Bone Metabolic Markers in Patients with Obstructive Sleep Apnea Syndrome.

I NTRODUCTION

Obstructive sleep apnea syndrome (OSAS) is a common disorder characterized by repeating episodes of upper airway obstruction during sleep. Previous studies indicated that OSAS was more prevalent among obese people and the incidence of OSAS increased with age.[ 1 2 3 ] At present, OSAS can seriously affect the quality of life in patients and has been considered as an independent risk factor of a variety of systemic diseases including hypertension and metabolic syndrome.[ 4 5 6 7 8 ]

Although the relationship between OSAS and bone mineral density (BMD) has been confirmed in many studies, the results are conflicting.[ 9 10 11 ] Only few studies have focused on the relationship between OSAS and bone metabolic markers. The bone is mainly composed of collagen fibers formed by type 1 collagen. Procollagen type 1 N-terminal propeptide (P1NP) is a specific marker for the deposition of type 1 collagen, whereas β-C-terminal telopeptide of type 1 collagen (β-CTX) is a degradation product of type 1 collagen. P1NP and CTX are recommended by the International Osteoporosis Foundation and the International Federation of Clinical Chemistry and Laboratory Medicine as the markers of bone formation and bone resorption, respectively.[ 12 13 ] N-terminal midfragment of osteocalcin (N-MID) is a degradation product of osteocalcin and its level in the peripheral blood can reflect the activity of osteoblasts and the rate of bone mineralization. Vitamin D (VD) is one of the essential and fat-soluble vitamins in the human body. Adequate daily intake of VD can increase the absorption of intestinal calcium and phosphorus by 30–40% and 80%, respectively. VD can also inhibit the secretion of parathyroid hormone (PTH), and the reduced PTH can mediate bone resorption and maintain the health of bone. Therefore, VD deficiency can now be considered as a high-risk factor of osteoporosis.[ 14 ] In this study, obesity was designed as the control group, we compared both the above bone metabolic markers and BMD in obesity and obese OSAS patients, and correlation analysis was performed to determine whether the levels of these bone metabolic markers and BMD in OSAS patients are correlated with the severity of OSAS, which is rarely seen in the previous studies.

M ETHODS

Ethical approval

The study was conducted in accordance with the Declaration of Helsinki and was approved by the Ethics Committee of the Second Hospital of Shanxi Medical University. Written informed consent was signed by all the participants before the study enrollment.

Patients

A total of 119 obese male individuals were enrolled in this study in spring months from 2015 to 2017. Polysomnography (PSG) and the collection of serum samples were conducted at the physical examination center, and the multi-guide sleep monitoring center of the Second Hospital of Shanxi Medical University. The age of the subjects ranged from 30 to 65 years, and after detailed medical history, physical and auxiliary examinations (including blood routine, urinalysis, electrocardiography, color Doppler echocardiography, thyroid color ultrasonography, abdominal color ultrasonography, chest radiography, pulmonary function test, serum biochemicals, and thyroid function test), candidates with smoking habit, alcoholism, cold, stuffy nose, acute illness, chronic obstructive pulmonary diseases, any other respiratory disorder, diabetes, hypertension, coronary heart diseases, heart failure, hyperthyroidism, hypothyroidism, hyperparathyroidism, hypoparathyroidism, Cushing syndrome, hypogonadism, hypopituitarism, growth hormone deficiency, acromegaly, gastrointestinal diseases, chronic renal disease, multiple myeloma, malignancy, bone metastasis, Paget's disease, rheumatologic disorders, stroke, paraplegia, hemiplegia, dementia, schizophrenia, depression, or a history of fractures were excluded from this study. In addition, the patients with positive airway pressure therapy were also excluded, as well as the candidates with a medication history that might affect bone metabolism, such as calcium, VD, bisphosphonates, corticosteroid, parathormone, calcitonin, antiepileptics, Vitamin K antagonist, and heparin. Moreover, all the participants were requested to avoid exercise for 48 h before blood collection. Before PSG monitoring, all participants wore loose clothes and maintained good personal hygiene. They were prohibited from using alcohol, coffee, tea, and sedative drugs.

