Effects of a Prolonged Submersion on Bone Strength and ...

Calcif Tissue Int (2010) 86:8–13
DOI 10.1007/s00223-009-9308-9

Effects of a Prolonged Submersion on Bone Strength
and Metabolism in Young Healthy Submariners

Tal Luria • Yinnon Matsliah • Yochai Adir •
Noam Josephy • Daniel S. Moran • Rachel K. Evans •
Amir Abramovich • Alon Eliakim • Dan Nemet

Received: 23 April 2009 / Accepted: 28 September 2009 / Published online: 31 October 2009
Ó Springer Science+Business Media, LLC 2009

Abstract Submariners taking part in prolonged missions exhibited continued decline at 4 weeks after return to shore
are exposed to environmental factors that may adversely and returned to baseline levels at the 6-month follow-up.
affect bone health. Among these, relatively high levels of There was a significant increase in circulating calcium
CO2, lack of sunlight exposure affecting vitamin D level. PTH and 25(OH)D levels decreased significantly.
metabolism, limited physical activity, and altered dietary Significant decreases were observed in both TRAP5b and
habits. The aims of this study were to examine the effect of CTx levels, markers of bone resorption, as well as in N-
a prolonged submersion (30 days) on changes in bone terminal propeptide of type I collagen (PINP), a bone
strength using quantitative bone speed of sound and in formation marker. Prolonged submersion led to a signifi-
markers of bone metabolism that include bone turnover cant decrease in bone strength, accompanied by an overall
(BAP, PINP, TRAP5b, and CTx) and endocrine regulators decrease in bone metabolism. Bone strength was regained
(serum calcium, PTH, and 25[OH]D) in a group of 32 only 6 months after return to shore. Prevention and/or
young healthy male submariners. The prolonged submer- rehabilitation programs should be developed following
sion led to increases in body weight and BMI and to a periods of relative disuse even for young submariners. The
decrease in fitness level. There was a significant decrease effects of repeated prolonged submersions on bone health
in bone strength following the submersion. Speed of sound are yet to be determined.

T. Luria Á Y. Matsliah Á Y. Adir Á N. Josephy Á A. Abramovich Keywords Bone density technology Á Ultrasound Á
Israel Naval Medical Institute, IDF Medical Corps, Haifa, Israel Osteoporosis Á Exercise Á Physical factors Á Mechanical
loading
Y. Matsliah
e-mail: [email protected] Prolonged submersion exposes submariners to environ-
mental factors that are thought to affect health [1]. Sub-
D. S. Moran mariners taking part in prolonged missions are exposed to
Heller Institute of Medical Research, Sheba Medical Center, relatively high levels of carbon dioxide (CO2); suffer from
Tel Hashomer, Israel lack of sunlight exposure, affecting vitamin D metabolism;
are limited in performing physical activity; and are sub-
R. K. Evans jected to altered dietary habits. All these factors are known
Military Performance Division, U.S. Army Research Institute to play an important role in the determination of bone
of Environmental Medicine, Natick, MA, USA health [2–4].

A. Eliakim Á D. Nemet Loss of bone density (bone mass per volume) weakens
Child Health and Sports Center, Meir General Hospital, its structural integrity, which may affect fracture suscep-
Sackler School of Medicine, Tel Aviv University, Tel Aviv, tibility. Remodeling of the bone is a process occurring
Israel throughout life, balancing between absorption of old bone
and deposition of new bone [5]. Bone density peaks in the
D. Nemet (&)
Department of Pediatrics, Meir General Hospital,
59 Tchernichovski St., Kfar-Saba 44281, Israel
e-mail: [email protected]

