Osteoporosis in South-East Asian Countries

Abstract

Osteoporosis (OP) is a condition where there is low bone density and microarchitectural deterioration which can predispose to fragility fractures. There is a wealth of literature on OP from the developed countries, but less so from Asia. This review will explore the field of OP research in South-East Asia with regard to the epidemiology, the diagnosis of OP and the role of laboratory tests in the management of OP, with emphasis on 25-dihydroxyvitamin D and bone turnover markers.

Introduction

Historical Background

Since the Battle of Hastings in 1066, the British and French have been great rivals in many spheres from the political (with numerous wars in Europe and in the ‘New World’ of North America) to sports (e.g. in rugby and football). To this rivalry, we can now add the question of who first described osteoporosis (OP). In 1822, Sir Astley Cooper, a British anatomist and surgeon, published his observations on hip fractures that occurred in ‘aged women’, defined as 50–80 years of age, after a fall/slip from a standing height, suggestive of a low trauma osteoporotic fracture.¹ He went on to describe the bones as ‘thin in their shell and spongy in their texture’ and with disordered architecture that made union of the fracture difficult, with more pathology in the older individuals.¹ At about the same time in the 1820s, the French pathologist Jean Georges Chretien Frederic Martin Lobstein noticed that some patients’ bones were punctured with larger-than-normal holes, which he described as OP, or porous bone.² The term was derived from the Greek words ‘ostéon’ (osteo) meaning bone and ‘poros’ meaning passage or little hole. In 1885, there was a further advance when Gustav Pommer, a German pathologist, described the distinction between OP and osteomalacia/rickets.³

The next development did not occur until more than 50 years later when an American endocrinologist, Fuller Albright, described 40 cases of OP occurring in postmenopausal women (and two in men) in a paper published in 1941.⁴ In his case series, the patients either had a low trauma fracture, developed severe back pain after minimal activity, or had kyphosis suggestive of vertebral fractures. He and his co-authors attributed the OP to ‘disuse and senescence’ as well as the loss of oestrogen after the menopause.⁴ In the 1960s, Christopher Nordin from Australia started to bring attention to the role of calcium deficiency as a cause of OP.⁵ In 1963, John Cameron and James Sorenson described a method of measuring peripheral bone mineral density (BMD) in vivo using single photon absorptiometry.⁶ By the late 1980s, commercially-manufactured bone densitometers became widely available. Thus the main elements in the science of OP – fragility fractures, the role of oestrogen and calcium in maintaining bone mass, and a method to measure BMD – were all present by the 1990s, facilitating an upsurge in OP research.

Definition of Osteoporosis

With the advent of easily available methods to measure bone density/bone mass, a definition of OP that incorporated bone mass measurements was needed. In 1991, a Consensus Development Conference defined OP as a ‘systemic skeletal disease characterised by low bone mass and microarchitectural deterioration of bone tissue, with a consequent increase in bone fragility and susceptibility to fracture risk’.⁷ This definition was adopted by the World Health Organization (WHO) in 1994⁸ and widely used until a revision in 2000 from the National Institutes of Health, which defined OP as a ‘skeletal disorder characterised by compromised bone strength predisposing to an increased risk of fracture’.⁹ This introduced the concept of bone strength, a combination of bone density and bone quality. From these definitions, OP could be classified based on BMD measurements and the idea of a ‘fracture threshold’ – a BMD cut-off that would include most patients at risk of, or who have had, osteoporotic fractures. Based on American white female data, the WHO defined OP as BMD ≥2.5 SD below the mean for healthy young women (i.e. T-score ≤−2.5) at any site (spine, hip or mid-radius). This level would identify 30% of all postmenopausal women as having OP, of which more than half would have sustained a previous osteoporotic fracture.⁸

Beginnings of Osteoporosis Research in SE Asia

By the late 1980s, research in OP in Asian populations was starting to be published, showing differences between Caucasian and Asian data in terms of absolute BMD values and fracture rates.¹⁰ Many of these studies were from countries in East Asia such as China, Hong Kong SAR, Japan and South Korea which have a long history of established research. Further south, is the South-East Asia (SEA) region, an area of Asia that encompasses Thailand, Singapore, Malaysia, Indonesia, Cambodia, Laos, Timor-Leste, Vietnam, the Philippines, Singapore, Brunei and Myanmar.¹¹ The SEA countries are ‘younger’ and less well-known in terms of their history of research, but many are rapidly developing their academic credentials. This paper aims to provide an update in selected aspects of OP research in the SEA countries.