Polysomnographic test

Among these 119 individuals, 32 subjects (based on the “Guidelines for prevention and control of adult obesity in China”) with body mass index (BMI) ≥28 kg/m 2 were assigned into the obesity group and the 87 OSAS patients were diagnosed by the multi guide sleep monitor (Sandman 64 guide sleep monitor, USA; Compumedics-E Type 44 guide sleep monitor, Australia). The OSAS patients were further divided into two groups based on their apnea-hypopnea index (AHI): the mild-to-moderate OSAS group contained 32 patients with 5 events/h ≤ AHI <30 events/h, and the severe OSAS group contained 55 patients with AHI ≥30 events/h. All subjects underwent PSG monitoring performed by experienced doctors, and their blood samples were collected on the following morning for further analysis.

Measurement of bone metabolic markers

The serum samples were collected between 08:00 a.m. and 09:00 a.m. after an overnight fast, and the levels of t-P1NP, N-MID, β-CTX, VD, and PTH were measured using a Roche Cobase601 electrochemical luminescence instrument (Roche Diagnostics, Mannheim, Germany), and the lumbar BMD was measured using a radiation absorption method.

Statistical analysis

Statistical analysis was performed using SPSS version 17.0 (SPSS Inc., Chicago, IL, USA). Data were expressed as mean ± standard deviation (SD). The analysis of variance was used to compare the difference among different groups based on the least significant difference. Pearson correlation analysis was performed to study the correlation among the data. A P < 0.05 was considered statistically significant.

R ESULTS

Evaluation of clinical information of the participants

As shown in Table 1 , the mean age of the individuals was 50.1 ± 7.3 years, 51.8 ± 8.1 years, and 48.2 ± 9.9 years for the obesity, mild-to-moderate OSAS, and severe OSAS groups, respectively, without significant difference ( P > 0.05). The mean values of BMI in the obesity ( t = 3.519, P = 0.001), and the severe OSAS groups ( t = 5.277, P < 0.001) were greater than that in the mild-to-moderate OSAS group.

Evaluation of the bone metabolic markers of the individuals

Subsequently, to investigate the effect of OSAS on bone metabolism, some serum bone metabolic markers were evaluated [ Table 2 ]. No significant differences were found in terms of the mean BMD values among the obesity, mild-to-moderate OSAS, and severe OSAS groups ( P > 0.05). The N-MID values among the three groups also showed no significant differences ( P > 0.05). The serum levels of t-P1NP in the obesity and mild-to-moderate OSAS groups were significantly lower than that in the severe OSAS group ( t = −4.138, P < 0.001; and t = −3.421, P = 0.001, respectively). The mean value of β-CTX in the severe OSAS group was significantly higher than that in the obesity group ( t = 2.575, P = 0.011), whereas the levels of β-CTX and t-P1NP between the obesity and mild-to-moderate OSAS groups showed no significant difference ( t = −1.839, P = 0.069; and t = −0.637, P = 0.525, respectively). In terms of serum VD levels, the mild-to-moderate and severe OSAS groups showed markedly lower results as compared to the obesity group ( t = −5.660, P < 0.001; and t = −10.862, P < 0.001, respectively), and the VD level in the mild-to-moderate OSAS group was significantly higher than that in the severe OSAS group ( t = 4.498, P < 0.001). The PTH level in the severe OSAS group was significantly higher than that in the mild-to-moderate OSAS and obesity groups ( t = 4.945, P < 0.001; and t = 7.056, P < 0.001, respectively), whereas no significant difference was observed in the mean values of PTH between the obesity and mild-to-moderate OSAS groups ( t = −1.877, P = 0.063).

