Circadian variation in bone resorption is not related to serum cortisol

A Systematic Review of the Circadian Rhythm of Bone Markers in Blood

There exists a marked circadian variation for several bone markers (BM), which is influenced by endogenous as well as exogenous factors including hormones, physical activity, and fasting. Consequently, was the aim of this review to provide an overview of the knowledge of the circadian variation of BM and which factors influence this rhythmicity. A systematic search of PubMed was performed for studies evaluating the circadian variation of BM and which factors influence this rhythmicity. The studies were screened for eligibility by a set of predetermined criteria including a list of relevant BM and a minimum study duration of 24 h with at least 3 blood samples of which two should be at least 6 h apart. In total were 29 papers included. There exists a marked circadian variation for most BM including Carboxy-terminal Cross-Linked Telopeptide of Type I Collagen (CTX) and osteocalcin (OC) with nighttime or early morning peak. Pro-collagen Type I N-terminal Propeptide (PINP) and PTH also showed circadian rhythm but with less amplitude. The inter-osteoblast-osteoclast regulatory markers such as OPG, RANKL, FGF23, and sclerostin showed no circadian rhythm. The markers were differently affected by exogenous factors like fasting, which greatly reduced the circadian variation of CTX but did not affect PINP or OC. The marked circadian variation and the factors which influence the rhythmicity, e.g., fasting are of great consequence when measuring BM. To reduce variation and heighten validity should circadian variation and fasting be kept in mind when measuring BM.

Bone Turnover Markers: Basic Biology to Clinical Applications

Bone turnover markers (BTMs) are used widely, in both research and clinical practice. In the last 20 years, much experience has been gained in measurement and interpretation of these markers, which include commonly used bone formation markers bone alkaline phosphatase, osteocalcin, and procollagen I N-propeptide; and commonly used resorption markers serum C-telopeptides of type I collagen, urinary N-telopeptides of type I collagen and tartrate resistant acid phosphatase type 5b. BTMs are usually measured by enzyme-linked immunosorbent assay or automated immunoassay. Sources contributing to BTM variability include uncontrollable components (e.g., age, gender, ethnicity) and controllable components, particularly relating to collection conditions (e.g., fasting/feeding state, and timing relative to circadian rhythms, menstrual cycling, and exercise). Pregnancy, season, drugs, and recent fracture(s) can also affect BTMs. BTMs correlate with other methods of assessing bone turnover, such as bone biopsies and radiotracer kinetics; and can usefully contribute to diagnosis and management of several diseases such as osteoporosis, osteomalacia, Paget's disease, fibrous dysplasia, hypophosphatasia, primary hyperparathyroidism, and chronic kidney disease-mineral bone disorder.

The role of circadian rhythm in osteoporosis; a review

Osteoporosis is characterized by a high incidence rate, with significant effects on people’s lives. The underlying mechanisms are complex, with no treatments for the condition. Recent studies have indicated that melatonin can be used to treat osteoporosis by promoting osteoblast proliferation and differentiation, and inhibiting osteoclast differentiation. Specifically, in vivo mechanisms are initiated by stabilizing biological rhythms in bone tissue. In healthy organisms, these biological rhythms are present in bone tissue, and are characterized by bone formation during the day, and bone resorption at night. When this rhythm is disrupted, osteoporosis occurs. Thus, taking appropriate medication at different times of the day could produce different effects on osteoporosis rhythms. In this review, we characterized these processes, and provided treatments and management strategies for individuals with osteoporosis.

Circadian rhythm of markers of bone turnover in patients with chronic kidney disease

Patients with chronic kidney disease (CKD) have a high risk of bone fractures. A circadian rhythmicity in turnover and mineralization of bone appears to be of importance for bone health. In CKD disturbances in the circadian rhythm of various functions has been demonstrated and indeed the circadian rhythm in the mineral metabolism is disturbed. The aim of the present study was to compare the circadian rhythm of bone turnover markers in ten patients with CKD to ten healthy controls. Bone turnover markers (C-terminal telopeptide of type I collagen, tartrate-resistant acid phosphatase 5b, N-terminal propeptide of type I procollagen, bone alkaline phosphatase and osteocalcin) were measured every third hour for 24 h. All bone turnover markers displayed a significant circadian rhythm in both groups and there were no significant differences in the rhythmicity between the two groups (no group*time interaction). As expected, due to the reduced renal clearance, the overall level of C-terminal telopeptide of type I collagen and osteocalcin was higher in CKD compared to the healthy controls. The present study suggests that disturbances in the circadian rhythm of bone turnover do not explain the metabolic bone disease and increased risk of fractures in CKD.

The contribution of serum cortisone and glucocorticoid metabolites to detrimental bone health in patients receiving hydrocortisone therapy

BACKGROUND

Glucocorticoid therapy is the most common cause of iatrogenic osteoporosis. Less is known regarding the effect of glucocorticoids when used as replacement therapy on bone remodelling in patients with adrenal insufficiency. Enhanced intracellular conversion of inactive cortisone to active cortisol, by 11 beta-hydroxysteroid dehydrogenase type 1(11β-HSD1) and other enzymes leading to alterations in glucocorticoid metabolism, may contribute to a deleterious effect on bone health in this patient group.

METHODS

Study design: An open crossover prospective study randomizing ten hypopituitary men, with severe ACTH deficiency, to three commonly used hydrocortisone dose regimens.

MEASUREMENTS

Following 6 weeks of each regimen, patients underwent 24-h serum cortisol/cortisone sampling, measurement of bone turnover markers, and a 24-h urine collection for measurement of urinary steroid metabolites by gas chromatography-mass spectrometry (GC-MS). Serum cortisone and cortisol were analysed by liquid chromatography-mass spectrometry (LC-MS).

RESULTS

Dose-related and circadian variations in serum cortisone were seen to parallel those for cortisol, indicating conversion of ingested hydrocortisone to cortisone. The median area under the curve (AUC) of serum cortisone was significantly higher in patients on dose A (20 mg/10 mg) [670.5 (IQR 621-809.2)] compared to those on dose C (10 mg/5 mg) [562.8 (IQR 520.1-619.6), p = 0.01]. A negative correlation was observed between serum cortisone and bone formation markers, OC [1-49] (r = - 0.42, p = 0.03), and PINP (r = - 0.49, p = 0.01). There was a negative correlation between the AUC of night-time serum cortisone levels with the bone formation marker, OC [1-49] (r = - 0.41, p = 0.03) but there were no significant correlations between day-time serum cortisone or cortisol with bone turnover markers. There was a negative correlation between total urinary cortisol metabolites and the bone formation markers, PINP (r = - 0.39, p = 0.04), and OC [1-49] (r = - 0.35, p = 0.06).