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T. Luria et al.: Bone Health in Submariners 9

second and third decades of life. Bone density in old age is Procedures
directly connected to the peak bone mass achieved in
young adulthood. Hence, failure to achieve peak bone mass With the exception of the fitness measures, which were
or exposing people to unfavorable conditions that affect taken within 1 week of the submersion (prior to
bone mass maintenance might increase risk for osteopo- deployment and after return), measures were taken on the
rotic fractures later in life [6]. day of deployment (baseline) and on the day of return to
shore (1 month post deployment). Bone strength was
In recent years quantitative ultrasound (QUS) measure- additionally measured 4 weeks and 6 months after return
ments of tibial speed of sound (SOS) were developed for to shore.
the diagnosis of osteoporosis [7, 8]. QUS is a relatively
inexpensive, portable method and involves no ionizing Experimental Condition
radiation. QUS measurements of bone were found to cor-
relate significantly with measurements of bone mineral The participants were submerged for 30 days in a Dolphin
density by dual-energy X-ray absorptiometry (DXA) in class submarine. This is a non-nuclear submarine, using
both adults and children [9–11]. QUS is used to derive an diesel-electric propulsion. The Dolphin-type submarine is
index of ‘‘bone strength’’ based on the speed of sound wave among the smallest submarines; its dimensions are
propagation through the heterogeneous bone tissue, 57 9 6.8 9 6.2 m. Submarine CO2 content ranged 0.8–
reflecting both quantitative (mineralization) and qualitative 1.2%.
(elasticity, microarchitecture, and fatigue damage) prop-
erties of bone [8, 10, 12]. QUS of bone has emerged as a Anthropometric Measurements
promising technique for evaluation of bone strength and
fracture risk [13, 14] and was shown to detect measureable Standard, calibrated scales and stadiometers were used to
changes in bone strength following relatively short periods determine height, weight, and body mass index (BMI,
of immobilization [15]. weight/height2). Thigh and calf circumferences were
measured 20 and 10 cm above and below the tibial tuber-
The use of laboratory markers for biochemical analysis osity, respectively. Measurements were conducted on the
of bone turnover [16, 17] provides additional information day of deployment and on the day of return to shore, by the
on the influence of different stressors on bone metabolism. same trained technician. Measurements were performed
Markers of bone formation include bone-specific alkaline three times, and their mean was recorded.
phosphatase (BAP) and N-terminal procollagen peptide
(PINP), which are released to the circulation during col- Evaluation of Fitness and Physical Activity
lagen synthesis. Serum levels of tartrate-resistant acid
phosphatase (TRAP) and type I carboxy-terminal telopep- Each volunteer performed a 2,000 m run. This outdoor
tide (CTx) are used as markers of bone resorption reflecting field test is a routine test in the army and participants are
osteoclastic activity [18]. well habituated to it. Recorded measures were endurance
time (seconds) and heart rate (before and immediately
The aims of this study were to examine the effect of following the run). The 2,000 m run was performed in the
prolonged submersion on changes in bone strength using morning, when ambient conditions are comfortable.
QUS and in markers of bone metabolism including bone
turnover markers (BAP, PINP, TRAP5b, and CTx) and During the submersion, participants were asked to fill a
endocrine regulators (serum calcium, PTH, and 25[OH]D) daily log of their physical activity (type, duration, and
in a group of young, healthy submariners. intensity), if performed.

Methods

Sample Population Blood Sampling Protocol

The study was approved by the Committee for Research on Early morning fasting (following a 12-hour fast) venous
Human Subjects, Israeli Defense Forces Medical Corps, blood samples were collected using standard phlebotomy
and informed consent was obtained from all participants. techniques. Samples were placed in an ice bath and
Thirty-two young (22.8 ± 3.8 years), healthy male sub- immediately taken to centrifugation. Aliquots of the
mariners were recruited to participate in this study. No resulting serum were stored at –80°C until analysis. All
subjects were on any medications or dietary supplements pre- and postsubmersion specimens were analyzed in the
before or at the time of study and follow-up. same batch by technicians who were blinded to the order of
the samples.

