The Journal of Clinical Endocrinology & Metabolism Vol. 86, No. 5 1884-1887
Copyright © 2001 by The Endocrine Society
Serum Leptin Levels Are Associated with Bone Mass in Nonobese Women1
Julie A. Pasco,
Margaret J. Henry,
Mark A. Kotowicz,
Gregory R. Collier,
Madeleine J. Ball,
Antony M. Ugoni and
Geoffrey C. Nicholson
The University of Melbourne, Department of Clinical and Biomedical
Sciences, Barwon Health (J.A.P., M.J.H., M.A.K., G.C.N.), and
Department of General Practice and Public Health (A.M.U.), Metabolic
Research Unit, School of Health Sciences (G.R.C.), Victoria 3220,
Australia; and School of Biomedical Sciences (M. J. B.),
University of Tasmania, Tasmania 7250, Australia
Address all correspondence and requests for reprints to: Dr. J. A. Pasco, The University of Melbourne, Department of Clinical and Biomedical Sciences, Barwon Health, P.O. Box 281, Geelong 3220, Australia. E-mail: juliep{at}barwonhealth.org.au
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Abstract
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Both serum leptin and bone mineral density are positively correlated
with body fat, generating the hypothesis that leptin may be a systemic
and/or local regulator of bone mass. We investigated 214 healthy,
nonobese Australian women aged 2091 yr. Bone mineral content,
projected bone area, and body fat mass were measured by dual energy
x-ray absorptiometry and fasting serum leptin levels by RIA.
Associations between bone mineral content (adjusted for age, body
weight, body fat mass, and bone area) and the natural logarithm of
serum leptin concentrations were analyzed by multiple regression
techniques. There was a significant positive association at the lateral
spine, two proximal femur sites (Wards triangle and trochanter), and
whole body (partial r2 = 0.019 to 0.036; all
P < 0.05). Similar trends were observed at the
femoral neck and posterior-anterior-spine. With bone mineral
density the dependent variable (adjusted for age, body weight, and body
fat mass), the association with the natural logarithm of leptin
remained significant at the lateral spine (partial r2
= 0.030; P = 0.011), was of borderline significance
at the proximal femur sites (partial r2 = 0.012 to
0.017; P = 0.058 to 0.120), and was not significant
at the other sites. Our results demonstrate an association between
serum leptin levels and bone mass consistent with the hypothesis that
circulating leptin may play a role in regulating bone mass.
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Introduction
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LEPTIN IS A hormone with a diverse range of
local and systemic physiological functions. In addition to its role as
a satiety factor and involvement in regulating energy balance, leptin
modulates the reproductive (1, 2), hematopoietic, and
immune systems (3, 4, 5); promotes angiogenesis (6, 7); and is involved in brain development (8) and
regulation of carbohydrate metabolism (9, 10, 11). In
vitro studies have demonstrated a direct effect of leptin on human
marrow stromal cells, stimulating osteoblast differentiation and
mineralization of bone matrix (12). A role for leptin in
fetal bone metabolism has been suggested (13). Leptin has
also been implicated in the development of the periosteal envelope in
growing bone (1), suggesting an anabolic effect on the
skeleton.
Both serum leptin (14, 15, 16, 17, 18) and bone mass (19, 20) are positively correlated with body fat. Mechanical loading
on the skeleton (21) and/or the actions of a mediator
between adipose tissue and bone may contribute to the association
between body fat and bone mass (19). We hypothesized that
leptin may be such a mediator and, therefore, we have evaluated the
relationship between serum leptin concentrations and bone mass in
women.
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Materials and Methods
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Subjects
Subjects were from a large age-stratified sample of women drawn
at random from electoral rolls spanning the Barwon Statistical Division
in southeastern Australia for participation in the Geelong Osteoporosis
Study (22, 23). Serum leptin levels were determined for a
subgroup encompassing a wide range of body mass index (BMI) for
involvement in other studies (24, 25). As the exponential
relationship between body fat mass and circulating leptin levels in
nonobese subjects diminishes among the obese (14, 16, 25),
we excluded obese women [BMI > 30.0 (26)]. Three
hundred sixty-five women were eligible to participate in our study.
Subjects were also excluded if they were currently exposed to
glucocorticoids (n = 4) or the oral contraceptive pill (n =
73); were breast-feeding (n = 18); had a fasting plasma
glucose > 7.0 mmol/liter (n = 4), incomplete sets of scans
(n = 23); or had prostheses (n = 7), pacemakers (n = 1),
silicon implants (n = 2), or nonremovable jewelry (n = 29)
that would affect scan interpretation. None of the subjects was
pregnant or using hormone replacement therapy. Of the 214 nonobese
subjects included in the study (aged 2091 yr), 133 were
premenopausal, 67 were postmenopausal, and 14 had indeterminate
menopausal status. All were free from drugs and diseases known to
affect bone metabolism. The study was approved by the Barwon Health
Research and Ethics Advisory Committee, and informed consent was
obtained from all participants.