CONCLUSION

Serum cortisol and cortisone and total urinary corticosteroid metabolites are negatively associated with bone turnover markers in patients receiving replacement doses of hydrocortisone, with nocturnal glucocorticoid exposure having a potentially greater influence on bone turnover.

TRIAL REGISTRATION

Irish Medicines Board Clinical Trial Number - CT900/459/1 and EudraCT Number - 2007-005018-37 . Registration date: 07-09-2007.

Bone turnover is altered during 72 h of sleep restriction: a controlled laboratory study

PURPOSE

The objective of the study was to evaluate how controlled, short-term sleep restriction (SR; 72 h) alters markers of bone formation and resorption and urinary calcium (Ca) output.

METHODS

Ten healthy, sleep-adequate, male soldiers were housed in the research facility one day prior to and for the duration of SR. Diet was controlled to provide adequate energy balance and macronutrient distribution, meeting the recommended dietary allowance (RDA) for Ca. Subjects engaged in light activities to maintain wakefulness and were allowed 2 h of sleep per night (0430-0630 hours). Blood samples were collected each morning at 0 h (baseline) and 24, 48, and 72 h of SR. Serum was assayed for parathyroid hormone (PTH), bone alkaline phosphatase (BAP), tartrate-resistant acid phosphatase (TRAP), and C-terminal telopeptide of type I collagen (CTX). Urine was collected in 24 h increments during SR for measurement of Ca and creatinine (Cr).

RESULTS

BAP was reduced at 24 h (P= 0.015) and resorption markers TRAP and CTX were increased after 48 and 72 h of SR compared to baseline (P < 0.05). The ratio of BAP:TRAP was significantly lower (P= 0.017) at 48 and 72 h of SR. In contrast, total 24 h urinary Ca and Ca/Cr excretion were unchanged.

CONCLUSIONS

Markers of bone formation and resorption are uncoupled in response to as little as 48 h of SR even when Ca intake is at the RDA. Sleep deprivation may be a risk factor for reduced bone health due to perturbations in bone turnover.

The 24-hour serum profiles of bone markers in healthy older men and women

The process of bone turnover displays variations over 24 h, with C-terminal cross-linked telopeptide of type 1 collagen (CTX) and osteocalcin exhibiting a nadir in the afternoon and a peak in the night. In contrast, N-terminal propeptide of type 1 procollagen (P1NP) did not display an apparent 24-hour rhythm. Other emerging novel biomarkers of bone, sclerostin and Dickkopf-related protein 1 (DKK1), are markers of osteocyte activity with limited data available regarding their 24-hour profiles. In this study, we aimed to extend available data on 24-hour profiles of CTX, osteocalcin, and P1NP and to assess the 24-hour profiles of sclerostin and DKK1 in healthy older men and women and to compare these between men and women. We measured these five bone markers in EDTA plasma collected every 4 h during 24 h in 37 healthy older men and women (range 52-76 years). Differences between time points were determined using repeated measures ANOVA and cosinor analyses were performed to determine circadian rhythmicity. The circadian rhythm of CTX was confirmed by the cosinor model, with women showing larger amplitude compared to men. Osteocalcin showed higher levels during nighttime compared to daytime in both men and women. For P1NP levels we observed a small but significant increase in the night in men. Sclerostin and DKK1 did not show a circadian rhythm, but sclerostin levels differed between time points. Because of the large intraindividual variation, DKK1 as measured in this study cannot be considered a reliable marker for diagnostic or research purposes. In conclusion, when measuring CTX, osteocalcin, P1NP, or sclerostin either in clinical practice or in a research setting, one should consider the 24-hour profiles of these bone markers.

The importance of the circadian system & sleep for bone health

Adequate sleep timed appropriately during the circadian night is important for numerous biological processes and systems. New evidence suggests that both sleep timing and duration may be important for optimal bone health as well. This review examines the diurnal variation of bone turnover markers (BTMs) and the importance of circadian clock genes in regulating bone mass. In addition, this review explores the evidence for a link between shift work (and its associated disturbances in sleep duration/quality and circadian alignment) and alterations in bone metabolism and bone health. Finally, we review how commonly used medications and over-the-counter substances (e.g. caffeine, melatonin) complicate the relationship between sleep and circadian disorders and bone health.

High Evening Cortisol Level Is Associated With Low TBS and Increased Prevalent Vertebral Fractures: OsteoLaus Study

CONTEXT

Increased evening cortisol levels have been implicated in bone mineral density (BMD) loss. The effect on bone microarchitecture and fracture risk has never been studied.

OBJECTIVE

To study the relationship between salivary cortisol circadian rhythm and (1) trabecular bone score (TBS) and (2) fracture prevalence.

DESIGN, SETTING, PATIENTS, AND INTERVENTIONS

Cross-sectional study including 608 women >50 years old (mean = 65.5) from the OsteoLaus cohort. Data included the FRAX© questionnaire, BMD, TBS and vertebral fracture (VFx) assessment by dual X-ray absorptiometry, and measures of salivary cortisol (awakening, 30 minutes thereafter, 11 am, and 8 pm).

RESULTS

In the multivariate model, participants in the highest tertile of 8 pm salivary cortisol (sc-8 pm) (mean = 5.7 ± 2.5 nmol/L) vs lowest tertile (1.7 ± 0.4 nmol/L) had lower TBS values (1.27 vs 1.29; P = 0.02), more prevalent VFx grades 2 and 3 (odds ratio = 5.34; P = 0.012), low-trauma fractures (odds ratio = 1.80; P = 0.036), and major osteoporotic fractures (odds ratio = 1.96; P = 0.042), without difference in lumbar spine BMD (0.91 vs 0.92 g/cm2; P = 0.431). VFx prevalence was associated with sc-8 pm and TBS independently of each other and of other risk factors. The cut-point for sc-8 pm correlating with the presence of >1 VFx was 3.62 nmol/L (sensitivity 0.74, specificity 0.66).

CONCLUSIONS

High sc-8 pm is associated with low TBS and an increased prevalence of radiologic VFx independently of other risk factors. Measurement of sc-8 pm may add relevant information in the assessment of fracture risk.

Diurnal Rhythms of Bone Turnover Markers in Three Ethnic Groups

CONTEXT

Ethnic groups differ in fragility fracture risk and bone metabolism. Differences in diurnal rhythms (DRs) of bone turnover and PTH may play a role.

OBJECTIVE

We investigated the DRs of plasma bone turnover markers (BTMs), PTH, and 1,25(OH)2D in three groups with pronounced differences in bone metabolism and plasma PTH.

PARTICIPANTS

Healthy Gambian, Chinese, and white British adults (ages 60-75 years; 30 per country).

INTERVENTIONS

Observational study with sample collection every 4 hours for 24 hours.