Correlation between the polysomnographic and bone metabolic markers in Obstructive sleep apnea syndrome patients

In the next step, a correlation analysis was performed to investigate the relationship between the severity of OSAS and bone metabolic markers in the OSAS patients. As shown in Table 3 , BMI was positively correlated with AHI ( r = 0.572, P < 0.001) and PTH ( r = 0.292, P = 0.006), but negatively correlated with minimum O 2 saturation (SaO 2 min) ( r = −0.339, P = 0.001) and BMD ( r = −0.261, P = 0.015) in OSAS patients. In addition, AHI was positively correlated with PTH ( r = 0.400, P < 0.001) and t-P1NP ( r = 0.396, P < 0.001) but negatively correlated with SaO 2 min ( r = −0.673, P < 0.001) and VD ( r = –0.404, P < 0.001). SaO 2 min was positively correlated with VD ( r = 0.258, P = 0.016) but negatively correlated with PTH ( r = −0.376, P < 0.001). PTH was positively correlated with t-P1NP ( r = 0.270, P = 0.011), but negatively correlated with VD ( r = −0.363, P = 0.001). The β-CTX was positively correlated with N-MID ( r = 0.616, P < 0.001) and t-P1NP ( r = 0.614, P < 0.001), and N-MID was positively correlated with t-P1NP ( r = 0.560, P < 0.001).

D ISCUSSION

There are many factors affecting bone metabolic markers, such as age, gender, menstrual variation, seasonal variation, circadian variation, fasting and food intake, physical activity, diseases, and drugs.[ 15 16 ] To eliminate the effects of these factors, we set strict exclusion criteria. In our study, only men were included, and no significant differences were found in terms of the mean age among the obesity, mild-to-moderate OSAS, and severe OSAS groups. We also excluded the patients with diseases and drugs that affect bone metabolic markers, such as thyroid and parathyroid diseases, Cushing syndrome, growth hormone deficiency, acromegaly, gastrointestinal diseases, chronic renal disease, multiple myeloma, malignancy, bone metastasis, and Paget's disease. In addition, in order to reduce the variability, samples were collected between 08:00 a.m. and 09:00 a.m. after an overnight fast only in spring, and exercise was avoided for 48 h before the collection.

The relationship between OSAS and obesity has been confirmed by many previous studies, and obesity is now considered as an independent risk factor for OSAS.[ 17 ] Zhang and Si[ 18 ] have reported that when body weight increased, the frequency of respiratory events would also increase during sleep. Some studies have suggested that if body weight gained by 10%, the value of AHI might increase by 32% and the risk of OSAS might also increase by 5 folds. In addition, obesity might also cause problems in bone metabolism by affecting the secretion of cytokines and hormones.[ 19 ] On the other hand, sleep disorders and chronic intermittent hypoxia (CIH) in OSAS patients might cause metabolic disorders, which could further aggravate the condition of obesity.[ 20 ] In this study, individuals with obesity were assigned into the control to reduce the influence of obesity. As expected, the results showed that the BMI in the severe OSAS group was higher than that in the mild-to-moderate OSAS group, and the correlation analysis indicated that BMI was positively correlated with AHI and PTH but negatively correlated with SaO 2 min and BMD. The results of this study showed no significant difference of BMD in subjects among the obesity, mild-to-moderate OSAS, and severe OSAS groups. The correlation analysis suggested no relationship between BMD and AHI or between BMD and SaO 2 min. The data obtained from this study were consistent with those shown in a previous report about BMD.[ 10 ] In summary, these results indicated that BMI was correlated to the severity and BMD of OSAS patients.

Previous studies have indicated that during osteoporosis, the changes in the levels of bone metabolic markers occurred much earlier than the change in BMD, suggesting that these metabolic markers might have an important value for the early diagnosis of osteoporosis. To investigate the role of bone metabolic markers in OSAS, the serum levels of these markers were compared among individuals from different groups. As mentioned above, our results indicated no significant differences in BMD among the obesity, mild-to-moderate OSAS, and severe OSAS groups, but some bone metabolic markers made the differences. As shown in Table 1 , the severe OSAS and obesity groups had the similar BMI, but the levels of β-CTX and t-P1NP shown in Table 2 were higher in the severe OSAS group than those in the obesity group. Many studies have reported that OSAS was associated with an increased level of bone resorption. In this study, it was also observed that the serum level of β-CTX, a marker of bone resorption, in the severe OSAS group was significantly higher compared to the obesity group. These results demonstrated that the activity of osteoclasts, bone resorption, and the release of β-CTX in the severe OSAS group were all increased. Tomiyama et al .[ 21 ] found that OSAS could be characterized by “an uncoupled state” between bone formation and bone resorption, because they observed that the level of urinary CTX was higher in severe OSAS patients than that in mild OSAS patients and controls, however, no such change was observed for bone formation markers (bone-specific alkaline phosphatase and osteocalcin). In this study, the severe OSAS group showed a higher level of t-P1NP, a marker of bone formation, than the obesity and mild-to-moderate OSAS groups. Although no significant differences were observed in the levels of N-MID among the obesity, mild-to-moderate OSAS, and severe OSAS groups, the N-MID level was on the uptrend in OSAS patients. These results indicated that the severe OSAS group was associated with an increased rate of bone collagen deposition. In addition, this study demonstrated that AHI and t-P1NP were positively correlated in OSAS patients, suggesting that the severity of OSAS might be associated with bone formation. In summary, all these results suggested an increased rate of bone turnover in the severe OSAS group. The correlation analysis also showed that β-CTX was positively correlated with N-MID and t-P1NP, and N-MID was positively correlated with t-P1NP, indicating “a coupled state” between bone formation and bone resorption in OSAS. Nevertheless, large-scale clinical studies are still needed to clarify the relationship between bone formation and bone resorption in OSAS patients.