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10 T. Luria et al.: Bone Health in Submariners

Serum Measurements number of small probes, designed to measure SOS at dif-
ferent sites. The measurement site was defined as the
Bone Turnover Markers midpoint between the apex of the medial malleolus and the
distal patellar apex. The probe was then moved across
BAP, a measure of bone formation, was assayed by the mid-tibial plane to search for the site with maximal
enzyme-linked immunosorbent assay (ELISA, OcteiaTM reading. The mean of at least three measurements was
OctaseÒ BAP Immunoenzymetric Assay; IDS, Tyne and selected for data analysis. All measurements were per-
Wear, UK), which is specific to the bone isoform. The formed by the same trained technician. The instrumental
interassay coefficient of variation (CV) was 6%. Reference accuracy is 0.25–0.5%, and the precision is 0.4–0.8%.
values (mean ± SD) for healthy men are 12.3 ± 4.3 lg/L,
with a reference range of 3.7–20.9 lg/L. Statistical Analysis

PINP was measured by the UNiQ radioimmunoassay Paired t-tests were used to compare pre and post mea-
from Orion Diagnostica (Espoo, Finland). The interassay surements. Linear (Pearson) correlation was used to eval-
CV was 9.5%. The reference range of values for healthy uate the relationship between bone SOS, bone metabolism
men is 22.0-87.0 lg/L. markers, and anthropometric measures. Data are presented
as mean ± SD. Statistical significance was set at P \ 0.05.
TRAP5b was measured by ELISA using the Bone-
TRAPÒ assay (IDS). The interassay CV was 8%. Refer- Results
ence values (mean ± SD) for young men are 3.06 ± 0.88
U/L and 2.59 ± 0.78 U/L, respectively. The upper limit for Anthropometric and Fitness Data
normal men is 4.82 U/L, with a reference range of 1.3-4.8
U/L. Characteristics of the study participants are summarized in
Table 1. Body weight and BMI increased significantly
CTx (Serum Crosslaps) were measured by ELISA kits following the deployment, while thigh and calf circum-
from Nordic Bioscience Diagnostics (Herlev, Denmark). ference, as well as fitness (assesed by 2,000 m run), were
The interassay CV was 5%. Reference values for healthy significantly lower in the submariners following the pro-
men are 0.115 ± 0.748 ng/mL, with a reference range of longed submersion.
0.010–0.712 ng/mL.
Bone SOS
Endocrine Regulators
There was a significant decrease in tibial SOS following the
Albumin and calcium were both measured using a DXC600 submersion (from 4,081 ± 17 to 4,044 ± 16, P \ 0.02).
Pro (Beckman Coulter, Fullerton, CA). Interassay CVs SOS exhibited continued decline at 4 weeks after return to
were 1.4% and 1.7%, respectively. Reference values shore (4,011 ± 1, P \ 0.0005 from baseline) and returned
for healthy men are 3.1–5.4 g/dL for albumin and to baseline levels at the 6-month follow-up (4,100 ± 19,
8.9–10.4 mg/dL for calcium. Fig. 1).

Vitamin D radioimmunoassay was used to measure Table 1 Subject characteristics and fitness level before and imme-
25(OH)D levels (DiaSorin, Stillwater, MN). The interassay diately after prolonged submersion (n = 32)
CV was 98.6%. Reference values for healthy men are
8.9–46.7 ng/mL. Pre Post P

Parathyroid hormone (PTH) was measured by immu- Age (years) 22.8 ± 3.8 176.7 ± 1.2 NS
noassay with chemiluminescent detection on the Immulite Height (cm) 176.7 ± 1.2 73.7 ± 1.9* 0.002
2000 (Diagnostics Products, Los Angeles, CA). The in- Weight (kg) 72.1 ± 1.9 23.4 ± 0.5* 0.002
terassay CV was 4.7%. Reference values for healthy men BMI (kg/m2) 23.05 ± 0.5 44.8 ± 0.6* 0.027
range 12.0–72.0 pg/mL. Thigh circumference (cm) 45.7 ± 0.6 31.8 ± 0.5* 0.002
Calf circumference 32.6 ± 0.5
Bone Ultrasound Measurements (cm) 529.2 ± 8.8* 0.005
2000 m run (s) 507.3 ± 11.6
Tibial ultrasound measures were taken on the right tibia
using Sunlight OmnisenseTM (Sunlight Medical, Somerset, * P \ 0.05
NJ), a QUS designed to measure SOS at different skeletal
sites, using the axial transmission method. Briefly, the SOS
measurement is based on the principle that ultrasound
waves propagate faster through bone than through soft
tissue. The device consists of a desktop main unit and a