MAIN OUTCOMES

Levels of plasma C-terminal telopeptide of type I collagen, procollagen type-1 N-propeptide, N-mid osteocalcin, bone alkaline phosphatase, PTH, and 1,25-dihydroxyvitamin D were measured. DRs were analyzed with random-effects Fourier regression and cross-correlation and regression analyses to assess associations between DRs and fasting and 24-hour means of BTMs and PTH.

RESULTS

Concentrations of BTMs, PTH, and 1,25-dihydroxyvitamin D were higher in Gambians compared to other groups (P < .05). The DRs were significant for all variables and groups (P < .03) and were unimodal, with a nocturnal peak and a daytime nadir for BTMs, whereas PTH had two peaks. The DRs of BTMs and PTH were significantly cross-correlated for all groups (P < .05). There was a significant positive association between C-terminal telopeptide of type I collagen and PTH in the British and Gambian groups (P = .03), but not the Chinese group.

CONCLUSIONS

Despite ethnic differences in plasma BTMs and PTH, DRs were similar. This indicates that alteration of rhythmicity and loss of coupling of bone resorption and formation associated with an elevated PTH in other studies may not uniformly occur across different populations and needs to be considered in the interpretation of PTH as a risk factor of increased bone loss.

Effect of Ramadan fasting in Saudi Arabia on serum bone profile and immunoglobulins

BACKGROUND

Each year Muslims fast from dawn to sunset for 1 month (Ramadan). In Saudi Arabia, the sleep-wake cycle during Ramadan is severely disturbed and is associated with abolition of the circadian cortisol rhythm, exposing Saudis to continuously increased cortisol levels, which may influence the immune response. In addition to cortisol, sleep and fasting affect the secretion of parathyroid hormone (PTH) and hence bone metabolism.

METHODS

Our objective was to investigate the effect of Ramadan type fasting on secretory patterns of PTH, markers of bone metabolism, and serum immunoglobulins. Blood samples from healthy young volunteers were collected at 9 a.m. and 9 p.m. (± 1 hour) before (Shaban) and 2 weeks into Ramadan. Calcium, phosphorus, magnesium, albumin, alkaline phosphatase, 25-OH vitamin D, intact PTH (iPTH), and immunoglobulin (Ig) A, M and G were measured.

RESULTS

During Ramadan, evening-adjusted calcium was higher (p = 0.036) and phosphate lower (p < 0.001) than the corresponding morning value. Moreover, the Ramadan mean morning phosphate was higher and the evening level lower was than Shabaan values (p = 0.010 and p <0.001, respectively), while mean iPTH level was decreased compared with the morning value (p = 0.001), and the evening mean during Shabaan (p = 0.029). Mean IgG concentration was significantly lower during Ramadan (p = 0.003 and p = 0.021 for morning and evening, respectively).

CONCLUSIONS

Changes in dietary practices during Ramadan modulated PTH secretion to a pattern which might be beneficial to bone health. Combined effects of fasting and disturbed sleep led to a noted decrease in IgG level. Therefore, a possible beneficial effect of fasting on bone turnover is combined with decreased immune response.

Obstructive sleep apnea and metabolic bone disease: insights into the relationship between bone and sleep

Obstructive sleep apnea (OSA) and low bone mass are two prevalent conditions, particularly among older adults-a section of the U.S. population that is expected to grow dramatically over the coming years. OSA, the most common form of sleep-disordered breathing, has been linked to multiple cardiovascular, metabolic, hormonal, and inflammatory derangements and may have adverse effects on bone. However, little is known about how OSA (including the associated hypoxia and sleep loss) affects bone metabolism. In order to gain insight into the relationship between sleep and bone, we review the growing information on OSA and metabolic bone disease and discuss the pathophysiological mechanisms by which OSA may affect bone metabolism/architecture.

Glucocorticoids mediate circadian timing in peripheral osteoclasts resulting in the circadian expression rhythm of osteoclast-related genes

Circadian rhythms are prevalent in bone metabolism. However, the molecular mechanisms involved are poorly understood. Recently, we suggested that output signals from the suprachiasmatic nucleus (SCN) are transmitted from the master circadian rhythm to peripheral osteoblasts through β-adrenergic and glucocorticoid signaling. In this study, we examined how the master circadian rhythm is transmitted to peripheral osteoclasts and the role of clock gene in osteoclast. Mice were maintained under 12-hour light/dark periods and sacrificed at Zeitgeber times 0, 4, 8, 12, 16 and 20. mRNA was extracted from femur (cancellous bone) and analyzed for the expression of osteoclast-related genes and clock genes. Osteoclast-related genes such as cathepsin K (CTSK) and nuclear factor of activated T-cells, cytoplasmic 1 (NFATc1) showed circadian rhythmicity like clock genes such as period 1 (PER1), PER2 and brain and muscle Arnt-like protein 1 (BMAL1). In an in vitro study, not β-agonist but glucocorticoid treatment remarkably synchronized clock and osteoclast-related genes in cultured osteoclasts. Chromatin immunoprecipitation (ChIP) assay showed the interaction between BMAL1 proteins and promoter region of CTSK and NFATc1. To examine whether endogenous glucocorticoids influence the osteoclast circadian rhythms, mice were adrenalectomized (ADX) and maintained under 12-hour light/dark periods at least two weeks before glucocorticoid injection. A glucocorticoid injection restarted the circadian expression of CTSK and NFATc1 in ADX mice. These results suggest that glucocorticoids mediate circadian timing to peripheral osteoclasts and osteoclast clock contributes to the circadian expression of osteoclast-related genes such as CTSK and NFATc1.

Comparison of β-adrenergic and glucocorticoid signaling on clock gene and osteoblast-related gene expressions in human osteoblast

Most living organisms exhibit circadian rhythms that are generated by endogenous circadian clocks, the master one being present in the suprachiasmatic nuclei (SCN). Output signals from the SCN are believed to transmit standard circadian time to peripheral tissue through sympathetic nervous system and humoral routes. Therefore, the authors examined the expression of clock genes following treatment with the β-adrenergic receptor agonist, isoprenaline, or the synthetic glucocorticoid, dexamethasone, in cultured human osteoblast SaM-1 cells. Cells were treated with 10(-6) M isoprenaline or 10(-7) M dexamethasone for 2 h and gene expressions were determined using real-time polymerase chain reaction (PCR) analysis. Treatment with isoprenaline or dexamethasone induced the circadian expression of clock genes human period 1 (hPer1), hPer2, hPer3, and human brain and muscle Arnt-like protein 1 (hBMAL1). Isoprenaline or dexamethasone treatment immediately increased hPer1 and hPer2 and caused circadian oscillation of hPer1 and hPer2 with three peaks within 48 h. hPer3 expression had one peak after isoprenaline or dexamethasone treatment. hBMAL expression had two peaks after isoprenaline or dexamethasone treatment, the temporal pattern being in antiphase to that of the other clock genes. Dexamethasone treatment delayed the oscillation of all clock genes for 2-6 h compared with isoprenaline treatment. The authors also examined the expression of osteoblast-related genes hα-1 type I collagen (hCol1a1), halkaline phosphatase (hALP), and hosteocalcin (hOC). Isoprenaline induced oscillation of hCol1a1, but not hALP and hOC. On the other hand, dexamethasone induced oscillation of hCol1a1 and hALP, but not hOC. Isoprenaline up-regulated hCol1a1 expression, but dexamethasone down-regulated hCol1a1 and hALP expression in the first phase.