It is well known that OSAS is related to obesity, a disorder closely associated with VD deficiency. Increased influx of 25-hydroxyvitamin D into adipose tissues is a plausible explanation for VD deficiency in obesity. A recent study has shown that if children suffered from both obesity and OSAS, their VD levels would be further reduced.[ 22 ] In this study, a lower VD level was also shown in the severe OSAS group, although it had a similar BMI as compared with that in the obesity group. Consistent with the findings from a previous study,[ 23 ] it was observed in this study that as the severity of OSAS increased, the level of VD would decrease and might be associated with the high bisphenol A (BPA) levels in OSAS patients. Erden et al .[ 24 ] found that the BPA level in the severe OSAS group was significantly higher than those in the mild-to-moderate OSAS and control groups, and the level of BPA was negatively correlated with VD level. BPA might be involved in the etiopathogenesis of OSAS by inducing obesity, androgen antagonist activities, estrogenic activities, and muscular dysfunctions. In addition, OSAS is associated with insulin resistance and diabetes, whereas a low level of VD can lead to glucose intolerance and IR, suggesting that VD may also play an important role in the pathophysiology of OSAS. The correlation analysis in this study also indicated that VD was negatively correlated with AHI but positively correlated with SaO 2 min in the OSAS patients, suggesting that the severity of OSAS might affect the level of VD in the body.

VD deficiency could also increase the secretion of PTH, while an increased level of PTH could induce bone resorption and bone loss.[ 25 ] Barceló et al .[ 26 ] reported that PTH was positively correlated with AHI. The results from this study also confirmed that PTH was positively correlated with AHI, but negatively correlated with SaO 2 min in the OSAS patients. In addition, the level of PTH in the severe OSAS group was significantly higher than that in the mild-to-moderate OSAS and obesity groups, suggesting that the patients with severe OSAS might have more active bone resorption.

Although the causes of increased bone resorption and formation markers in severe OSAS patients are still unclear, it has been considered that hypoxia, oxidative stress, inflammation, and increased sympathetic tone might be the contributing factors.

OSAS is characterized by CIH. Hypoxia could accelerate the resorption of bone by affecting the proliferation and differentiation of osteogenic cells as well as the activity of osteoclasts.[ 27 28 29 ] However, some studies showed that hypoxia could promote osteoblasts differentiation into osteocytes and inhibit sclerostin expression,[ 30 31 ] which was a negative regulatory factor for bone formation. Acidosis induced by hypoxia could inhibit mineral deposition and activate osteoclasts.[ 32 33 ] Oxidative stress which is also present in OSAS patients can affect bone health through increasing bone resorption. In addition, CIH can activate the NF-kB signaling pathway and induce inflammatory conditions through up-regulating the expression of downstream cytokines including interleukin 6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha), which are called osteolytic cytokines, and subsequently increase the risk of bone mineral loss. Repeated intermittent hypoxia and micro arousal during the sleep of OSAS patients could increase the sympathetic tone.[ 34 35 ] Most scholars believe that the excitation of sympathetic nerves could inhibit the activity of osteoblasts, thus leading to reduce bone formation and ultimately lowering the bone mass and BMD.[ 36 ]

