ISSN: 2469-5718

Sasaki et al. Int J Sports Exerc Med 2019, 5:152
DOI: 10.23937/2469-5718/1510152
Volume 5 | Issue 11
Open AccessInternational Journal ofSports and Exercise Medicine
Research Article

Complex High Turnover in Bone Metabolism Impairs Bone Quality
in Female Long-Distance Runners with Menstrual Disorders
Eiji Sasaki1*, Yuki Fujita1, Katsuro Yoneda2, Shoko Kinugasa3, Ken Kato4, Eiichi Tsuda1, Yasuyuki
Ishibashi1 and Takashi Umeda2
Department of Orthopedic Surgery, Hirosaki University Graduate School of Medicine, Japan
Meijo University, Japan
3
Public Health Center, Okazaki Medical Association, Japan4
Milk Science Research Institute, Megmilk Snow Brand Co Ltd, Japan1
2

Check forupdates*Corresponding author: Eiji Sasaki, MD, PhD, Department of Orthopaedic Surgery, Hirosaki University Graduate School of
Medicine, 5 Zaifu-cho, Hirosaki, Aomori 036-8562, Japan, Tel: +81-172-39-5083, Fax: +81-172-36-3826
Abstract
Objectives: Menstrual disorder is a risk factor for stressfracture. Stress fractures impede running activity in female long-distance runners. However, the bone metabolism of female long-distance runners, and its relationshipwith bone quality, are not understood. We examined thecharacteristics of bone metabolism and bone quality infemale long-distance runners with menstrual disorder.
Methods: Sixteen female university long-distance runners
and 31 young female non-athlete volunteers were enrolled.
Self-reported menstrual condition was recorded. Serum estradiol, collagen type 1 cross-linked N-telopeptide (NTx),
propeptide of procollagen type I (P1NP) and pentosidine
concentrations were measured and compared between thegroups. Factors associated with serum pentosidine concentration were examined by regression analysis.
Results: Fifteen (93.8%) athletes and five (16.3%) controls had a menstrual disorder (p < 0.001). Serum NTx (p <0.001), total P1NP (p = 0.010) and pentosidine (p < 0.001)
concentrations were significantly higher in athletes. There
was a positive correlation between serum NTx and total
P1NP concentrations (ρ = 0.448, p = 0.002). Multivariate
linear regression analysis identified a significant positive
correlation between serum pentosidine and NTx concentrations (p = 0.030), and negative correlations between serum
pentosidine and serum estradiol (p = 0.035) and total P1NP
(p = 0.030) concentrations.
Conclusion: Markers of bone absorption and formation
were elevated in female long-distance runners with menstrual disorder compared with healthy controls, and there
were positive correlations between the markers. The com

plex high state was distinct to female long-distance runners
with menstrual disorder, and higher bone absorption impaired bone quality.

Introduction
Female athletes are at risk of menstrual disorderand osteoporosis as a result of the relative energy
deficiency caused by their sporting activities [1,2].
Stress fracture occurs when there is an imbalance between bone formation and bone absorption as a result of repeated cyclic loading [3], which is common
in female athletes and may prevent them from running for prolonged periods. The prevalence of stressfracture is significantly higher in female long-distance runners than women who participate in othersports [4,5], its incidence is reportedly as high as 50%
per year [6]. Female athletes are at risk as a result of
reduced bone mineral density, low body weight andchronic menstrual disorders [7-9]. Low body weight
and fat content are critical for elite endurance athletes. It has been challenging to identify athletes athigh risk for stress fracture so as to prevent them, because the underlying pathophysiologic mechanisms
causing stress fracture remain elusive [10].
Bone metabolic markers are attracting an increasing amount of attention as objective tools for the