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T. Luria et al.: Bone Health in Submariners 11

Discussion

Fig. 1 Bone SOS before and immediately after the prolonged In this study, we demonstrated a significant decrease in
submersion and 1 and 6 months following return. * P \ 0.02, bone SOS following a 30-day submersion. A further
** P \ 0.0001 decrease in bone strength was noted 4 weeks after the end
of submersion. Subjects regained bone strength only
Albumin 6 months after return to shore. The decrease in bone SOS
was accompanied by a reduction in bone turnover (both
There was a significant change in albumin levels following bone formation and resorption markers), reduced muscle
the exercise (from 4.56 ± 0.25 to 4.84 ± 0.30 g/dL, mass, and fitness level.
P \ 0.0005); consequently, assay results for albumin were
used to adjust for plasma volume shifts, to accurately Prolonged submersions are relatively common in naval
assess changes in production. forces throughout the world. During operations inside the
submarine, submariners are exposed to environmental
Bone Formation Markers factors significant to health. Some of these factors that may
adversely affect bone health include lack of sunlight
PINP significantly decreased by 10.4 ± 1.8 lg/L following exposure to the skin (required for vitamin D metabolism)
the submersion (from 76.74 ± 34.92 to 66.30 ± 29.41 lg/ [19], limited space that restricts physical activity, subop-
L, P \ 0.0005), while there was no significant change in timal nutritional and high coffee consumption, as well as
BAP levels (from 17.31 ± 7.32 to 16.89 ± 5.22 lg/L, elevated CO2 concentrations due to the closed atmospheric
P = 0.59). environment [20].

Bone Resorption Markers The present study demonstrates, for the first time using
QUS assessment of tibial SOS, a significant decrease in
A significant decrease of 0.43 ± 0.09 U/L was observed in bone strength in a group of young, healthy male submari-
TRAP5b (from 4.26 ± 1.08 to 3.83 ± 0.91 U/L, ners following prolonged submersion. This decrease in
P \ 0.0005). CTx levels significantly decreased by bone strength was accompanied by a reduction in calf and
0.24 ± 0.04 ng/dL (from 0.83 ± 0.26 to 0.59 ± 0.28 ng/ thigh circumferences, probably reflecting a decrease in
dL, P \ 0.0005). muscle mass. Moreover, further decrease in bone SOS was
noted 4 weeks after return to shore.
Endocrine Regulators
This prolonged effect is in agreement with data from
There was a significant increase in circulating calcium level both human and experimental animal studies demonstrat-
by 0.30 ± 0.04 mg/dL (from 9.53 ± 0.25 to 9.83 ± 0.32 ing a delayed return of bone strength to baseline following
mg/dL, P \ 0.0005). PTH significantly decreased by 6.0 ± periods of relative disuse (limb immobilization, bed rest,
2.0 pg/mL (from 30.30 ± 8.88 to 24.25 ± 11.01 pg/mL, spaceflight, or unloading) [21–23]. Indeed, the finding of a
P \ 0.007) and 25(OH)D levels decreased significantly significant performance decrement in 2,000 m run time
by 3.9 ± 0.7 ng/mL (from 25.54 ± 7.30 to 21.66 ± 5.38 following the deployment supports the notion that the
ng/mL, P \ 0.0005) following the submersion. submariners were ‘‘detrained’’ over the course of the
deployment. Our data suggest that 30 days of unfavorable
No significant correlations were found between changes environmental conditions, such as those that exist in the
in bone SOS and bone markers. submarine, lead to significant muscle and bone alterations
and that these changes may be detected using bone strength
measurements.