As seen in our study, the levels of bone resorption and formation markers in patients with severe OSAS were significantly increased compared to obese men, while no significant differences were found in terms of the mean BMD values among the obesity, mild-to-moderate OSAS, and severe OSAS groups. This was mainly because age, gender, and weight were more important for BMD. In our study, all participants were obese men and age-matched, and there was no significant difference in terms of BMI between the obesity and severe OSAS groups. In the obese people included in our study, the BMI values in the obesity and severe OSAS groups were higher than that in the mild-to-moderate OSAS group, it seemed that the mean BMD levels in the obesity and severe OSAS groups were lower than that in the mild-to-moderate OSAS group, but no significant differences were found among the three groups. On the other hand, the changes in bone metabolic markers are much earlier than the change in BMD, so clinical follow-up will have to be done to disclose the truth.

There were some limitations in our study. The sample was small, and it was mainly because of the strict exclusion criteria. The concentrations of IL-6 and TNF-alpha which are related to bone metabolism and CIH existed in OSAS should be measured and analyzed. Follow-up evaluation was not involved in the present study, long-term follow-up of OSAS patients will provide more evidence, and may also help us investigate the bone metabolism in OSAS patients with positive airway pressure therapy.

In conclusion, increased bone resorption and formation markers in patients with severe OSAS compared to obese men indicated high bone turnover, and the severity of OSAS was correlated with the serum levels of t-P1NP, VD, and PTH. In the future, large-scale clinical studies are needed, and the specific mechanisms underlying such observations remain to be further clarified.

Financial support and sponsorship

This study was supported in part by grants from the National Natural Science Foundation of China (No. 81570086), and Research Project Supported by Shanxi Scholarship Council of China (No. 2017-120).

Conflicts of interest

There are no conflicts of interest.

Edited by: Xin Chen

R EFERENCES

1 Kielb SA Ancoli-Israel S Rebok GW Spira AP Cognition in obstructive sleep apnea-hypopnea syndrome (OSAS): Current clinical knowledge and the impact of treatment Neuromolecular Med 2012 14 180 93 doi: 10.1007/s12017-012-8182-1 22569877 10.1007/s12017-012-8182-1 PMC3823054

2 Wang XY Wang S Liu X Ding X Li M Han DM Retinal vascular morphological changes in patients with extremely severe obstructive sleep apnea syndrome Chin Med J 2017 130 805 10 doi: 10.4103/0366-6999.202728 28345544 10.4103/0366-6999.202728 PMC5381314

3 Chen JH Huang R Luo JM Xiao Y Zhang Y Adult monosymptomatic nocturnal enuresis with obstructive sleep apnea syndrome Chin Med J 2016 129 2011 2 doi: 10.4103/0366-6999.187853 27503032 10.4103/0366-6999.187853 PMC4989438

4 Asghari A Mohammadi F Kamrava SK Jalessi M Farhadi M Evaluation of quality of life in patients with obstructive sleep apnea Eur Arch Otorhinolaryngol 2013 270 1131 6 doi: 10.1007/s00405-012-2157-6 22903757 10.1007/s00405-012-2157-6

5 Parati G Ochoa JE Bilo G Mattaliano P Salvi P Kario K Obstructive sleep apnea syndrome as a cause of resistant hypertension Hypertens Res 2014 37 601 13 doi: 10.1038/hr.2014.80 24804613 10.1038/hr.2014.80

6 Fusetti M Fioretti AB Valenti M Masedu F Lauriello M Pagliarella M Cardiovascular and metabolic comorbidities in patients with obstructive sleep apnoea syndrome Acta Otorhinolaryngol Ital 2012 32 320 5 23326012 PMC3546405

7 Xin C Zhang W Wang L Yang D Wang J Changes of visual field and optic nerve fiber layer in patients with OSAS Sleep Breath 2015 19 129 34 doi: 10.1007/s11325-014-0978-9 24807117 10.1007/s11325-014-0978-9