Measurements
Body weight and height were measured to the nearest 0.1 kg and
0.1 cm, respectively, and BMI calculated as
weight/height2 (kg/m2).
Dual-energy x-ray absorptiometry was performed using a Lunar Corp. (Madison, WI). DPX-L densitometer and analyzed with
Lunar Corp. DPX-L software version 1.31. Bone mineral
content (BMC) (g), areal bone mineral density (BMD)
(g/cm2), and projected bone area
(cm2) were measured at the spine in the
posterior-anterior (PA, L24) and lateral projections (L3), proximal
femur (femoral neck, Wards triangle, trochanter), whole body,
ultradistal (UD), and midforearm sites. In vivo short-term
precision for BMC, BMD, and projected area, respectively, was 1.4%,
0.6%, 1.5% for PA-spine; 2.9%, 3.4%, 4.0% for lateral spine;
2.9%, 1.6%, 2.3% for the femoral neck; 3.0%, 2.1%, 2.8% for
Wards triangle; 4.3%, 1.6%, 3.9% for the trochanter; 0.6%, 0.4%,
0.9% for the whole body; 1.6%, 2.1%, 1.7% for UD-forearm; and
0.8%, 1.1%, 0.9% for the midforearm. Body fat mass (g) was
determined from whole-body scans, with a precision of 3.8%. Venous
blood samples were collected following an overnight fast, separated by
centrifugation and stored at -80 C until analysis. Serum leptin
concentrations were determined by a commercial RIA (Linco Research, Inc., St. Louis, MO). The interassay coefficient of
variation ranged from 4.18.2%, and the intraassay coefficient of
variation was 5%.
Statistics
Serum leptin concentrations were transformed to the natural
logarithm (ln) to normalize the data before analysis. Regression
techniques (27) were used to develop equations for
predicting BMC and BMD at each site. Higher than linear adjustments for
age, centered about the mean to reduce collinearity (27),
were included for the forearm sites. Linear adjustments were made for
age at the other sites and for body weight and body fat mass at all
sites. BMC was also adjusted for projected bone area to correct for
differences in areas scanned. All between-predictor correlations were
<0.9, as required for valid regression analysis (27).
Menopause status was not included in the models for the entire sample
because its contribution was negligible. Partial
r2 values of the predictors (27)
were calculated for each predictor using site-specific models.
Significance was set at P < 0.05 and all statistical
analyses were performed using Minitab (release 12) software
package.
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Results
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Table 1
lists subject
characteristics. Median serum leptin (range) was 11.3 ng/ml
(2.189.3). Univariate analysis indicated that leptin (ln) was
correlated with all indices of body fatness: r = 0.60 for body
weight; r = 0.80 for body fat mass; r = 0.83 for per cent
body fat and r = 0.74 for BMI (all P < 0.001).
Correlations between leptin (ln) and BMC or BMD in the entire sample
and according to menopausal status displayed no consistent pattern
(Table 2
).
At the lateral spine and two proximal femur sites (Wards triangle and
trochanter), there was an independent positive association of leptin
(ln) with BMC (Table 3
) adjusted for age,
body weight, body fat mass and projected bone area (partial
r2 = 0.019 to 0.036; all P <
0.05). Similar trends were observed at the femoral neck and PA-spine
(partial r2 = 0.013, 0.011; P =
0.103, 0.137; respectively). At the whole body, body weight and body
fat mass were not significant predictors of BMC after adjusting for age
and bone area. Whether these variables were forced into the regression
equation, leptin (ln) remained a significant predictor of BMC
(P < 0.05). No association was detected between leptin
(ln) and BMC at the UD-forearm (P = 0.825), and a trend
toward a negative association was observed at the midforearm
(P = 0.158).
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Table 3. Multiple regression analysis with BMC (g) as the
dependent variable and age (yr), body weight (kg), body fat mass (g),
bone area (cm2), and leptin (ng/mL) (ln) as independent
variables in the models for the proximal femur sites (femoral neck,
Wards triangle, and trochanter), spine (PA and lateral), and whole
body for the whole cohort
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When BMD (adjusted for age, body weight, and body fat mass) was
analyzed (Table 4
), the effect of leptin
(ln) remained significant at the lateral spine (partial
r2 = 0.030, P = 0.011). The
association was of borderline significance at the proximal femur sites
(partial r2 = 0.012 to 0.017, P =
0.058 to 0.120) and was not significant at the other sites
(P > 0.05).
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Table 4. Multiple regression analysis with areal BMD
(g/cm2) as the dependent variable and age (yr), body weight
(kg), body fat mass (g), and leptin (ng/ml) (ln) as independent
variables in the models for the proximal femur sites (femoral neck,
Wards triangle, and trochanter), spine (PA and lateral), and whole
body for the whole cohort
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Among postmenopausal women, significant positive associations
between leptin (ln) and BMC were observed at the PA-spine and whole
body (P = 0.011 and 0.016, respectively); associations
at other sites, and using BMD as the dependent variable, were not
significant (P = 0.0820.443). No significant
associations were observed among premenopausal women.
