the area (or size), which may represent the change of the shape in the region. The present study demonstrated that in the femoral neck, the aBMD decline in aging occurs not only due to the decline of BMC, but also due to the increase in the area, for both men and women. In fact, the increase of the femoral neck area represents the physiological compensating effect of the weakened bone tolerance [
From this perspective, the meaning or significance of aBMD change should be diverse depending on the sites measured and gender. Moreover, the apparent decrease of aBMD may not simply represent the weakness of that measured region (e.g., in the femoral neck), since the greater diameter can make the cylindrical structure stronger [
The limitation of this study is that the measurements were carried out by the ordinary DXA method without using elaborate software like hip structure analysis or CT. DXA has an inherent inaccuracy [
The strength of our study is its random selection of our samples from people in the local community with very little bias in the process. NILS-LSA is one of the few major epidemiological studies investigating the aging mechanism that is designed to select subjects in a completely random manner. The results of this study should therefore reveal characteristics of the entire Japanese population.
In summary, we investigated the meaning of aBMD changes in aging through separate analyses of BMC and area change. The results revealed that the significance of aBMD changes were very divergent among the sites measured, and between sexes. This may explain the dissociation of aBMD change and bone strength, which encourages one to be more cautious when interpreting the meaning of aBMD change.
This work was supported by the research fund for Longevity Sciences [