Bone-turnover markers in fracture healing

Biochemical markers of bone-turnover have long been used to complement the radiological assessment of patients with metabolic bone disease. Their implementation in daily clinical practice has been helpful in the understanding of the pathogenesis of osteoporosis, the selection of the optimal dose and the understanding of the progression of the onset and resolution of treatment. Since they are derived from both cortical and trabecular bone, they reflect the metabolic activity of the entire skeleton rather than that of individual cells or the process of mineralisation.

Quantitative changes in skeletal-turnover can be assessed easily and non-invasively by the measurement of bone-turnover markers. They are commonly subdivided into three categories; 1) bone-resorption markers, 2) osteoclast regulatory proteins and 3) bone-formation markers. Because of the rapidly accumulating new knowledge of bone matrix biochemistry, attempts have been made to use them in the interpretation and characterisation of various stages of the healing of fractures. Early knowledge of the individual progress of a fracture could help to avoid delayed or nonunion by enabling modification of the host’s biological response.

The levels of bone-turnover markers vary throughout the course of fracture repair with their rates of change being dependent on the size of the fracture and the time that it will take to heal. However, their short-term biological variability, the relatively low bone specificity exerted, given that the production and destruction of collagen is not limited to bone, as well as the influence of the host’s metabolism on their concentration, produce considerable intra- and inter-individual variability in their interpretation. Despite this, the possible role of bone-turnover markers in the assessment of progression to union, the risks of delayed or nonunion and the impact of innovations to accelerate fracture healing must not be ignored.

Biochemical assessment of bone turnover and bone fragility in men

Osteoporosis in men is less studied than in women. Few data concern biochemical bone turnover markers (BTM) in men and their potential use.

METHODOLOGY

We evaluated papers concerning BTM in men cited on Medline. Selection of studies were based on the number of subjects, age range, group homogeneity, follow-up duration, number of BTM.

RESULTS

BTM levels are high in young men, then decrease with age. In elderly men, bone resorption increases with age more than bone formation. Variability of individual values is high and their significance is unclear. In elderly men, BTM levels correlate negatively with bone mineral density suggesting that accelerated bone turnover underlies age-related bone loss. Data on the prediction of accelerated bone loss and fractures by BTM in men are scant. Testosterone treatment induces a decrease in bone resorption followed by a decrease in bone formation. Bisphosphonates and calcitonin decrease BTM levels in osteoporotic men. Parathyroid hormone 1-34 and growth hormone induce a rapid increase in bone turnover followed by a progressive slowdown.

CONCLUSIONS

Few studies concern BTM in men. Currently available data are not sufficient to suggest guidelines for the practical use of BTM in the clinical management of the osteoporosis in elderly men.

Circadian oscillation of gene expression in murine calvarial bone

The genes encoding the core circadian transcription factors display an oscillating expression profile in murine calvarial bone. More than 26% of the calvarial bone transcriptome exhibits a circadian rhythm, comparable with that observed in brown and white adipose tissues and liver. Thus, circadian mechanisms may directly modulate oxidative phosphorylation and multiple metabolic pathways in bone homeostasis.

INTRODUCTION

Although circadian rhythms have been associated historically with central regulatory mechanisms, there is emerging evidence that the circadian transcriptional apparatus exists in peripheral tissues. The aim of this study was to determine the presence and extent of circadian oscillation in the transcriptome of murine calvarial bone.

MATERIALS AND METHODS

Cohorts of 8-week-old male AKR/J mice were maintained in a controlled 12-h light:12-h dark cycle on an ad libitum diet for 2 weeks. Groups of three mice were killed every 4 h over a 48-h period. The level of gene expression at successive times-points was determined by quantitative RT-PCR and Affymetrix microarray. Data were analyzed using multiple statistical time series algorithms, including Cosinor, Fisher g-test, and the permutation time test.

RESULTS

Both the positive (Bmal1, Npas2) and negative (Cry1, Cry2, Per1, Per2, Per3) elements of the circadian transcriptional apparatus and their immediate downstream targets and mediators (Dbp, Rev-erbalpha, Rev-erbbeta) exhibited oscillatory expression profiles. Consistent with findings in other tissues, the positive and negative elements were in antiphase relative to each other. More than 26% of the genes present on the microarray displayed an oscillatory profile in calvarial bone, comparable with the levels observed in brown and white adipose tissues and liver; however, only a subset of 174 oscillating genes were shared among all four tissues.

CONCLUSIONS

Our findings show that the components of the circadian transcriptional apparatus are represented in calvarial bone and display coordinated oscillatory behavior. However, these are not the only genes to display an oscillatory expression profile, which is seen in multiple pathways involving oxidative phosphorylation and lipid, protein, and carbohydrate metabolism.

An update on biomarkers of bone turnover and their utility in biomedical research and clinical practice

BACKGROUND

Maintenance of the structural and functional integrity of the skeleton is a critical function of a continuous remodeling driven by highly associated processes of bone resorption and synthetic activities driven by osteoclasts and osteoblasts, respectively. Acceleration of bone turnover, accompanied with a disruption of the coupling between these cellular activities, plays an established role in the pathogenesis of metabolic bone diseases, such as osteoporosis. During the past decades, major efforts have been dedicated to the development and clinical assessment of biochemical markers that can reflect the rate of bone turnover. Numerous studies have provided evidence that serum levels or urinary excretion of these biomarkers correlate with the rate of bone loss and fracture risk, proving them as useful tools for improving identification of high-risk patients.

OBJECTIVE

The aim of the present review is to give an update on biomarkers of bone turnover and give an overview of their applications in epidemiological and clinical research.

DISCUSSION

Special attention is given to their utility in clinical trials testing the efficacy of drugs for the treatment of osteoporosis and how they supplement bone mass measurements. Recent evidence suggests that biochemical markers may provide information on bone age that may have indirectly relates to bone quality; the latter is receiving increasing attention. A more targeted use of biomarkers could further optimize identification of high-risk patients, the process of drug discovery, and monitoring of the efficacy of osteoporosis treatment in clinical settings.