Epidemiology

BMD Differences Between Caucasians and Asians

In clinical practice, BMD is usually measured using dual-energy x-ray absorptiometry (DXA). OP can be diagnosed following a fragility fracture, or when there is a T-score of ≤−2.5 as measured by DXA.¹² The WHO has suggested that the reference standard for diagnosis of OP in both genders and all ethnic groups be the femoral neck (FN) measured with DXA, where T-scores are derived from the US National Health and Nutrition Examination Survey (NHANES) III database of white women aged 20–29 y.¹³

However, many studies have shown that there are ethnic differences in BMD.^14,15 Generally, Asian populations have lower BMDs. The BMDs of Cambodian, Lao and Vietnamese women in Rochester, MN, US were found to be lower than those of white women.¹⁶ Part of the reason for this is the way BMD is measured. DXA measures BMD using a 2-dimensional calculation, without being able to measure depth; thus a larger bone (in Caucasians) will tend to have a higher measured BMD compared to a smaller bone (in Asians).¹⁴ Some, but not all, of these differences can disappear after adjustment for weight/body size.^15,17 In the Rochester study, after calculation of the volumetric BMD (vBMD; previously BMAD), the prevalence of OP as measured by DXA dropped in the Asian women, but it was still substantially higher than in white women. At the lumbar spine (LS), the prevalence of OP in Asian women dropped from 35.2% based on BMD to 25.1% based on vBMD, but it was still higher than that of white women (10.8%). For the FN, the figures were 45.1%, 32.1% and 18.3% respectively.¹⁶

However, in clinical practice, the DXA BMD reading is used as measured, without the facility to make adjustments. Thus, using the NHANES III database to calculate the T-score in an Asian population can potentially over-diagnose patients with OP. In SEA, two studies illustrate this point. A study of postmenopausal women from Thailand showed that using the reference intervals supplied by the (American) DXA manufacturer led to a much higher prevalence of OP which increased with age.¹⁸ For example, in the 55–59 y age range, the prevalence of OP at the LS was 22.6% using a Thai BMD reference database but 42.7% using the American reference database. For the FN, the figures were 10.3% and 29.1% respectively in the 55–59 y age group. Using the American database, in the >75-year-olds, 78% would have OP at the LS and 80.3% at the FN, which would mean having to treat the majority of the elderly population!¹⁸ A study from Vietnam similarly showed that the prevalence of OP was higher when the T-score was based on the NHANES III database compared to using a local database.¹⁹ For postmenopausal women, the prevalence of OP was 29% using a Vietnamese-derived T-score compared to 44% using the NHANES III database. Similarly for men >50 y, the prevalence of OP was 10% compared to 30% respectively.¹⁹

There is a conundrum here. In studies on OP, the International Osteoporosis Foundation (IOF) recommends analysing FN BMD using T-scores derived from the NHANES III reference database for FN measurements in Caucasian women aged 20–29 y.²⁰ The International Society of Clinical Densitometry (ISCD) recommends using a uniform Caucasian (non-race-adjusted) female normative database for women and men of all ethnic groups to determine the T-score; however, ISCD do allow that the ‘application of recommendation may vary according to local requirements’.²¹ If clinicians in SEA use the Caucasian/NHANES III database, it will lead to a high rate of over-diagnosis of OP. Thus the logical step would be to use a local or Asian database for clinical practice, and consider the use of the Caucasian database for research.

BMD Differences Within Individual SE Asian Countries

There are two countries in SEA with multi-ethnic populations: Malaysia and Singapore. In those populations, there are Malays (from the Malay peninsula, Sumatra and parts of Borneo), Chinese (many descended from ancestors in China) and Indians (many descended from ancestors in India). Similar to other multi-ethnic studies, the different races have different BMD and fracture risks within the same country. In Singapore, Chinese women aged 45–69 y have significantly lower FN BMD and higher LS BMD than Malays and Indians after adjustment for age, height and body mass index.²² For Singaporean men, Malay and Indian men had significantly higher LS, FN and hip BMD than Chinese men.²³ In Malaysia, in a study of women >45 y, LS BMD was similar between the major ethnic groups, although the Chinese had significantly less bone mass than Malays and Indians at the hip.²⁴ In a more recent study of female rheumatoid arthritis patients, Chinese patients had lower FN and total hip BMDs compared to Malay and Indian patients, with no difference in LS BMD.²⁵ Thus it would seem that in multi-ethnic populations in SEA, the Chinese will have a lower hip BMD compared to Malays and Indians with no difference in LS BMD, suggesting the need for ethnicity-based normative data for BMD.