Citation: Sasaki E, Fujita Y, Yoneda K, Kinugasa S, Kato K, et al. (2019) Complex High Turnover in BoneMetabolism Impairs Bone Quality in Female Long-Distance Runners with Menstrual Disorders. Int J
Sports Exerc Med 5:152. doi.org/10.23937/2469-5718/1510152
Accepted: November 07, 2019; Published: November 09, 2019
Copyright: © 2019 Sasaki E, et al. This is an open-access article distributed under the terms of theCreative Commons Attribution License, which permits unrestricted use, distribution, and reproduction
in any medium, provided the original author and source are credited.
Sasaki et al. Int J Sports Exerc Med 2019, 5:152• Page 1 of 8 •DOI: 10.23937/2469-5718/1510152

monitoring of bone condition. Conventional evaluation techniques such as Bone Mineral Density (BMD),
micro-computed tomography, scintigraphy and magnetic resonance imaging are well-established meansof diagnosing stress fracture [11-13], but do not reflect the metabolic condition of bone. It has beenreported, however, there is a relationship between
high bone absorption and stress fracture in athletes[14], raising the possibility that athletes at risk ofstress fracture could be identified before they wereaffected, and preventative measures put in place.
Nevertheless, there is conflicting evidence that bonemetabolic markers do not predict stress fracture incombat [15]. The inconsistency of the evidence has
meant that bone metabolic markers are not routinely used in clinical practice, but we hypothesized thatthey may help illuminate the etiology and mechanismof stress fracture in elite long-distance runners.
It is well recognized that bone quality contributes
to bone strength as much as bone mineral density [16].
Postmenopausal women with primary osteoporosis
have lower bone mineral density, and elevated serum
pentosidine (sPEN) concentration, compared with agematched controls without osteoporosis [17]. Poor bonequality is reportedly significantly associated with the incidence of femoral fractures, even if BMD is adequate[18]. Few studies have investigated bone quality in athletes, although there has been a report suggesting that
there is a relationship between some (but not all) bonequality markers and history of stress fracture in elite
university lacrosse players [19]. The nature of the relationship between bone metabolism in female athletes
and bone quality remains unclear.
A fuller understanding of bone metabolism andbone condition in female athletes would illuminate
the underlying mechanisms of stress fracture, andmay allow stress fracture to be predicted, and thus
avoided. We undertook a study to investigate theinfluence of menstrual disorder on systemic bonemetabolic markers and bone quality markers in female long-distance runners, due to their high risk ofstress fracture, and compare these with healthy female volunteers as a control group. We also sought
to examine the relationship between bone metabolic
markers and bone quality markers. We hypothesizedthat female long-distance runners would have specific bone metabolism features that influenced bonequality, and that these would be influenced by menstrual disorders.

Methods
Subjects
We enrolled 19 elite female long-distance runners
belonging to the ekiden club of our institution, and 32
healthy young women who had volunteered to participate in the Iwaki Health Promotion Project, having
Sasaki et al. Int J Sports Exerc Med 2019, 5:152

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responded to mass media advertisements and been
recruited by public health nurses. The Iwaki Health
Promotion Project is a community-based preventative medicine program that aims to improve life expectancy by performing general health check ups andprophylactic interventions [20-22]. Female long-distance runners were defined as the athlete group, andthe volunteers as the control group. In the athletegroup, three subjects under treatment for fracture
or menstrual disorders by hormone or oral contraceptive supplementation were excluded. Subjects inthe control group were aged 18-25 years; one was
excluded as she was taking an oral contraceptive. Ultimately, there were 16 subjects in the athlete groupand 31 in the control group. The study was conductedin May 2015 in athlete group, and May 2014 and May
2015 in the control group. All participants gave written, informed consent to participate, and the study
was conducted with the approval of our institutional
ethics committee.
Demographic data, including height and weight,
were measured in all subjects by the investigators, andBody Mass Index (BMI) was calculated. The percentage
of fat volume per body weight (% fat) was estimated by
the impedance method (MC-190, Tanita Co. Ltd., Tokyo,
Japan). The history of stress fracture was established bymeans of a self-reported questionnaire regarding fracture site and frequency.