Circadian phase difference of leptin in android versus gynoid obesity

A circadian rhythm in serum leptin, measured every 4 h for 24 h, characterizes normal-weight women (N = 14), and women with gynoid (N = 17) or android (N = 26) obesity, peaking around midnight (P < 0.05), but differing by about 3 h between android and gynoid women (P < 0.01). Obesity is associated with a higher MESOR (rhythm-adjusted mean; P < 0.001) and a smaller relative circadian amplitude (P < 0.05). Gynoid obesity is associated with a larger circadian amplitude of cortisol (P < 0.05), whereas android obesity is associated with a larger circadian amplitude and a higher MESOR of insulin (P < 0.05). Understanding putative mechanisms underlying different body fat distribution may lead to improved chronotherapeutic measures.

Circadian variation in biochemical markers of bone cell activity and insulin-like growth factor-I in two-year-old horses

Studies in humans have found circadian changes to be one of the most important sources of controllable preanalytical variability when evaluating bone cell activity using biochemical markers. It remains unclear whether similar circadian changes influence bone marker concentrations in the horse. The aim of this study was to characterize changes in serum concentrations of three biochemical markers of bone cell activity over a 24-h period in six 2-yr-old Thoroughbred mares, and to determine circadian variability in IGF-I, which regulates bone turnover. Three bone markers were measured in serum: osteocalcin, a marker of bone formation, the carboxy-terminal propeptide of type-I collagen (a marker of bone formation), and the carboxy-terminal telopeptide of type-I collagen (a marker of bone resorption). Data were analyzed using the cosinor technique, which fits a 24-h cycle to each dataset. A significant circadian rhythm was observed for osteocalcin (P = 0.028), with an estimated amplitude of 7.6% of the mean (95% confidence interval 1.3% to 16.3%), and an estimated peak time of 0900. However, the observed rhythm for the carboxy-terminal telopeptide of type-I collagen (amplitude = 7.4%) was not significant (P = 0.067), and there were no significant changes in concentrations of the carboxy-terminal propeptide of type-I collagen over the 24-h study period (P = 0.44). There was a small but significant circadian rhythm for IGF-I (P = 0.04), with an estimated amplitude of 3.4% (95% confidence interval 0.2 to 7.1%) and peak at 1730. Further studies are now required to determine the potential association between circadian changes in IGF-I and osteocalcin in the horse. Although no significant circadian variation was found in concentrations of the car-boxy-terminal propeptide of type-I collagen and the carboxy-terminal telopeptide of type-I collagen, this may in part be a result of the age of the animals that were still skeletally immature. Future studies should aim to determine whether these markers develop a circadian rhythm at a later age when growth is complete. In the meantime, consistency in time of sampling should continue to be considered best practice when measuring biochemical markers of bone turnover in the horse.

Reduction of nocturnal rise in bone resorption by subcutaneous GLP-2

We have previously shown that a subcutaneous injection of glucagon-like peptide-2 (GLP-2) at 9 a.m. in fasting postmenopausal women results in a dose-dependent decrease in the serum concentration of fragments derived from the degradation of the C-terminal telopeptide region of collagen type I (s-CTX), a marker of bone resorption. In contrast, GLP-2 was found to have a neutral effect on bone formation, as assessed by serum osteocalcin. Since increased s-CTX levels are normally observed at night, we conducted bedtime studies in healthy postmenopausal women. The objective was to study the effect of GLP-2 injection on bone turnover given at bedtime. A total of 81 postmenopausal women were included in two randomised placebo-controlled studies. In conclusion, we found a dose-related reduction of s-CTX after injection of GLP-2 (P < 0.05) and osteocalcin levels was increased as compared to placebo (P = 0.07) by the treatment, suggestive of a stimulative effect on bone formation. An area under the curve (AUC0-10 h) analysis for s-CTX after GLP-2 injection confirmed the dose-related decrease as compared to placebo (P < 0.05).

Octreotide abolishes the acute decrease in bone turnover in response to oral glucose

Feeding or oral intake of glucose results in an acute suppression of bone turnover. This does not appear to be mediated by insulin. Several gastrointestinal hormones modulate bone turnover in vitro and may mediate this response. We examined whether inhibiting the production of gastrointestinal hormones using octreotide could block glucose-mediated suppression of bone turnover. Fifteen subjects were each studied on four occasions in a randomized, single-blind, crossover study after receiving 1) oral placebo, iv saline; 2) oral glucose, iv saline; 3) oral glucose, iv octreotide; or 4) iv octreotide alone. We measured serum C-terminal telopeptide of type I collagen, urinary N-terminal telopeptide of type I collagen, osteocalcin, procollagen type I N-terminal propeptide, PTH, insulin, ionized calcium, and glucose over 4 h. All bone turnover markers decreased significantly after oral glucose (P < 0.001). At 120 min serum C-terminal telopeptide decreased by 45 +/- 2%, urinary N-terminal telopeptide by 31 +/- 7%, osteocalcin by 16 +/- 1%, and procollagen type I N-terminal propeptide by 8 +/- 1%. There was no significant decrease in bone turnover in response to oral glucose during octreotide infusion. Octreotide alone resulted in a significant increase in all bone turnover markers (P < 0.05) and PTH (P < 0.01). We conclude that octreotide completely abolishes the bone turnover response to glucose intake and increases PTH secretion. The apparent bone turnover response to feeding is probably mediated by an octreotide-inhibitable endocrine factor.

Circadian variation in the serum concentration of C-terminal telopeptide of type I collagen (serum CTx): effects of gender, age, menopausal status, posture, daylight, serum cortisol, and fasting

We examined the diurnal variation in serum concentration of C-terminal telopeptide of type I collagen (serum CrossLaps, sCTx) under various conditions. The studies included a total of 100 individuals. Blood samples were collected every 3 h over 27 h. sCTx levels varied over the 24 h with a maximum at about 05:00 in the morning and a minimum of about 14:00 in the afternoon. The variation had a magnitude of about +/-40% around the 24 h mean and was similar in premenopausal and early and late postmenopausal women with normal and low bone mass. Furthermore, it was not affected by 5 days of bed-rest, by absence of a normal diurnal variation in cortisol production, or by absence of a normal light cycle (blindness). Nasal salmon calcitonin, an antiresorptive drug used for treatment of osteoporosis, was not able to break the circadian pattern whether the treatment was administered in the morning or the evening. The only parameter that showed a pronounced influence on the circadian variation was fasting, which reduced the variation significantly to about one fourth. From a practical point of view the results of this study demonstrate that samples for sCTx should be taken in the fasting state.