One of the consequences of low bone density is an osteoporotic fracture. The typical sites of osteoporotic fractures are at the spine/vertebra, hip and wrist/Colles’ fracture. Surprisingly, despite the generally lower BMD in Asians, the rate of osteoporotic fractures is not higher compared to Caucasians. This was shown in a large, multi-ethnic study from the US, where 197,848 postmenopausal women had their peripheral BMD measured and were followed up for one year. Black women had the highest BMDs and Asian women had the lowest BMDs. At one year, the white and Hispanic women had the highest rate of fractures, relative risk (RR) 1.0 (the referent group), followed by Native Americans (RR 0.87), blacks (RR 0.52) and Asian Americans (RR 0.32).²⁶ The next two sections will discuss what is known about osteoporotic fractures in SEA.

Hip Fractures

Worldwide, there are marked differences in the incidence rates of hip fractures. The highest age-adjusted rates of hip fractures are found in the Scandinavian countries and North America, with the rates in southern Europe 7-fold lower.²⁷ The hip fracture incidence rates are also lower in Asian and Latin American populations.²⁷

In SEA, the Asian Osteoporosis Study was the first study to look at hip fracture rates in Malaysia, Singapore and Thailand.²⁸ The results showed age-adjusted rates for men and women (per 100,000) as follows: Singapore, 164 and 442; Malaysia, 88 and 218; Thailand, 114 and 289; compared with 1989 US white rates of 187 in men and 535 in women.²⁸ There are no data for the other SEA countries.

In a 2014 paper, the authors found declining hip fracture rates in North America, Oceania, Northern Europe, Hong Kong, Taiwan and in most of Central Europe, but increasing rates of hip fractures in much of Asia (data from Japan, Korea, Lebanon and Singapore), Southern Europe and South America.²⁹ A recent study looking at projections for hip fracture rates in men and women for 2050 in a number of Asian countries suggests that the rates are increasing. Compared to 2018, in 2050 the hip fracture rates in both men and women were projected to increase 3.55-fold in Malaysia, 3.53-fold in Singapore and 2.79-fold in Thailand.³⁰ Possible reasons for this increase would be the demographic shift towards an ageing population and increasing urbanisation of the population which tend to increase hip fracture rates.

Vertebral Fractures

The worldwide variation in vertebral fracture rates has been found to be lower than hip fracture rates.³¹ For women, the vertebral fracture prevalence rates are: 20–24% in North American white women,³¹ 9% in Indonesia,³² 23.6% in Thailand,³³ and 26.5% in Vietnam.³⁴ Similarly in men, vertebral fracture prevalence rates are comparable in US men (21.5%),³¹ Thai (29.1%)³³ and Vietnamese men (23%),³⁴ but much lower in Indonesian men (16%).³² There are no data for the other SEA countries.

Diagnosis of Osteoporosis

Clinical practice guidelines are available for medical practitioners to assist in the diagnosis of OP. Within the SEA countries, Indonesia,³⁵ Malaysia,³⁶ Myanmar,³⁷ the Philippines,³⁸ Singapore,³⁹ Thailand⁴⁰ and Vietnam⁴¹ have local Clinical Practice Guidelines for OP management. In contrast, Brunei, Cambodia, Laos and Timor Leste have no local OP guidelines. Overall, there are more similarities than differences between the guidelines.

In practice, OP can be diagnosed following a low-trauma fragility fracture, or with a BMD measurement using DXA showing a T-score of ≤−2.5.^36,38–40 It is recommended that, for the purpose of diagnosing OP, BMD is measured using DXA.^35–40

Role of Laboratories in Osteoporosis Management

First-line laboratory investigations in patients with OP aim to identify common aetiologies of OP. Further selected investigations may be performed if clinically indicated (Table).⁴² More than half of premenopausal women and about 30% of postmenopausal women are found to have a secondary cause of OP.⁴³ Almost 75% of men who were referred for evaluation of OP had secondary causes.⁴⁴

Table.

Laboratory tests for osteoporosis.

Initial/first-line blood tests	Clinical rationale
Full blood count	The presence of anaemia as a marker of chronic disease. Triad of anaemia, hypercalcaemia and raised creatinine may indicate multiple myeloma
ESR	A high value may indicate inflammatory diseases causing increased bone loss or multiple myeloma
Creatinine, eGFR	As a baseline to determine treatment options. It may also indicate the presence of CKD-MBD if elevated
Adjusted calcium (total calcium and albumin), inorganic phosphate	To assess for osteomalacia. To detect conditions associated with hypercalcaemia such as primary hyperparathyroidism or malignancy or hypocalcaemia and consequent secondary hyperparathyroidism causing bone loss. Low phosphate levels could also be due to renal phosphate wasting disorders
Alkaline phosphatase	Elevated levels could be due to increase in bone formation due to a recent fracture, or a sign of liver disease
Other tests as required
Vitamin D - 25(OH)D	To assess the baseline level and, if low, to supplement as required
PTH	Required if serum calcium is abnormal, to help investigate the cause of the calcium abnormality
Liver function tests	To exclude chronic liver disease
Thyroid function tests	To exclude hyperthyroidism
Serum testosterone, FSH, LH	To exclude hypogonadism
Serum protein electrophoresis/urine Bence-Jones protein/serum free light chain assays	To exclude multiple myeloma
24 h urine cortisol or overnight dexamethasone suppression test	If Cushing’s syndrome is suggested clinically, then these screening tests could be performed
Tissue transglutaminase antibody (together with IgA)	To rule out coeliac disease (rare in SEA)

CKD-MBD, chronic kidney disease-mineral and bone disorder; ESR, erythrocyte sedimentation rate; FSH, follicle stimulating hormone; LH, luteinising hormone; PTH, parathyroid hormone; SEA, South-East Asia.