Based on these data, it appears that the bone/muscle
complex might be compromised following prolonged
submersions, potentially increasing the risk for musculo-
skeletal injury (i.e., fracture and stress fractures) until
normalization of bone properties occurs. As most of the
submarine soldiers are in their second to third decade of
life, prolonged, recurrent periods of insult to the bone may
potentially influence the peak bone mass achieved during
these years and may place them at risk for earlier onset or
more severe osteoporosis later in life.

Mechanical strain is one of the most powerful stimulators
of bone formation and growth. Several studies have

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12 T. Luria et al.: Bone Health in Submariners

demonstrated that physical activity increases bone mass in maintained in the submarine breathing atmosphere. High
children, adolescents, and adults [24–26], while inactivity CO2 content in breathed air is a unique feature of the diesel
results in bone resorption and decreased bone mineral density submarine, and in the present study submariners were
[27]. Overall, submariners in the present study were inactive exposed to relatively high CO2 content. It is also possible
(self-reported activity diaries). In the confined environment that the reduction in vitamin D levels is a result of
of the submarine, submariners were unable to run or jump decreased intake. In this case, supplementation of vitamin
and did not perform any type of resistance training. The D may prove beneficial [30]. Unfortunately, vitamin D
only activity performed was short-distance walking, and the intake was not evaluated during this study.
cumulative time spent walking was at most 10 minutes a day.
Therefore, prolonged submersion with reduction of physical On the contrary, serum calcium levels increased during
stimulation may lead to reduced bone strength. the submersion. Prolonged immobilization is a known
cause of hypercalcemia [31]. In addition, previous studies
The results of the present study suggest that in subma- have demonstrated that the increase in serum Ca levels in
riners maintenance of physical activity during prolonged submariners may be a result of decreased urinary excretion
submersions and especially following return to shore may of Ca [32, 33]. For example, Davies and Morris [2] dem-
assist in bone strength preservation. Moreover, QUS onstrated that continuous exposure to raised environmental
measurements may serve as a simple, noninvasive, sensi- levels of CO2 resulted in a decrease of 50% in urinary Ca
tive, and cost-effective tool in the evaluation and follow-up secretion. The decrease in vitamin D levels probably also
of bone strength of this unique population. contributes to the reduction in urinary Ca secretion. The
increase in Ca was accompanied by a reduction in PTH.
Commonly used methods for the evaluation of bone
densitometry (e.g., DXA and computed tomography) mea- There are several limitations in the present study. Usage
sure quantitative aspects of bone mineral density. However, of activity diaries may be problematic when evaluating
these methods do not assess additional qualitative factors physical activity levels. We were unable to objectively
that contribute to bone strength such as bone elasticity, determine the levels of physical activity in the submarine
microarchitecture, and fatigue damage [7, 28]. It is possible, or during the month after return to shore. Further, we did
therefore, that the decrease in bone strength resulted from not anticipate the finding of a prolonged decrease in bone
both qualitative and quantitative bone alterations. SOS (i.e., 1 month after return to shore); and therefore, in
our study design, we did not include additional blood
In our study, the adverse effects of prolonged submer- sampling or fitness evaluation at later time points. Retro-
sion on bone SOS were accompanied by reduced circu- spectively, there is no doubt that additional measurement in
lating levels of bone remodeling markers. Markers of bone these time points (1 and 6 months after return to shore)
remodeling are usually classified according to the meta- could have been beneficial.
bolic pathway they reflect. Actively resorbing osteoclasts
secrete proteases, which act to degrade the collagenous and In summary, prolonged submersion (30 days) leads to a
noncollagenous bone matrix [5]. In the present study CTx significant decrease in bone strength, accompanied by an
represented collagen breakdown while TRAP5b repre- overall decrease in bone metabolism. Bone strength is
sented the noncollagenous matrix proteins. Both resorption regained only months after return to shore. Prevention and/
markers significantly decreased following the submersion. or treatment programs should be developed even for young
Under normal circumstances, bone resorption is followed submariners deployed to prolonged periods of relative
by the formation of a new bone, a function of active disuse. The long-term effects of repeated prolonged sub-
osteoblasts. In the present study, the levels of PINP, a mersions on bone health are yet to be determined.
product of collagen synthesis, decreased following sub-
mersion, while the levels of BAP, a protein released by References
osteoblasts, did not change. Overall, a decrease in both
bone formation and bone resorption markers occurred, 1. Tansey WA, Wilson JM, Schaefer KE (1979) Analysis of health
probably reflecting a lower turnover of bone metabolism. data from 10 years of Polaris submarine patrols. Undersea Bio-
med Res 6(Suppl):S217–S246
Numerous studies discuss the connection between lack
of sun exposure (indoor confinement), long-sleeved dress, 2. Davies DM, Morris JE (1979) Carbon dioxide and vitamin D
or seasonal sun deprivation and calcium and vitamin D effects on calcium metabolism in nuclear submariners: a review.
metabolism [19]. Serum levels of 25(OH)D decreased Undersea Biomed Res 6(Suppl):S71–S80
during the submersion. This change probably reflects a
decrease in body stores of vitamin D due to the absence of 3. Meier C, Liu PY, Handelsman DJ, Seibel MJ (2005) Endocrine
sunlight exposure. Gilman et al. [29] previously suggested regulation of bone turnover in men. Clin Endocrinol (Oxf)
that in submariners the decrease in vitamin D levels may 63:603–616
also be mediated by the relatively high CO2 levels
4. Meier C, Woitge HW, Witte K, Lemmer B, Seibel MJ (2004)
Supplementation with oral vitamin D3 and calcium during winter