8 Koren D Chirinos JA Katz LE Mohler ER Gallagher PR Mitchell GF Interrelationships between obesity, obstructive sleep apnea syndrome and cardiovascular risk in obese adolescents Int J Obes (Lond) 2015 39 1086 93 doi: 10.1038/ijo.2015.67 25913504 10.1038/ijo.2015.67 PMC4496293

9 Uzkeser H Yildirim K Aktan B Karatay S Kaynar H Araz O Bone mineral density in patients with obstructive sleep apnea syndrome Sleep Breath 2013 17 339 42 doi: 10.1007/s11325-012-0698-y 22467193 10.1007/s11325-012-0698-y

10 Mariani S Fiore D Varone L Basciani S Persichetti A Watanabe M Obstructive sleep apnea and bone mineral density in obese patients Diabetes Metab Syndr Obes 2012 5 395 401 doi: 10.2147/DMSO.S37761 23152692 10.2147/DMSO.S37761 PMC3496369

11 Sforza E Thomas T Barthélémy JC Collet P Roche F Obstructive sleep apnea is associated with preserved bone mineral density in healthy elderly subjects Sleep 2013 36 1509 15 doi: 10.5665/sleep.3046 24082310 10.5665/sleep.3046 PMC3773200

12 Wheater G Elshahaly M Tuck SP Datta HK van Laar JM The clinical utility of bone marker measurements in osteoporosis J Transl Med 2013 11 201 doi: 10.1186/1479-5876-11- 23984630 10.1186/1479-5876-11-201 PMC3765909

13 Vasikaran S Eastell R Bruyère O Foldes AJ Garnero P Griesmacher A Markers of bone turnover for the prediction of fracture risk and monitoring of osteoporosis treatment: A need for international reference standards Osteoporos Int 2011 22 391 420 doi: 10.1007/s00198-010-1501-1 21184054 10.1007/s00198-010-1501-1

14 Lamberg-Allardt C Vitamin D in children and adolescents Scand J Clin Lab Invest Suppl 2012 243 124 8 doi: 10.3109/00365513.2012.682885 22536773 10.3109/00365513.2012.682885

15 Shetty S Kapoor N Bondu JD Thomas N Paul TV Bone turnover markers: Emerging tool in the management of osteoporosis Indian J Endocrinol Metab 2016 20 846 52 doi: 10.4103/2230-8210.192914 27867890 10.4103/2230-8210.192914 PMC5105571

16 Greenblatt MB Tsai JN Wein MN Bone turnover markers in the diagnosis and monitoring of metabolic bone disease Clin Chem 2017 63 464 74 doi: 10.1373/clinchem.2016.259085 27940448 10.1373/clinchem.2016.259085 PMC5549920

17 Romero-Corral A Caples SM Lopez-Jimenez F Somers VK Interactions between obesity and obstructive sleep apnea: Implications for treatment Chest 2010 137 711 9 doi: 10.1378/chest.09-0360 20202954 10.1378/chest.09-0360 PMC3021364

18 Zhang W Si LY Obstructive sleep apnea syndrome (OSAS) and hypertension: Pathogenic mechanisms and possible therapeutic approaches Ups J Med Sci 2012 117 370 82 doi: 10.3109/03009734.2012.707253 23009224 10.3109/03009734.2012.707253 PMC3497224

19 Cao JJ Effects of obesity on bone metabolism J Orthop Surg Res 2011 6 30 doi: 10.1186/1749-799X-6- 21676245 10.1186/1749-799X-6-30 PMC3141563

20 Attal P Chanson P Endocrine aspects of obstructive sleep apnea J Clin Endocrinol Metab 2010 95 483 95 doi: 10.1210/jc.2009-1912 20061419 10.1210/jc.2009-1912

21 Tomiyama H Okazaki R Inoue D Ochiai H Shiina K Takata Y Link between obstructive sleep apnea and increased bone resorption in men Osteoporos Int 2008 19 1185 92 doi: 10.1007/s00198-007-0556-0 18224268 10.1007/s00198-007-0556-0

22 Kheirandish-Gozal L Peris E Gozal D Vitamin D levels and obstructive sleep apnoea in children Sleep Med 2014 15 459 63 doi: 10.1016/j.sleep.2013.12.009 24684979 10.1016/j.sleep.2013.12.009 PMC4247539