Vitamin D

Vitamin D is a fat-soluble prohormone pivotal to calcium homeostasis.⁴⁵ It thus plays an important role in the healthy mineralisation, growth and remodelling of bone. In humans, 90% of vitamin D is obtained from 7-dehydrocholesterol via conversion in the skin on exposure to ultraviolet B (UVB) light, and the rest is from the diet.⁴⁶ Vitamin D from both the skin and diet is metabolised in the liver to 25-hydroxyvitamin D (25(OH)D), which is further metabolised in the kidneys to its active form, 1,25-dihydroxyvitamin D (1,25(OH)₂D).⁴⁷ Hydroxylation of vitamin D by the liver to form 25(OH)D is determined almost entirely by the concentration of the precursors.⁴⁸ The major circulating metabolite of vitamin D is 25(OH)D and its concentration is a reflection of the body’s vitamin D stores.⁴⁸ Therefore, the current standard method of assessing vitamin D status is measuring the serum concentration of 25(OH)D.⁴⁹

Measurement of 25(OH)D

Although 1,25(OH)₂D is the biologically active form of vitamin D, 25(OH)D is the recommended biomarker of vitamin D nutritional status. The limited utility of 1,25(OH)₂D in reflecting vitamin D body stores is due to its low circulating concentration (1000 times lower than 25(OH)D) and its short half-life of 4 h. In addition, in vitamin D deficiency, 1,25(OH)₂D levels are often normal due to stimulation of renal 1α-hydroxylase expression by secondary hyperparathyroidism. Furthermore, there is no reference material or reference method available. However, measurement of 1,25-(OH)₂D is useful in some conditions such as chronic kidney disease, hereditary phosphate-losing disorders, vitamin D-resistant rickets and granulomatous disease.⁵⁰

Significant limitations exist in 25(OH)D assays. Vitamin D metabolites are tightly bound to vitamin D binding proteins (VDBP) in plasma. Hence, analytes need to be dissociated from VDBP in order to measure the total 25(OH)D by immunoassay.⁵¹ The variation in VDBP concentrations in pregnancy or dialysis patients interferes with 25(OH)D measurement on immunoassay.⁵² There are also structurally-related hydrophobic compounds that may interfere with 25(OH)D measurement. Further, cross-reactivity with various vitamin D metabolites affects assay selectivity. Equimolar measurement of 25(OH)D₂ (calcidiol) and 25(OH)D₃ (calcitriol) is ideally required i.e. the assay cross-reacts equally for the two compounds. If a method only partially cross-reacts with 25(OH)D₂, the reported total 25(OH)D result for a patient on calcidiol supplementation will be lower using that method.⁵¹

The gold standard for vitamin D measurement is liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) that selectively detects vitamin D metabolites based on their mass-to-charge ratios (m/z).⁵¹ However, LC-MS/MS is not without issues. It is complex, not fully automated, and a number of instrumentation parameters differ between laboratories.⁵³ A potential interference in LC-MS/MS, but not in immunoassay, is the 3-epi-25-hydroxyvitamin D (3-epi-25(OH)D) metabolite that causes an analytical artefact. MS is unable to differentiate 3-epi-25(OH)D from 25(OH)D unless they are chromatographically separated from one another.^54,55

To achieve comparable results across different methods and manufacturers, steps were taken towards standardisation of assays, which was crucial for the establishment of a common cut-off value to define vitamin D status.⁵⁰ Harmonisation of 25(OH)D assays began in 1989 with the Vitamin D External Quality Assessment Scheme (DEQAS) which significantly reduced the variability between participating laboratories.⁵⁶ This was followed by the commercial availability of standard reference materials designed for method validation in clinical laboratories and by the development of reference method procedures.⁵¹ In 2010, the Vitamin D Standardization Program, a joint effort by NIH, CDC, NIST and Ghent University, was introduced to standardise 25(OH)D measurements, including extension of standardisation to assay manufacturers that chose to participate.⁵⁷ Results from participating laboratories should be comparable to the values obtained using reference method procedures and the method is considered standardised if the CV is <10% and the bias <5%.⁵⁸