Biochemical examination
Fasting venous blood specimens were taken fromall subjects in the early morning for biochemical examination. Serum was obtained by centrifugation,
separated into aliquots and stored at -80 °C for later
analysis. The serum concentrations of the following
gonadotropins were examined; luteinizing hormone(LH, mIU/ml); follicle stimulating hormone (FSH, mIU/
ml), prolactin (ng/ml), estradiol (pg/ml) and testosterone (ng/ml) by our institution’s clinical laboratory. Bone metabolism markers consisted of indices ofbone absorption, bone formation and bone quality
markers, in addition to serum calcium (mg/dl) and inorganic phosphate (mg/dl) concentrations. The boneabsorption marker was serum type I collagen crosslinked N-telopeptide (sNTx, nM bone collagen equivalents/l (BCE/l)) concentration, measured by enzyme
immunoassay. The bone formation marker was total
N-terminal propeptide of procollagen type I (P1NP,µg/l) concentration, also measured by enzyme immunoassay. The bone quality marker was serum pentosidine (sPEN, pmol/ml) concentration, measured byhigh performance liquid chromatography. We referred to Japanese Osteoporosis Society guidelines
for the normal ranges of sNTx (7.5-16.5 nM BCE/l)and sP1NP (14.9-68.8 µg/l) [23].

Bone mineral density measurement• Page 2 of 8 •DOI: 10.23937/2469-5718/1510152

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Bone mineral density was measured by the Dual-Energy X-Ray Absorptiometry (DEXA) method. In a pilot
study in the athlete group, we investigated the relationship between serum estradiol concentration and BMD
in the lumbar vertebral body, femoral neck and distal radius. Spearman’s correlation analysis identified correlation coefficients of 0.45 (p = 0.082) in the lumbar vertebral body, 0.53 (p = 0.034) in the femoral neck and 0.69
(p < 0.001) in the distal radius. Consequently, we judgedthat BMD in the distal radius most strongly reflected the
gonadotropic status, and adopted it for further analyses(Table 1).

Menstruation status
Menstruation status was surveyed by self-reported
questionnaire. Presence or absence of menstruation,
cycle length, regularity, first menstruation and oral contraceptive use were recorded. We defined normal menstruation as regular menstruation in a cycle of 25-38
days, and irregular menstruation as a delayed menstruation cycle from 39 days to 3 months. Primary amenorrhea was defined as never having menstruated, andsecondary amenorrhea was defined as absence of menstruation for > 3 months [24].

Statistical analysis
We performed power analysis to estimate thesample size sufficient for regression analysis of theconcentrations of sNTx and sPEN using data from another pilot study of 16 healthy young female participants in the Iwaki Health Promotion Project in 2014.
These data indicated that the Standard Deviation (SD)of the x-axis was 3.37, and the SD of the regression
Table 1: Relationship between serum estradiol concentration
and bone mineral density in lumbar vertebra, femur and distal
radius.
Serum estradiol concentrationρ

p-value

Lumbar vertebra

0.45

0.082

Femoral neck

0.53

0.034

Distal radius

0.69< 0.001

Spearman’s correlation coefficients between bone mineraldensity and serum estradiol concentration were calculated in
the athlete group as a pilot study.

errors was 5.59. If the true slope of the line obtainedby regressing the concentration of sPEN against thatof sNTx was 0.888, we would have needed to study
29 subjects to be able to reject the null hypothesisthat the slope equalled zero with a probability (power) of 0.8. The type I error associated with this testof the null hypothesis was 0.05. As 47 subjects wereenrolled to this study, we were able to reject the null
hypothesis with a power of 0.999 [25].
The demographic and biologic data of the athlete and control groups were compared using theMann-Whitney U test. The proportions reporting
each menstrual disorder were compared by the chisquared test. Spearman’s correlation coefficient (ρ)
was calculated to examine the relationship betweensNTx and P1NP concentrations to examine the extent
of their contribution to bone metabolic turnover. The
BMD in the distal radius was examined according toparticipants’ menstruation status, and compared using analysis of variance (ANOVA) and Tukey’s post hoc
test. To identify risk factors for impaired bone quality, linear regression analysis was performed with thesPEN concentration as the dependent variable, andage, BMI, and serum gonadotropin and bone metabolism marker concentrations as dependent variables.
Data input and analyses were undertaken with SPSS(version 12.0J, SPSS Inc., Chicago, IL, USA). A p-value< 0.05 was considered statistically significant.