23 McCarty DE Chesson AL Jr Jain SK Marino AA The link between Vitamin D metabolism and sleep medicine Sleep Med Rev 2014 18 311 9 doi: 10.1016/j.smrv.2013.07.001 24075129 10.1016/j.smrv.2013.07.001

24 Erden ES Genc S Motor S Ustun I Ulutas KT Bilgic HK Investigation of serum bisphenol A, Vitamin D, and parathyroid hormone levels in patients with obstructive sleep apnea syndrome Endocrine 2014 45 311 8 doi: 10.1007/s12020-013-0022-z 23904340 10.1007/s12020-013-0022-z

25 Sai AJ Walters RW Fang X Gallagher JC Relationship between Vitamin D, parathyroid hormone, and bone health J Clin Endocrinol Metab 2011 96 E436 46 doi: 10.1210/jc.2010-1886 21159838 10.1210/jc.2010-1886 PMC3047227

26 Barceló A Esquinas C Piérola J De la Peña M Sánchez-de-la-Torre M Montserrat JM Vitamin D status and parathyroid hormone levels in patients with obstructive sleep apnea Respiration 2013 86 295 301 doi: 10.1159/000342748 23154407 10.1159/000342748

27 Knowles HJ Hypoxic regulation of osteoclast differentiation and bone resorption activity Hypoxia (Auckl) 2015 3 73 82 doi: 10.2147/HP.S95960 27774484 10.2147/HP.S95960 PMC5045091

28 Srinivasan S Koenigstein A Joseph J Sun L Kalyanaraman B Zaidi M Role of mitochondrial reactive oxygen species in osteoclast differentiation Ann N Y Acad Sci 2010 1192 245 52 doi: 10.1111/j.1749-6632.2009.05377.x 20392243 10.1111/j.1749-6632.2009.05377.x PMC2856121

29 Utting JC Flanagan AM Brandao-Burch A Orriss IR Arnett TR Hypoxia stimulates osteoclast formation from human peripheral blood Cell Biochem Funct 2010 28 374 80 doi: 10.1002/cbf.1660 20556743 10.1002/cbf.1660

30 Hirao M Hashimoto J Yamasaki N Ando W Tsuboi H Myoui A Oxygen tension is an important mediator of the transformation of osteoblasts to osteocytes J Bone Miner Metab 2007 25 266 76 doi: 10.1007/s00774-007-0765-9 17704991 10.1007/s00774-007-0765-9

31 Genetos DC Toupadakis CA Raheja LF Wong A Papanicolaou SE Fyhrie DP Hypoxia decreases sclerostin expression and increases Wnt signaling in osteoblasts J Cell Biochem 2010 110 457 67 doi: 10.1002/jcb.22559 20336693 10.1002/jcb.22559 PMC2881162

32 Arnett TR Gibbons DC Utting JC Orriss IR Hoebertz A Rosendaal M Hypoxia is a major stimulator of osteoclast formation and bone resorption J Cell Physiol 2003 196 2 8 doi: 10.1002/jcp.10321 12767036 10.1002/jcp.10321

33 Arnett TR Acidosis, hypoxia and bone Arch Biochem Biophys 2010 503 103 9 doi: 10.1016/j.abb.2010.07.021 20655868 10.1016/j.abb.2010.07.021

34 Daulatzai MA Pathogenesis of cognitive dysfunction in patients with obstructive sleep apnea: A hypothesis with emphasis on the nucleus tractus solitarius Sleep Disord 2012 2012 251096 doi: 10.1155/2012/251096 10.1155/2012/251096 PMC3581091 23470865

35 Andreou G Vlachos F Makanikas K Effects of chronic obstructive pulmonary disease and obstructive sleep apnea on cognitive functions: Evidence for a common nature Sleep Disord 2014 2014 768210 doi: 10.1155/2014/768210 10.1155/2014/768210 PMC3932644 24649370

36 He JY Jiang LS Dai LY The roles of the sympathetic nervous system in osteoporotic diseases: A review of experimental and clinical studies Ageing Res Rev 2011 10 253 63 doi: 10.1016/j.arr.2011.01.002 21262391 10.1016/j.arr.2011.01.002