Acute changes of bone turnover and PTH induced by insulin and glucose: euglycemic and hypoglycemic hyperinsulinemic clamp studies

Bone turnover is acutely suppressed after feeding or oral glucose. Insulin infusion suppresses bone turnover and might mediate this effect, but this is confounded by a possible direct effect of hypoglycemia. We examined the effect of euglycemic hyperinsulinemia and hypoglycemic hyperinsulinemia on bone turnover using an insulin clamp. Sixteen men participated in this double-blind crossover study. Clamp induction involved infusion of insulin (80 mU/m(2).min) while maintaining euglycemia (5 mmol/liter) for 40 min with a variable rate dextrose infusion. Glucose was lowered to 2.5 mmol/liter (hypoglycemic clamp) or maintained at 5 mmol/liter (euglycemic clamp) for a further 105 min. Nine controls received a matched saline infusion. Measurements included serum C-terminal telopeptide of type I collagen, procollagen type I N-terminal propeptide, osteocalcin, and PTH. Induction of hyperinsulinemia resulted in a reduction in PTH (27% +/- 5; P < 0.01), but no significant change in bone turnover from baseline. Hypoglycemic clamp resulted in suppression of serum C-terminal telopeptide of type I collagen by 34% +/- 3, procollagen type I N-terminal propeptide by 15% +/- 1, osteocalcin by 5% +/- 1, and PTH by a further 12% +/- 5 (all P < 0.05). By contrast, there was no significant change in any marker of bone turnover during euglycemic clamp. Postprandial hyperinsulinemia is unlikely to explain the acute suppression of bone turnover with feeding. The reduction in bone turnover during hypoglycemia may be related to hypoglycemia itself, acute changes in PTH, or other hormones released in response to hypoglycemia.

Hypercorticism blunts circadian variations of osteocalcin regardless of nutritional status

Anorexia nervosa (AN) and Cushing's syndrome (CS) are both responsible for osteoporosis. The mechanisms leading to osteoporosis in AN include hypogonadism, nutritional depletion, and in some cases hypercorticism. Osteocalcin circulating level is a serum marker of osteoblastic activity that follows a circadian rhythm (OCR). Serum osteocalcin is decreased in both CS and AN and can be increased with treatment. In this study we analyzed the influence of combined cortisol and nutritional status on osteocalcin levels and its circadian rhythm in these two different models of hypercorticism, one nutritionally replete (CS) and one nutritionally deplete (AN), and we evaluated the effects of their treatment (surgical cure and weight gain, respectively). Before treatment, osteocalcin levels were lower in CS (n = 16) and AN (n = 42) than in controls and in the AN patient subgroup with hypercorticism (n = 13) compared to those without (n = 29). OCR was absent in CS and in AN patients with hypercorticism, whereas their circadian cortisol cycle was maintained. In CS, successful surgical treatment increased osteocalcin levels (n = 5) and restored OCR. In AN, weight gain (n = 13) induced a significant decrease in cortisol levels in hypercortisolic AN patients, and restored normal osteocalcin levels and OCR. In conclusion, we found that hypercorticism was associated with a decrease in osteocalcin levels in nutritionally replete or deplete patients and that OCR was more affected by cortisol levels than by cortisol cycle.

Assessment and Recommendations on Factors Contributing to Preanalytical Variability of Urinary Pyridinoline and Deoxypyridinoline

Background: Pyridinoline (PYD) and deoxypyridinoline (DPD) are two of the most extensively characterized biochemical bone markers, but the interpretation of results is hampered by biologic and other preanalytical variability. We reviewed factors contributing to preanalytical variation of pyridinium cross-links in urine.

Methods: We searched four databases for English-language reports on PYD and/or DPD in urine. Searches were restricted to humans, except for studies of stability, when the search was expanded to other species. The 599 identified articles were supplemented with references from those articles and with articles known to the authors.

Results: The mean reported within-day variability was 71% for PYD (range, 57–78%) and 67% for DPD (range, 53–75%). The mean interday variability was 16% for both DPD and PYD (range for PYD, 12–21%; range for DPD, 5–24%). The mean intersubject variabilities across studies were 26% for PYD (range, 12–63%) and 34% for DPD (range, 8–98%) for healthy premenopausal women and 36% (range, 22–61%) and 40%, (range, 27–54%) for postmenopausal women, respectively. Specimen instability and errors in creatinine measurements were additional sources of variability.

Conclusions: Intra- and intersubject variability can be reduced by collecting specimens at a specific time of the day and by maintaining similar patient status at each specimen collection regarding factors such as medications and dietary supplements.

Mechanism of circadian variation in bone resorption

The diurnal variation in bone resorption markers is poorly understood and may contain essential information about regulation of bone resorption. To explore the acute regulation of bone resorption we studied bone turnover in 14 postmenopausal women during a randomized, crossover, 24 h study of oral glucose tolerance test (OGTT), normal diet, or fasting. Whereas fasting counteracted variation in bone resorption, as measured by serum C-telopeptide fragments of collagen type 1 degradation (s-CTx), OGTT and normal diet induced a 50% reduction (p < 0.001) over 2 h. For OGTT, s-CTx reverted to baseline levels after 6 h, and for normal diet s-CTx remained suppressed during the afternoon and returned to baseline overnight. Repeated OGTT at 8:00 A.M. and 8:00 P.M. in nine postmenopausal women demonstrated that identical reductions in s-CTx could be obtained at both timepoints with an intermediate return to baseline between tests. A 2 h randomized, crossover study of OGTT and fasting in 23 men and premenopausal women similarly revealed a 50% decrease in s-CTx. A randomized, crossover 2 h study of insulin tolerance test compared with fasting in six men and premenopausal women demonstrated a 20%-30% decrease in s-CTx (p < 0.01-0.05). Nine hour follow-up of ten healthy individuals during a crossover experiment of OGTT, protein, and fat intake revealed a comparable 50% reduction in s-CTx, but distinct profiles of serum glucose and serum insulin. Bone resorption was reduced by intake of food, glucose, fat, and protein and counteracted by fasting, and this seems to have been be independent of age and gender. Both exogenous and endogenous insulin stimulation tests induced a reduction in bone resorption, but this was only partial when compared with the reduction observed during food intake.

Circadian and longitudinal variation of serum C-telopeptide, osteocalcin, and skeletal alkaline phosphatase in C3H/HeJ mice