Results
The mean age of the athletes was 19.7 ± 1.1 years,
compared with 22.7 ± 1.6 years in the control group (p< 0.001). Long-distance runners had significantly lower
body weight (p < 0.001), lower BMI (p < 0.001) and lower % fat (p < 0.001) than controls (Table 2). Three subjects in the athlete group (17.6%) reported a history ofstress fracture. Indeed, these athletes reported multiple
incidences of stress fracture, affecting the fibula, metatarsal, sacrum, tibia and navicular. Fifteen of the 16 athletes (93.8%) reported menstrual disorders, comparedwith 16.1% of the control group (p < 0.001, Table 3). Six
athletes had irregular menstruation, seven secondary
amenorrhea and two primary amenorrhea. Mean age atfirst menstruation (excluding those with primary amenorrhea) was significantly delayed by 3 years in the ath-

Table 2: Demographic data of subjects.
Control

Athletep-value

Sample number

31

16

-

Age (y.o.)22.7 ± 1.6

19.7 ± 1.1< 0.001

Height (cm)157.4 ± 6.2

158.8 ± 5.2

0.363

Body weight (kg)51.8 ± 6.8

44.9 ± 4.6< 0.001

BMI (kg/m )21.0 ± 3.1

17.8 ± 1.2< 0.001% fat (%)

28.4 ± 6.7

15.2 ± 4.6< 0.001

2

Data are presented as the mean ± standard deviation. The Mann-Whitney U test was performed to compare groups. A p-value <0.05 was considered significant. Abbreviation: BMI: Body Mass Index.
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Table 3: Comparison of gonadotropin, bone metabolism markers and bone quality markers in athlete and control groups.
Control

Athletep-value

LH (mIU/ml)

6.4 ± 7.6

3.9 ± 3.3

0.281

FSH (mIU/ml)

4.3 ± 2.5

5.3 ± 2.5

0.217

Gonadotropin concentration

Prolactin (ng/ml)

20.4 ± 16.4

8.6 ± 2.2

< 0.001

Estradiol (pg/ml)

80.5 ± 61.9

20.8 ± 10.5

< 0.001

Testosterone (ng/ml)

0.4 ± 0.1

0.4 ± 0.1

0.225
P < 0.001

Menstrual status
Normal menstruation

26 (83.9%)

1 (6.3%)

Irregular menstruation

5 (16.1%)

6 (37.4%)

Secondary amenorrhea

0 (0%)

7 (43.8%)

Primary amenorrhea

0 (0%)

2 (12.5%)

First menstruation (y.o.)11.9 ± 1.2

14.8 ± 2.0< 0.001

Gonadotropin values are presented as the mean ± standard deviation. Values in parentheses indicate the proportion (%) in eachgroup. The concentrations of gonadotropin were compared using the Mann-Whitney U test. The proportions reporting menstrual
disorders were compared using the chi-square test. A p-value < 0.05 was considered significant.
Abbreviations: LH: Luteinizing Hormone; FSH: Follicle Stimulating Hormone.