Novel Markers of Vitamin D Status

Although considerable progress has been made, some issues remain unresolved. The associations between 25(OH)D and several clinical parameters, including BMD and fracture risk, are inconsistent between races. Recent studies have provided some insight into novel markers of vitamin D status including the vitamin D metabolite ratio (24,25(OH)₂D:25(OH)D), bioavailable 25(OH)D (25(OH)D not bound to VDBP), and free 25(OH)D (circulating 25(OH)D not bound to VDBP or albumin). Genetic studies have demonstrated polymorphisms in VDBP, 7-dehydrocholesterol synthase and the vitamin D receptor (VDR), which are the key proteins of vitamin D metabolism. However, these markers are not ready for use in clinical practice as there are unanswered questions on their measurement and the interpretation of results.⁵⁰

Some studies have been done in SEA with regard to these novel markers of vitamin D status. In a first study of a multi-ethnic female Malaysian population, free and bioavailable 25(OH)D were found to reflect total 25(OH)D, and were not superior to total 25(OH)D in the correlation with BMD. The median values of calculated free and bioavailable 25(OH)D were significantly higher in Chinese compared with Malays and Indians, consistent with their median total 25(OH)D serum levels. The study concluded that, since the calculated free and bioavailable 25(OH)D levels did not correlate with BMD, they may not be helpful in clinical practice when assessing bone health in Malaysian women.⁵⁹

A study of young healthy Thai adults demonstrated that the interaction between vitamin D status, as measured by circulating 25(OH)D, and VDBP rs2282679 genotypes modified the association between total 25(OH)D and BMD and bone turnover markers (BTMs).⁶⁰ Another recent study in Thailand also found that VDBP gene polymorphisms were significantly associated with vitamin D deficiency, proposing that these polymorphisms are a risk factor for vitamin D deficiency in Thais.⁶¹

Levels of 25(OH)D in SE Asian Countries

The definition of vitamin D deficiency based on 25(OH)D is still a matter of debate as it depends on the cut-off values as well as the assays used to measure the levels. It has been shown that these factors will alter the frequency of vitamin D deficiency in different populations.⁶² Vitamin D deficiency has been defined as a serum 25(OH)D level <50 nmol/L (<20 ng/mL).⁶³

Determination of sufficient or optimal levels of vitamin D has been more difficult. One method to determine sufficient levels of 25(OH)D is based on the inverse relationship of 25(OH)D to parathyroid hormone (PTH). 25(OH)D deficiency leads to low serum 1,25(OH)₂D as there is less production and, consequently, low (ionised) calcium levels due to less absorption from the gut. This, in turn, will stimulate the production of PTH, leading to a mild functional secondary hyperparathyroidism. To ‘switch off’ this unfavourable feedback loop, 1,25(OH)₂D has to bind to the parathyroid VDR and inhibit PTH gene transcription.⁶⁴ Thus, the optimal level of 25(OH)D would be a concentration where there is maximal suppression of PTH. In addition, ‘sufficient’ 25(OH)D levels have been defined according to other criteria including maximum calcium absorption, peak BMD, and lowest rates of bone loss, falls and fractures.⁶⁵

However, not all studies have agreed on the concentration of 25(OH)D at which PTH levels are at their lowest. In studies on healthy adults in France⁶⁶ and the US,^67–69 a 25(OH)D level of ≥75 nmol/L (≥30 ng/mL) maximally suppressed PTH levels. In contrast, a lower 25(OH)D level of around 50 nmol/L was enough to suppress PTH levels to the nadir in studies of Irish⁷⁰ and Malaysian⁷¹ women. However, a study in Vietnamese men and women did not show a 25(OH)D threshold at which PTH plateaued.⁷² An interesting study from Malaysia used two BTMs, C-terminal telopeptide of type I collagen (CTX) and procollagen type I N propeptide (P1NP), to determine the optimum level of vitamin D needed to maintain adult bone health. On the LOESS plots, CTX and P1NP plateaued at 25(OH)D levels of 35 nmol/L (14 ng/mL) and 20 nmol/L (8 ng/mL) respectively. In contrast, PTH increased steeply when the 25(OH)D level was 20 nmol/L (8 ng/mL). The study concluded that the required 25(OH)D concentration for maintenance of adult bone health is 20–35 nmol/L (8–14 ng/mL).⁷³ Therefore, the ideal concentration of 25(OH)D in a population may need to be adapted at a local level. For the SEA countries, the only guidance as to what would be an optimal 25(OH)D level comes from Singapore, which advises to ‘aim for >20 ng/mL for optimal bone and muscle strength’.³⁹

One of the difficulties with comparing the prevalence of vitamin D deficiency across different countries is that the definitions can vary. Some papers will regard 25(OH)D levels of <50 nmol/L (<20 ng/mL) to be deficient, but others will take 75 nmol/L (30 ng/mL) as the cut-off. In addition, the populations studied are different, ranging from pregnant women to postmenopausal women and middle-aged men. As this paper is focussed on OP, we will discuss 25(OH)D levels in postmenopausal women and older men.