123

T. Luria et al.: Bone Health in Submariners 13

prevents seasonal bone loss: a randomized controlled open-label deprivation in the Antarctic. Eur J Appl Physiol Occup Physiol
prospective trial. J Bone Miner Res 19:1221–1230 79:141–147
5. Manolagas SC (2000) Birth and death of bone cells: basic regu- 20. Dimai HP, Domej W, Leb G, Lau KH (2001) Bone loss in
latory mechanisms and implications for the pathogenesis and patients with untreated chronic obstructive pulmonary disease is
treatment of osteoporosis. Endocr Rev 21:115–137 mediated by an increase in bone resorption associated with
6. Riggs BL, Melton LJ III (1986) Involutional osteoporosis. N Engl hypercapnia. J Bone Miner Res 16:2132–2141
J Med 314:1676–1686 21. Bloomfield SA, Allen MR, Hogan HA, Delp MD (2002) Site- and
7. Foldes AJ, Rimon A, Keinan DD, Popovtzer MM (1995) Quan- compartment-specific changes in bone with hindlimb unloading
titative ultrasound of the tibia: a novel approach for assessment of in mature adult rats. Bone 31:149–157
bone status. Bone 17:363–367 22. Pavy-Le TA, Heer M, Narici MV, Rittweger J, Vernikos J (2007)
8. Prins SH, Jorgensen HL, Jorgensen LV, Hassager C (1998) The From space to Earth: advances in human physiology from
role of quantitative ultrasound in the assessment of bone: a 20 years of bed rest studies (1986–2006). Eur J Appl Physiol
review. Clin Physiol 18:3–17 101:143–194
9. Bauer DC, Gluer CC, Cauley JA, Vogt TM, Ensrud KE, Genant 23. Rittweger J, Simunic B, Bilancio G, De Santo NG, Cirillo M,
HK, Black DM (1997) Broadband ultrasound attenuation predicts Biolo G, Pisot R, Eiken O, Mekjavic IB, Narici M (2009) Bone
fractures strongly and independently of densitometry in older loss in the lower leg during 35 days of bed rest is predominantly
women. A prospective study. Study of Osteoporotic Fractures from the cortical compartment. Bone 44:612–618
Research Group. Arch Intern Med 157:629–634 24. Beck BR, Snow CM (2003) Bone health across the lifespan—
10. Jaworski M, Lebiedowski M, Lorenc RS, Trempe J (1995) exercising our options. Exerc Sport Sci Rev 31:117–122
Ultrasound bone measurement in pediatric subjects. Calcif Tissue 25. Eliakim A, Raisz LG, Brasel JA, Cooper DM (1997) Evidence for
Int 56:368–371 increased bone formation following a brief endurance-type
11. Kang C, Speller R (1998) Comparison of ultrasound and dual training intervention in adolescent males. J Bone Miner Res
energy X-ray absorptiometry measurements in the calcaneus. Br J 12:1708–1713
Radiol 71:861–867 26. Slemenda CW, Miller JZ, Hui SL, Reister TK, Johnston CC Jr
12. Prevrhal S, Fuerst T, Fan B, Njeh C, Hans D, Uffmann M, (1991) Role of physical activity in the development of skeletal