Inbred strains of mice are increasingly being used as an animal model to investigate skeletal disorders relevant to humans. In the bone field, one of the most convenient endpoints for evaluating genetic, physiological, or pharmaceutical perturbations is the use of biochemical markers. To apply biochemical markers in an effective manner, it is of key importance to establish the biological variation and appropriate sampling time. In this study, we evaluate two components: (i) circadian changes, and (ii) longitudinal variation for three serum markers, osteocalcin, C-telopeptide, and skeletal alkaline phosphatase (sALP), using 6-week-old C3H/HeJ (C3H) mice. To study circadian rhythms, the mice were randomly divided into eight groups of 15 mice each. Blood was collected at 3 h intervals, starting at 9:00 A.M. and continuing until 6:00 A.M. the next day. To determine whether circadian rhythm is intrinsically regulated or influenced by restricted food intake, it was also studied after a 12 h fasting period. Serum osteocalcin and C-telopeptide levels were measured by enzyme-linked immunoassay (ELISA) and skeletal alkaline phosphatase by a kinetic assay. The results demonstrated significant circadian variations in osteocalcin and C-telopeptide levels with a peak value between 0900 and 1200 h during daytime and a nadir between 15:00 and 18:00 h. The peak levels of C-telopeptide and osteocalcin were 26%-66% higher as compared with 24 h mean values. The pattern of the circadian variation of C-telopeptide and osteocalcin was similar in female and male animals and was not significantly affected by restricted food intake. The sALP levels were only marginally affected by the circadian rhythm. Longitudinal variations, expressed as coefficient of variation (CV), for osteocalcin, C-telopeptide, and sALP concentrations were 17%, 14%, and 16%, respectively. In addition, the longitudinal variations were not significantly influenced by the time of blood collection in sALP and osteocalcin levels, whereas C-telopeptide levels showed significantly higher within-subject day-to-day variation in morning samples, as compared with blood samples collected in the afternoon. The results highlight the importance of: (i) the timing of sample collection for appropriate interpretation of the bone marker data; and (ii) using the appropriate number of samples based on the variance obtained herein.

Circadian variation of serum leptin in healthy and diabetic men

Leptin, from the Greek leptos, meaning thin (in reference to its ability to reduce body fat stores), is a hormone secreted primarily by adipocytes. At one time, leptin was portrayed as a potential means of combating obesity. Recently, leptin has been identified as a potent inhibitor of bone formation, acting through the central nervous system. Since numerous studies clearly show that bone remodeling is circadian rhythmic with peak activity during sleep, it is of interest to explore circadian variability in serum leptin. Accordingly, circadian characteristics of serum leptin were examined in 7 clinically healthy men and 4 obese men with type II diabetes. Blood samples were collected for 24 h at 3 h intervals beginning at 19:00. The dark (sleep) phase of the light-dark cycle extended from 22:30 to 06:30, with brief awakening for sampling at 01:00 and 04:00. Subjects consumed general hospital meals (2400 calories) at 16:30, 07:30, and 13:30. Serum leptin levels were determined by a R&D Systems enzyme immunoassay technique. Data were analyzed by linear least-squares estimation using the population multiple components method. A statistically significant (P < .018) circadian rhythm modeled by a single 24 h cosine curve characterized the data of each group. The 24 h mean leptin level was statistically greater (P < .001) in the obese diabetic men than in the healthy men (9.47 +/- 0.66 ng/mL vs. 24.07 +/- 1.71 ng/mL, respectively). Higher leptin levels occurred between midnight and roughly 02:30, and lowest leptin levels occurred between noon and the early afternoon. The phasing of this rhythm is similar to the circadian rhythm in bone remodeling previously described. Our results suggest the findings from a single morning blood sampling for leptin may be misleading since it may underestimate the mean 24 h and peak concentrations of the hormone.

Space Flight Is Associated with Rapid Decreases of Undercarboxylated Osteocalcin and Increases of Markers of Bone Resorption without Changes in Their Circadian Variation: Observations in Two Cosmonauts

Background: Microgravity induces bone loss by mechanism(s) that remain largely unknown.

Methods: We measured biochemical markers related to bone remodeling in two cosmonauts before, during, and after 21- and 180-day space flights, respectively.

Results: During both flights, type I procollagen propeptide and bone alkaline phosphatase decreased as early as 8 days after launch. Undercarboxylated osteocalcin percentage increased early and remained high during both flights. Vitamin K supplementation restored carboxylation of osteocalcin during the long-term flight. Urinary and serum C-telopeptide of type I collagen (CTX) increased as early as day 8 of the flights; the increase was greater in serum than in urine. Pyridinoline, free deoxypyridinoline, and N-telopeptide increased less than CTX during the short-term space flight. The circadian rhythm of bone resorption assessed by urine CTX and free deoxypyridinoline was not altered by microgravity.

Conclusion: Vitamin K metabolism or action and bone remodeling may be altered in cosmonauts.

Weight gain reverses bone turnover and restores circadian variation of bone resorption in anorexic patients

OBJECTIVE

The present study was conducted in order to describe the variations and circadian rhythm of biochemical markers of bone remodelling at baseline and after weight gain in patients with anorexia nervosa (AN).

SUBJECTS

We studied 9 women (mean age 21 years, range: 16-30) with established AN who remained amenorrhoeic during the study and with a low body mass index (BMI) after refeeding and 6 female controls (mean age 20 years, range, 18-24 and BMI: 20.6 +/- 1.1 kg/m2). Refeeding was not associated with any other intervention or treatment, especially oestrogen replacement or hormonal contraception. Serum levels of oestradiol remained below 70 pmol/l before and after refeeding.

MEASUREMENTS

During the study, PTH and 25-hydroxyvitamin D measurements were performed. Markers of bone formation: serum intact osteocalcin (iBGP) and serum intact BGP + fragments (iBGP+F) and markers of bone resorption: urine C-teloptide of type I collagen (uCTX) and serum C-telopeptide ofvtype 1 collagen (s-CTX) were measured.

RESULTS

At baseline, PTH and 25 OH-vitamin D concentrations were within the normal range in AN patients and no significant variation was observed after refeeding. Bone formation markers were found to be significantly different at baseline between AN patients and controls. After refeeding, iBGP and iBGP+F levels increased by 172% and 154%, respectively, to values no different from controls. Intact BGP and iBGP+F exhibited a significant circadian variation in controls (P < 0.05 and P < 0.002, respectively), whereas we did not find any such circadian rhythm in AN patients. After refeeding no significant circadian variation was observed; however, iGBP+F tended to peak in early morning and exhibited a nadir in the afternoon. At baseline, sCTX was 2-fold higher in AN patients than in controls. After weight gain sCTX decreased significantly and reached control values. Refeeding induced a non-significant 40% decrease in uCTX. We found positive correlations between uCTX and the 24-h mean value of sCTX levels (r2 = 0.93, P < 0.0001) and between uCTX and the mean value of sCTX peak levels at 0800 h (r2 = 0.65, P < 0.0003). Serum CTX exhibited a significant circadian variation in controls (P < 0.001) with a peak at 0800 h and a nadir at 1600 h with a 60% decrease between peak and nadir values. We found that anorexia nervosa suppressed the sCTX circadian variation which was restored by refeeding. We found a significant non-linear relationship between BMI and sCTX/iBGP ratio in AN (r2 = 0.6, P < 0.0001), thus illustrating the influence of nutritional status on bone remodelling.

CONCLUSIONS

In this study we found that weight gain, related to refeeding only, reversed the anorexia nervosa-induced uncoupling of bone remodelling and restored circadian variation of a bone resorption marker.