In a study of postmenopausal women from the Philippines with a mean age of 70 y, the mean level of 25(OH)D was 87.0 nmol/L (34.9 ng/mL), with 36% of the subjects having levels <75 nmol/L (<30 ng/mL).⁷⁴ In a population survey of Filipino adults >20 y, men had significantly higher mean levels of 25(OH)D compared to women, 93.0 nmol/L (37.3 ng/mL) and 70.7 nmol/L (28.3 ng/mL) respectively.⁷⁵ When looking at the older population, the mean 25(OH)D level for both men and women in the 40–59 y group was 85.3 nmol/L (34.2 ng/mL) with 43.5% having levels <75 nmol/L. In the >60 y age group, the mean 25(OH)D level was 87.4 nmol/L (35.0 ng/mL) with 38.1% having levels <75 nmol/L.⁷⁵

In Thailand, there were two studies of postmenopausal women. The first study’s subjects had an average age of 67.5 y with a mean 25(OH)D level of 67.6 nmol/L (27.1 ng/mL). Further, 54.0% of the subjects were vitamin D insufficient at <75 nmol/L and 31.8% at <50 nmol/L.⁷⁶ The second study showed a mean 25(OH)D level of 64.4 nmol/L (25.8 ng/mL) in a group of women with a mean age of 75.2 y. Of those, 77.4% were vitamin D insufficient at <75 nmol/L and 21.5% at <50 nmol/L.⁷⁷

In Vietnam, a cross-sectional study of men (mean age 43.8 y) and women (mean age 47.7 y), the mean 25(OH)D was significantly higher in men compared to women, 91.9 nmol/L (36.8 ng/mL) and 75.1 nmol/L (30.1 ng/mL) respectively.⁷² In the >60 y group, the prevalence of vitamin D insufficiency (<75 nmol/L) was significantly higher in women (56%) compared to men (23%).⁷² Thus, even within these three countries which are all tropical with abundant sunshine, the prevalence of vitamin D insufficiency ranges widely from 36% up to 77.4%.

Studies from Cambodia, Malaysia, Singapore and Indonesia have looked at 25(OH)D levels in premenopausal/<50 y individuals. In Cambodian women, the median 25(OH)D level was 64.9 nmol/L (26.0 ng/mL).⁷⁸ In two studies of premenopausal Indonesian women, the mean 25(OH)D levels were 44.2 nmol/L (17.7 ng/mL)⁷⁹ and 51.9 nmol/L (20.8 ng/mL).⁸⁰ Interestingly, there was no difference in 25(OH)D levels between women who wore the hijab (a veil covering the whole body except the face and hands worn by Muslim women to preserve their modesty) and those who did not.⁷⁹ In Malaysia and Singapore, 25(OH)D levels vary between the different races, with the Chinese generally having higher levels compared to the Malays and Indians. Malaysian Chinese females had higher 25(OH)D levels, 58 nmol/L (23.2 ng/mL), compared to Malay, 43 nmol/L (17.2 ng/mL), and Indian, 45 nmol/L (18.0 ng/mL), females.⁷¹ Similarly, in Singapore, the Chinese females had the highest levels of 25(OH)D, 49.5 nmol/L (19.8 ng/mL), compared to Malay, 37.8 nmol/L (15.1 ng/mL), and Indian, 34.0 nmol/L (13.6 ng/mL), females.⁸¹ One possible reason for this finding could be skin pigmentation. Darker-skinned individuals have more melanin in their skin, a pigment produced by melanocytes. Melanin is an effective UVB sunscreen and thus will reduce vitamin D production in the skin. For example, African Americans who are heavily pigmented require at least 5–10 times longer exposure than whites to produce adequate cholecalciferol in their skin.⁴⁶ Thus, Malays and Indians, who have more pigmented skin colouring compared to Chinese, could be expected to have lower levels of 25(OH)D production for similar amounts of sun exposure.