Bone mineral density of radius (g/cm2)

＊
＊

0.8
0.6
0.4
0.2
0
Normalmenstruation

Irregular
menstruation

Secondaryamenorrhea

Primary
amenorrhea

Menstrual condition
Figure 1: Comparison of bone mineral density in radius among menstrual conditions.
Values were mean and standard deviations of bone mineral density in normal menstruation, irregular menstruation, secondary menstruation, and primary amenorrhea, and were compared by one-way analysis of variates, and Tukey test was
performed as a post hoc. A p-value below 0.05 compared with normal menstruation (*), irregular menstruation (†), andsecondary menstruation (#) were significant.
Table 4: Correlation between concentration of bone metabolism markers and bone mineral density in the distal radius.
Distal radiusρp-value

Serum NTx

-0.60

0.001

Serum total P1NP

-0.17

0.395

Serum pentosidine

-0.59

0.001

Spearman’s correlation coefficients between bone mineraldensity in radius and concentrations of bone metabolic markers
were calculated in all subjects.
Sasaki et al. Int J Sports Exerc Med 2019, 5:152

lete group compared with controls (Table 3).
The distal radial BMD in the athlete group was 0.60
± 0.04 g/cm2, compared with 0.68 ± 0.06 g/cm2 in thecontrol group: ANOVA indicated that BMD was significantly reduced with increased severity of menstrual
disorder (p = 0.002, Figure 1). Also, BMD in subjects
with amenorrhea was significantly lower than those
who menstruated normally. There were significant
relationships between distal radial BMD (estimatedby serum estradiol concentration) and both sNTx con• Page 4 of 8 •DOI: 10.23937/2469-5718/1510152

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Table 5: Comparison of bone metabolism markers and bone quality marker between athlete and control groups, and the proportion with values over the normal range.

Calcium

Normative

Control group (n = 31)

range

Mean

Out of range Mean

Out of range

8.4 - 10.4

9.5 ± 0.4

02

(mg/dl)iPhosphate

2.5 - 4.5

3.9 ± 0.6

7.5 - 24.0

16.4 ± 3.8

14.9 - 68.8

51.4 ± 17.5

sPEN

4

4.1 ± 0.4

1

26.7 ± 5.8

4

15.6 - 43.0

21.4 ± 6.2

12< 0.001(75.0%)
76.0 ± 36.8

0

0.192

(18.8%)

7(12.9%)(pmol/ml)< 0.0013(3.2%)(µg/l)

p-value(12.5%)(12.9%)(nM BCE/l)
Total P1NP

10.0 ± 0.4(0%)(mg/dl)sNTxAthlete group (n = 16)

0.031(43.8%)
37.4 ± 7.8

3

(0%)

< 0.001(18.8%)

The concentration of bone metabolism markers and bone quality markers were compared using the unpaired Mann-Whitney U
test. The normal ranges of each bone metabolism marker were as follows: NTx, 7.5 - 24.0 nM bone collagen equivalents [BCE]/l;total P1NP, 14.9 - 68.8 µg/l; pentosidine, 15.6 - 43.0 pmol/ml. A p-value < 0.05 was considered significant.

180

AthleteSerum P1NP concentration (µg/l)

160

Control

140
120
100
80
60
40
20
0
0

10

20

30

40

50

sNTx concentration (nM BCE/l)
Figure 2: Scatter plot of bone metabolism markers.
Abbreviations: NTx: Type I collagen cross-linked N-telopeptide; P1NP: N-terminal propeptine of procollagen type I. Cut
offline was cited from reference data for diagnosis of osteoporosis [23].

centration (ρ = -0.60, p = 0.001) and sPEN concentration (ρ = -0.59, p = 0.001) in the athlete group (Table4).
For indices of bone metabolism, the concentrationsof sNTx (p < 0.001) and total P1NP (p = 0.010) were significantly higher in the athlete group than the control
group (Table 5). The sNTx concentration was above thenormal range in all athletes (100%), and serum P1NP
was above the normal ranges in eight (47.1%) athletes,
respectively. For indices of bone quality, the concentration of sPEN in the athlete group was significantly highSasaki et al. Int J Sports Exerc Med 2019, 5:152

er than controls (p < 0.001), and was above the normal
range in three subjects (17.6%). There was a positive
correlation between sNTx and total P1NP concentrations (ρ = 0.448, p = 0.002; Figure 2).
Multivariate linear regression analysis identified asignificant positive correlation between higher sPEN
concentration and increasing sNTx concentration (p =0.030), and a negative correlation between higher sPEN
concentration and reducing serum estradiol concentration (p = 0.035) and serum total P1NP concentration (p= 0.030). There was no significant relationship between• Page 5 of 8 •DOI: 10.23937/2469-5718/1510152