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Discussion
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To our knowledge, this is the first reported association between
bone mass and serum leptin concentrations, independent of body weight,
and body fat mass. The regression models we used to test the
association between BMC and leptin included adjustments for body
weight, to account for the mechanical loading on the skeleton caused by
gravitational forces (21), and bone area, to correct for
differences in areas scanned. We also adjusted for body fat mass to
allow for the potential influence of other humoral factors associated
with adipose tissue (28). Using these models, significant
positive associations were demonstrated at two proximal femur sites
(Wards triangle and trochanter), the lateral spine, and whole body.
Leptin may have contributed to the variance attributed to body fat
mass; however, leptin alone explained a small proportion (14%) of
the variance in adjusted BMC. Significant associations between leptin
and adjusted BMC were observed at the PA-spine and whole body among
postmenopausal women. Smaller numbers in the separate analyses of
premenopausal and postmenopausal women may have limited our ability to
detect significant associations at other sites.
The high correlations between circulating leptin concentrations
and indices of adiposity have been reported previously
(14, 15, 16, 17, 18). In a study of 54 postmenopausal women (BMI
15.842.9 kg/m2), the positive association
between plasma leptin levels and BMC was no longer significant after
adjusting for body fat mass (17). The inclusion of obese
subjects may have diminished the association. There is a different
relationship between body fat mass and circulating leptin levels among
the obese, who display a wide range of serum leptin concentrations that
overlap those observed in lean subjects (14, 15). The
findings in the present study are reported for nonobese women alone for
whom there is an exponential relationship between body fat mass and
serum leptin. In accordance with results from our study, another study
of 94 adult women (18) reported no relationship between
leptin and BMD or bone geometry at the distal radius. The reasons for
the lack of association at non-weight-bearing sites remain unclear.
In cross-sectional studies (17, 18), no correlation was
observed between circulating leptin and markers of bone turnover.
Because the subjects were likely to be in a steady state with bone
formation coupled to resorption (29), it would seem
unlikely that any association would be observed. Changes in leptin
concentrations and bone turnover at the individual bone remodeling
units might not produce measurable systemic changes. However, the
hypothesis that there is a relationship between serum leptin and bone
turnover could be tested by producing pharmacological alterations in
serum leptin and measuring turnover response.
Unlike a recent study (30), earlier studies in mice
(31) and pubertal girls (1) suggested that
the effect of leptin on the skeleton occurs in cortical bone, whereas
leptin-treated ob/ob mice were shown to gain both trabecular
and cortical bone (32). Our data indicate the associations
between bone mass and serum leptin were not as strong when BMD rather
than BMC was used as the dependent variable. BMD is influenced by bone
size (33). The association between leptin and BMD
(i.e. the ratio of BMC/bone area) is conceptually different
from the association with BMC after adjusting for bone area, which
partially compensates for the confounding influence of bone size.
Furthermore, if leptin promotes periosteal bone apposition, the amount
of mineral at the bone-soft tissue interface might increase, resulting
in an increase in apparent bone area. Thus, BMD may remain relatively
unchanged because any increase in BMC would be offset by increases in
bone size. Further studies on the association among leptin
concentration, cortical thickness, and medullary width may indicate
whether an anabolic effect occurs at the endosteal or periosteal
surface of bone.
Weight loss or gain in adult women is associated with corresponding
changes in circulating leptin levels (14, 34) and bone
mass (35). Furthermore, reduced body fat
(36), reduced leptin levels (25), and reduced
bone mass (37) have been observed among smokers. These
patterns may suggest that changes in body fat may, in part, be
translated into changes in bone mass through fluctuations in
circulating leptin levels and/or other mediators of adipose tissue
origin.
Raised peripheral leptin levels may favor bone formation while
suppressing adipogenesis. At a local level, bone marrow adipocytes
produce leptin, which may enhance osteogenic activity and inhibit
adipogenic activity (12). Failure to identify leptin
receptors or leptin effects on osteoblasts in primary osteoblast
cultures from calvaria (30) may reflect changes associated
with osteoblast differentiation. Recent data suggest that leptin may
also inhibit fetal bone resorption (13).
This cross-sectional study of the relationship between bone mass
and circulating leptin levels does not address the question as to
whether leptin is involved in skeletal growth and the development of
peak bone mass. However, the results suggest that serum leptin may play
a role in regulating skeletal mass in nonobese adult women, and these
findings need to be explored in men. Further studies on the association
between circulating leptin levels and bone geometry may provide insight
into the mechanism of leptins effect on bone.
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Acknowledgments
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Our thanks are extended to S. Panahi and B. Skoric for their
help scanning subjects, A. de Silva and M. S. Solin for measuring
serum leptin concentrations, and B. M. Burgess for assistance in
collating data.
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Footnotes
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1 The project was supported by the Victorian Health Promotion
Foundation. 
Received August 9, 2000.
Revised December 7, 2000.
Accepted December 8, 2000.
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