Bone Turnover Markers

Measurement of BTMs in blood or urine estimate the rate of bone turnover in OP. Changes in BTMs may be useful for monitoring treatment; however, there is insufficient data to include BTMs in fracture risk prediction. BTMs cannot be used for the diagnosis of OP.⁸² In Asia-Pacific, BTMs are suggested for the short-term monitoring of osteoporosis treatment; that is, to help clinicians assess treatment response and adjust treatment as required.⁸³ In the Clinical Practice Guidelines from SEA countries, BTMs are recommended for assessment of fracture risk,^{35–37,39,40} treatment response^{35,36,38–40} and compliance to treatment.^36–38

Measurement of Bone Turnover Markers

The IOF and IFCC Working Group on Bone Standards,⁸⁴ the National Bone Health Alliance (NBHA) in the US,⁸⁵ and the Asia-Pacific group⁸³ endorse serum/plasma P1NP and CTX, markers of bone formation and bone resorption respectively, as reference analytes for BTMs in clinical studies. However, further studies are required for the routine use of BTMs in OP due to their broad biological and analytical variability, amounting to an approximately 7.3-fold difference.⁸⁶

Currently, two assays for CTX measurement by ELISA method and automated immunoassay are available: Beta-CrossLaps Roche Elecsys (ECLIA, Roche Diagnostics, Mannheim, Germany) and CTX-1 (CrossLaps) IDS-iSYS (CLIA, Immunodiagnostic Systems, Tyne and Wear, UK). In both immunoassays, antibodies are against the same epitope (β-isomerised octapeptide EKAH(β)DGGR) on the non-helical CTX-1. However, despite the good between-assay correlation, there appears to be some bias.⁴² In addition, a study by Chubb et al. showed a significant difference in the patients’ results produced by these commercially-available CTX assays, and restricted commutability of calibrators between assays.⁸⁷

CTX is affected by renal function,⁴² and has a diurnal variation (±30–35%) with an early morning peak and a nadir in the afternoon.⁸⁸ Food intake partly mediates this variation, decreasing CTX. Hence, to reduce the within-individual variation, the recommended sample collection for CTX is a morning, fasting sample. A fasting sample during the middle of the day is preferable, however this timing may be inconvenient and almost all clinical studies have used morning sample collection. Extreme exercise routines and non-recreational exercise produce significant changes in CTX. Oral contraceptive use and phase of menstrual cycle among premenopausal women also have an influence on CTX levels.⁸⁸ CTX is more stable in EDTA plasma than serum.⁴² A study in Malaysia demonstrated that the mean CTX level was significantly lower in premenopausal compared to postmenopausal Malay women. Plasma CTX was also found to be more sensitive than urine N-terminal telopeptide (NTX), with lower variability than NTX. Both markers were influenced by the duration of menopause, body mass index, physical activity, education level and marital status.⁸⁹

Currently available P1NP automated immunoassays include the total P1NP (trimeric molecule and monomer) and the intact P1NP (trimeric form only) assays measured on Elecsys (Roche Diagnostics) and IDS-iSYS (Immunodiagnostic Systems), respectively. A radioimmunoassay is also available for intact P1NP (UniQ P1NP RIA Orion Diagnostica, Epsoo, Finland). Monomeric fragments accumulate in renal failure, affecting the measured P1NP concentration of total but not intact P1NP.⁴² P1NP has little diurnal variation, is not affected by food intake and is stable at room temperature; both serum and plasma are suitable for its measurement. The inter-individual variation, however, is significant. Hence, the use of individual baseline values acquired pre-treatment is more relevant than population-based reference intervals when monitoring treatment. Age influences P1NP; levels are significantly higher in infants, children and adolescents than in adults, reflecting their somatic growth.⁹⁰

A study was performed to determine age-related reference intervals for serum P1NP and CTX in the Australian population.⁹¹ In premenopausal women, the serum P1NP and CTX values were similar to previous studies. However, in males and post-menopausal women, the serum CTX data were quite different than previously described, emphasising the importance of establishing reference intervals for different populations. Using the data mainly from this study, but also other published reference interval studies, consensus reference intervals for P1NP and CTX were developed by the Australasian Association for Clinical Biochemistry (AACB) Reference Intervals Harmonisation Project.⁹² There are limited data on reference intervals for BTMs in SEA, but a recent study constructed reference intervals for CTX and P1NP for the Vietnamese population.⁹³

Clinical Use

The use of BTMs for the diagnosis and monitoring of Paget’s disease of bone is well established, ⁹⁴ and shows promise for malignant bone disease.⁹² In OP, the only recognised clinical use for BTMs is monitoring treatment, although optimum treatment targets, improvement in fracture outcomes or compliance to therapy are yet to be established.⁹⁵ After the initiation of OP treatment, the change in BTMs is large and occurs by 3–6 months, compared to BMD where the change is small and slow.