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Table 6: Correlation coefficients between serum pentosidine concentration and bone metabolism markers.
Correlation coefficients

p-value

Adjusted R2

Age

-0.13

0.416

0.52

Body mass index

-0.26

0.032

Prolactin

-0.14

0.230

Estradiol

-0.28

0.035

Testosterone

0.16

0.167

Serum NTx

0.39

0.030

Serum total P1NP

-0.29

0.030

Linear regression analysis was performed with the serum pentosidine concentration as the dependent variable, and age, bodymass index, concentration of terminal gonadotropin and bone metabolic markers as independent variables. Values of correlation
coefficients indicated standardized coefficients with serum pentosidine concentration, and minus values in coefficients indicatedthe negative correlation. Abbreviations: NTx: Type I collagen cross-linked N-telopeptide; P1NP: N-terminal propeptine of procolla.

sPEN concentration and serum total P1NP concentration (Table 6).

Discussion
Our cross-sectional comparison study found thatindices of bone absorption and bone formation weresignificantly higher in female long-distance runners
with menstrual disorder than healthy young female
volunteers. We also found that there were significant
relationships between markers of bone absorption
and bone formation. The bone metabolism of female
long-distance runners exhibited high turnover but
differed from the bone metabolism typical of osteoporosis. We have elected to describe this distinct pattern of bone metabolism as ‘complex high turnover’,
mainly a combination of high bone absorption related
to menstrual disorder and high bone formation as aresult of repeated microdamage caused by running.
We also found that high bone absorption was significantly correlated with deterioration in bone quality
reflected by sPEN concentration.

Complex high turnover bone metabolism
The bone metabolism of female long-distance runners was characterized by high bone absorption andhigh bone formation, which we defined as a ‘complex
high turnover’ state when associated with menstrual
disorder. It is well-recognized that bone metabolism
changes to a high turnover state after menopause[26], when diminished estrogen secretion induces anincrease in the expression of receptor activation of
nuclear factor-κB ligand (RANKL) in osteoclasts, which
in turn accelerates bone absorption [27]. This process
also induces a weak increase in bone formation ability via the RANK-RANKL pathway [28], but one that is
insufficient to shift the bone metabolic balance fromthe bone absorption responsible for osteoporosis. In
the female athletes in our study, higher bone formation ability was induced by the repeated stimulationof running as well as activation of bone absorption
by menstrual disorder. Repeated stimulation reportedly promotes bone formation ability [29]. UltimateSasaki et al. Int J Sports Exerc Med 2019, 5:152

ly, in these elite endurance runners the combinationof high bone absorption and bone formation led to acomplex high turnover state unlike that seen in postmenopausal osteoporosis.

Problems with complex high turnoverComplex high turnover bone metabolism would be
likely to predispose athletes to stress fracture. Bonefragility reportedly begins with the micro-cracks andvoids [30], the accumulation of which cause non-traumatic fracture in osteoporosis [31]. Barr and colleagues
reported that accumulation of microdamage is a consistent risk for fracture in dogs, when the number of fracture sites increased when bone metabolism was strongly suppressed by high-dose bisphosphonate administration [32]. High turnover bone metabolism is rarely seenin health young adults, and the bone remodeling systemgenerally acts to repair microdamage when it occurs.
However, more rapid bone remodeling leads to the development of wider remodeling spaces, which become
weak points. Thus, the repetitive loading of long-distance running may cause substantial microdamage tothe lower extremity, resulting in elevated bone formation and pace of bone remodeling. We propose that an
excessive state of bone metabolic turnover would likely
increase the risk for stress fracture in athletes.