Responses of markers of bone and collagen turnover to exercise, growth hormone (GH) administration, and GH withdrawal in trained adult males

To examine the interactions between acute exercise and GH on markers of bone and collagen turnover and to assess the potential for detecting GH abuse in athletes using these markers, we studied 17 aerobically trained males (age, 26.9+/-1.5 yr). Sequential studies of exercise, GH administration, and GH withdrawal were undertaken. A randomized, controlled study of rest vs. exercise showed that exercise did not change serum osteocalcin; other markers of formation increased transiently (each P<0.001): bone-specific alkaline phosphatase (+16.1%), carboxyterminal propeptide of type I procollagen (+14.1%), and procollagen III N-terminal extension peptide (+5.0%). The carboxyterminal cross-linked telopeptide of type I collagen, a bone resorption marker, increased 9.7% (P = 0.018) in response to exercise. A randomized, double blind, placebo-controlled, parallel study of recombinant human GH treatment (0.15 IU/kg x day) for 1 week increased serum osteocalcin (net increase preexercise, +/-10.0%; P = 0.017), carboxyterminal propeptide of type I procollagen (+17.6%; P = 0.002), procollagen III N-terminal extension peptide (+48.4%; P = 0.001), and carboxyterminal cross-linked telopeptide of type I collagen (53.3%; P = 0.009). Disappearance half-times after cessation of recombinant human GH for pre- and postexercise markers ranged from 248-770 h. We conclude 1) endurance exercise transiently activates bone and collagen turnover; 2) brief GH administration results in similar but quantitatively greater augmentation; and 3) these data will assist in designing a GH detection strategy.

Total and Free Deoxypyridinoline after Acute Osteoclast Activity Inhibition

Background: Deoxypyridinoline (Dpd) is one of the two pyridinium cross-links that provide structural rigidity to type I collagen in bone. During osteoclastic resorption, Dpd is released into circulation and is excreted in the urine in free and peptide-bound forms. Free and total Dpd are highly correlated, but whether the free-to-total cross-link ratio is constant in both normal and high bone turnover states remains controversial. To compare free and total Dpd performance in a physiological condition, urinary free and total Dpd were measured after a short-term inhibition of osteoclast activity such as that induced by an oral calcium load.

Methods: Total and free Dpd were measured by HPLC and by immunosorbent assay, respectively, in two groups of subjects, one (calcium-treated; n = 16) taking calcium and the other not (control; n = 9).

Results: The urinary excretion of total Dpd at 2 and 4 h after oral calcium loading was decreased compared with controls. By contrast, changes in free Dpd were similar in the calcium-treated and control groups, reflecting only circadian rhythm.

Conclusions: Total and free Dpd do not show comparable sensitivity in detecting short-term inhibition of osteoclast activity. The degradation process of peptide-bound to free Dpd could render free Dpd insensitive to acute changes of osteoclast activity.

Evaluation of a nonhuman primate model to study circadian rhythms of calcium metabolism

We evaluated primate models for the study of circadian rhythms in calcium and bone metabolism. Blood and urine were collected from two cynomolgus macaques every 4 h for 24 h. Studies were initiated at three different clock times to separate the effects of repeated experimental sampling from circadian effects. Also, samples were collected from seven monkeys at times of expected maxima and minima. Some parameters exhibited the expected circadian rhythm with increases at night (serum total calcium) or in the early morning (urinary collagen cross-links). Others displayed the effects of the experimental procedure, either increasing (urinary creatinine and phosphorus) or decreasing (osteocalcin, urinary calcium) with repeated sampling. Serum phosphorus, cortisol, and type I procollagen were influenced by both clock time and experimental procedures. Alkaline phosphatase and parathyroid hormone did not show any differences with time or sampling. This data is consistent with findings in humans that bone resorption increases at night and that endogenous corticosteroids decrease bone formation. The usefulness of the monkey model is limited by the physiological stress of sample collection in these subjects.

Fast and slow bone losers. Relevance to the management of osteoporosis

A low bone mineral density (BMD) is presently regarded as the most important risk factor for the development of osteoporosis. BMD is a function of peak bone mass attained during growth and subsequent age-related bone loss. BMD can be measured accurately and precisely, although the rate of bone loss is more difficult to assess. When axial BMD was measured, the rate of bone loss was shown to increase by 2- to 4-fold at the menopause. Although this rate varies markedly between individuals, it is symmetrically distributed, which argues against the existence of a subpopulation of fast bone losers. Levels of biochemical markers of bone turnover (e.g. osteocalcin, bone specific alkaline phosphatase, deoxypyridinoline) also increase markedly at the menopause, and individuals with a high turnover tend to lose bone more rapidly. Moreover, since increased bone resorption also results in qualitative changes regardless of BMD, a high bone turnover constitutes an independent risk factor. Currently, large intraindividual variations (10 to 40%) in levels of biochemical markers and assay errors still limit our ability to correctly classify individual patients as fast or slow bone losers. The routine use of these markers as a screening tool to predict the risk of osteoporosis in individuals is of limited value, although their selective use in therapeutic decision-making is more promising.

Effects of the circadian variation in serum cortisol on markers of bone turnover and calcium homeostasis in normal postmenopausal women

Bone turnover has a circadian pattern, with bone resorption and, to a lesser extent, bone formation increasing at night. Serum cortisol also has a circadian pattern and is a potential candidate for mediating the circadian changes in bone turnover. Thus, we measured bone formation and resorption markers before (study A) and after (study B) elimination of the morning peak of cortisol. We also assessed effects of the circadian cortisol pattern on serum calcium, PTH, and urinary calcium excretion. Ten normal postmenopausal women, aged 63-75 yr (mean, 69 yr), were studied. Metyrapone was administered to block endogenous cortisol synthesis and either a variable (study A) or a constant (study B) infusion of cortisol was given to reproduce and then abolish the morning cortisol peak. Blood was sampled every 2 h for serum cortisol, ionized calcium, PTH, and bone formation markers [osteocalcin and carboxyl-terminal propeptide of type I collagen (PICP)], and timed 4-h urine samples were collected for measurement of calcium, phosphorus, sodium, potassium, and bone resorption markers (N-telopeptide of type I collagen and free deoxypyridinoline). During study A, serum osteocalcin had a circadian pattern, with a peak at 0400 h and a nadir at 1400 h. During study B, however, the afternoon nadir of serum osteocalcin was eliminated (P < 0.001 and P < 0.005 for the difference in the patterns of peak and nadir, respectively, on the 2 study days). In contrast, the circadian patterns of serum PICP and urinary N-telopeptide of type I collagen and free deoxypyridinoline were virtually identical during the two studies. Urinary calcium excretion declined after the cortisol peak, without differences between the 2 study days in phosphorus or sodium excretion or in serum PTH. We conclude that the circadian variation in serum cortisol is responsible for the circadian pattern of serum osteocalcin, but not that of PICP or bone resorption markers. The physiological variation in serum cortisol may also reduce urinary calcium excretion.