The direction, magnitude and time course of the response vary by treatment and by BTM.⁸⁴ Significant reductions in CTX are seen after 1 month of bisphosphonate treatment, indicating inhibition of osteoclastic activity, and reach a plateau from 3 months onwards.⁹⁶ Reductions are seen earlier with intravenous compared to oral therapy.⁸⁴ The decrease in P1NP is delayed by about 4 weeks compared to CTX, reaching a plateau after 3–6 months of treatment.⁹⁶ In contrast, with teriparatide, an anabolic agent, there is an initial increase in P1NP reflecting direct stimulation of bone formation followed by a later increase in CTX.⁸⁴

The goal of anti-resorptive treatment is to achieve a decrease in BTMs into the lower half of the premenopausal range 1–3 months after intravenous treatment or 3–6 months after oral treatment.⁹⁴ Chubb et al. recommended plasma CTX cut-off values corresponding to a urine NTX absolute value of 21 nmol BCE/mmol, defined as the fracture risk reduction target post risedronate treatment: CTX 230 ng/L and 271 ng/L for the automated Roche and IDS i-SYS assays, respectively. Expressing CTX as an absolute treatment target value is an advantage as it does not require a baseline value.⁹⁷ In contrast, following OP treatment with teriparatide, an increase in P1NP by >10 mg/L from baseline within 1–3 months has been proposed as an indication of response to therapy.⁹⁴ The advantage of serum P1NP is its ability to assess both osteoanabolic and anti-remodelling therapies in monitoring postmenopausal OP.⁹⁸ Lack of response in BTMs following treatment may be due to non-adherence with treatment or a secondary cause for continuing bone loss. BTMs are not beneficial for monitoring strontium ranelate therapy, and are not recommended in prediction of osteonecrosis of the jaw post bisphosphonate treatment.⁹⁴

For BTM levels within the premenopausal range prior to treatment, a reduction by more than the least significant change (LSC) confirms the response to treatment in OP patients.⁴² When monitoring OP treatment, a one-sided rather than two-sided probability of 0.05 is appropriate since the direction of change is known. In this case, an 80% probability (p<0.2) is adequate and the LSC is 1.19 × CV.⁹⁹ Changes of >30% should be of clinical significance as these changes exceed the LSC of 25–30% for these serum-based markers. If the change in BTMs is equivocal, another measurement is recommended in 3 months before modifying treatment based on an insufficient BTM response. Successive BTM levels should be taken on more than two separate instances before a clinical decision is made.⁹⁴ In assessing BTM response to treatment, both the LSC and the reference interval approach have limitations.¹⁰⁰

A study, performed by secondary analysis of trial data in women with OP treated with zolendronic acid, compared the clinical validity and the detectability of response of short-term changes in BMD (hip and spine) and BTMs (s-P1NP and s-CTX). Hip BMD and s-P1NP ranked highly for prediction of clinical fracture. Both s-P1NP and s-CTX ranked highly for detectability of response to treatment. S-P1NP had the highest overall ranking.¹⁰¹

Although epidemiologic studies suggest that BTMs may predict hip fracture risk, findings are inconsistent and Asian data are lacking. There is no agreed recommendation for BTM use in fracture risk prediction, either for case finding or for population screening. A study in Singapore showed that higher levels of serum osteocalcin, P1NP, CTX and NTX were associated with subsequent risk of hip fracture in an Asian population, and established P1NP and CTX as the best BTM predictors for incident hip fractures.¹⁰² A cross-sectional study in Vietnam showed that, for a given age and weight, the elevation in P1NP and CTX in postmenopausal women was higher than in elderly men. However, only CTX was significantly associated with BMD in males and females, indicating BTMs have limited utility in the identification of high-risk individuals.⁹³

Lack of population-based prospective studies precludes BTMs from FRAX algorithms. Other uses for BTMs include prediction of rate of bone loss, identification of secondary OP, prediction of response to therapy and improving adherence. However, interventional BTM thresholds to prevent bone loss in menopausal and elderly individuals have not been defined. In addition, there is no systematic study on identification of secondary OP or in which individuals were stratified according to BTMs and then randomised to treatment or placebo accordingly. Limited research on improving adherence gave contradicting results.⁸⁴

Standardisation of commercial BTM assays is required to establish a reference system and to attain universally comparable measurements for the use of globally agreed decision limits and target values, irrespective of the laboratory or the method used. A working group of the IFCC and IOF is working in collaboration with commercial manufacturers to achieve this.⁸⁴

Conclusions

The amount of research on osteoporosis in SEA has been increasing but knowledge gaps remain, especially in the less-developed countries in the region. BMD in SEA populations is generally lower than in Caucasians but, paradoxically, hip fracture rates are not higher, although projected to increase. In contrast, vertebral fracture rates in SEA are similar to those in North America. 25(OH)D levels and the prevalence of vitamin D insufficiency/deficiency varied widely across SEA populations, with differences even within the same country with a multi-ethnic population. In the Asia-Pacific region, the BTMs s-P1NP and s-CTX can be used for the monitoring of OP treatment.