Degradation of bone quality
We found that the concentration of sPEN, a systemic bone quality marker, rose in parallel with sNTx
concentration. Pentosidine is a part of the structureof an Advanced Glycation End product (AGE), and its
serum concentration reflects the concentration in collagen in bone. In primary osteoporosis, deteriorationin bone quality is induced by age and gonadotropin
deficit [18]. Furthermore, oxidative stress, glycationstress related to renal failure and diabetes mellitus
all promote excessive production of AGEs [33]. These
data support our finding that the sPEN concentration was positively correlated with serum estradiol
and sNTx concentrations. Mitome and colleagues reported that accumulation of pentosidine in collagen• Page 6 of 8 •DOI: 10.23937/2469-5718/1510152

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correlated with a reduction in bone formation [33].
Given these findings, it was thought that, of the bonemetabolic markers, only those of bone absorption
would be significantly correlated with sPEN concentration, essentially the opposite to the negative correlation with bone formation. Increasing sPEN concentration with age is an independent risk factor for
reduced bone strength [34]. Shiraki and colleagues
have reported the influence of genetic determinants
upon impaired bone quality. The hazard ratio for
fracture with the combination of the methylenetetrahydrofolate reductase polymorphism (C677T) disorder of bone quality and osteoporosis was 7.21 (95%
confidence interval 4.34-11.97) [35]. Furthermore, ininjured sites with micro damage, macrophages produce tumor necrosis factor-α and interleukin-1β [36],which would further reduce bone quality.

loading and increased remodeling space) that likely
increase the risk for stress fracture. While long-term
follow-up and further investigation of symptoms
and training intensity are necessary, bone metabolic markers may contribute to our understanding ofstress fracture and its risk factors.

Limitations and future directions

Conflict of Interest

Our study had several limitations besides its small
sample size. First, most long-distance runners had
menstrual disorders. In previous studies, the proportion of female athletes experiencing menstrual
disorder ranged from 3-66%, compared with 2-5%in the general female population [37]. The likely explanation is the nature of junior athletics in Japan,
where the intensity of physical training required tocompete in national competitions is so high that menstrual disorders are very common. Our findings, andthose of others, underline the importance of managing menstrual disorder appropriately, as amenorrhea
has consistently been identified as a risk factor for
stress fracture [38]. Second, we found no significant
difference in LH or FSH between the groups, whenmenstrual disorder would be expected to suppress
hypothalamic function. Physiologic cyclical changes
in serum gonadotropin concentration may have influenced our results. Third, relatively few of our athletes
had experienced stress fracture. While we were able
to illuminate the complex high turnover bone metabolism state associated with menstrual disorder in female long-distance runners, our sample was too small
to establish whether athletes with stress fractures
exhibited the same bone metabolic pattern. Further
research is needed to establish the characteristics ofbone metabolism in athletes with stress fracture, andthe nature of the relationships between complex highturnover, bone quality and stress fracture. A large,
prospective cohort study will be needed to identify
markers of bone metabolism and quality that predict
stress fracture.

This study was supported in a part by Megmilk Snow
Brand Co., Ltd. (Tokyo, Japan). The authors have nocompeting interests to declare.

Despite these limitations, we found that female
long-distance runners with menstrual disorder exhibited a complex high turnover bone metabolism state,consisting of a combination of high bone absorption(caused by menstrual disorder) and high bone formation (caused by microdamage provoked by repeated
Sasaki et al. Int J Sports Exerc Med 2019, 5:152

Conclusions
Markers of bone absorption and formation weresignificantly elevated in female long-distance runners
with menstrual disorders compared with healthy young
women, and there were positive correlations between
the markers. The complex high turnover bone metabolism state that we identified appears to be distinct to female long-distance runners. Furthermore, higher boneabsorption impaired bone quality, creating the conditions required for stress fracture.

Acknowledgements
The authors would like to thank the long-distancerunners in Meijo University Women’s Ekiden Club, whokindly agreed to participate in this study.

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