Srivastav S, Genant HK (2001) Quantitative ultrasound of the mass in children. J Bone Miner Res 6:1227–1233
tibia depends on both cortical density and thickness. Osteoporos 27. Mazess RB, Whedon GD (1983) Immobilization and bone. Calcif
Int 12:28–34 Tissue Int 35:265–267
13. Bauer DC, Gluer CC, Cauley JA, Vogt TM, Ensrud KE, Genant 28. Prevrhal S, Fuerst T, Fan B, Njeh C, Hans D, Uffmann M,
HK, Black DM (1997) Broadband ultrasound attenuation predicts Srivastav S, Genant HK (2001) Quantitative ultrasound of the
fractures strongly and independently of densitometry in older tibia depends on both cortical density and thickness. Osteoporos
women. A prospective study. Study of Osteoporotic Fractures Int 12:28–34
Research Group. Arch Intern Med 157:629–634 29. Gilman SC, Biersner RJ, Bondi KR (1982) Effect of a 68-day
14. Moayyeri A, Kaptoge S, Dalzell N, Bingham S, Luben RN, submarine patrol on serum 25-hydroxyvitamin D levels in healthy
Wareham NJ, Reeve J, Khaw KT (2009) Is QUS or DXA better men. Int J Vitam Nutr Res 52:63–67
for predicting the 10-year absolute risk of fracture? J Bone Miner 30. Duplessis CA, Harris EB, Watenpaugh DE, Horn WG (2005)
Res 24:1319–1325 Vitamin D supplementation in underway submariners. Aviat
15. Eliakim A, Nemet D, Friedland O, Dolfin T, Regev RH (2002) Space Environ Med 76:569–575
Spontaneous activity in premature infants affects bone strength. 31. Sato Y, Kaji M, Higuchi F, Yanagida I, Oishi K, Oizumi K
J Perinatol 22:650–652 (2001) Changes in bone and calcium metabolism following hip
16. Delmas PD (1995) Biochemical markers of bone turnover. Acta fracture in elderly patients. Osteoporos Int 12:445–449
Orthop Scand Suppl 266:176–182 32. Dlugos DJ, Perrotta PL, Horn WG (1995) Effects of the sub-
17. Eriksen EF, Charles P, Melsen F, Mosekilde L, Risteli L, Risteli J marine environment on renal-stone risk factors and vitamin D
(1993) Serum markers of type I collagen formation and degra- metabolism. Undersea Hyperb Med 22:145–152
dation in metabolic bone disease: correlation with bone histo- 33. Messier AA, Heyder E, Braithwaite WR, McCluggage C, Peck A,
morphometry. J Bone Miner Res 8:127–132 Schaefer KE (1979) Calcium, magnesium, and phosphorus
18. Watts NB (1999) Clinical utility of biochemical markers of bone metabolism, and parathyroid-calcitonin function during pro-
remodeling. Clin Chem 45:1359–1368 longed exposure to elevated CO2 concentrations on submarines.
19. Zerath E, Holy X, Gaud R, Schmitt D (1999) Decreased serum Undersea Biomed Res 6(Suppl):S57–S70
levels of 1,25-(OH)2 vitamin D during 1 year of sunlight

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