Our results showed consistent differences between prevalence of osteoporosis and osteopenia when using the reference data provided by the densitometry manufacturer and our derived reference data. With higher reference values, the densitometry provided reference data has the tendency to over-estimate osteoporosis in Singapore population. It is well established that measured BMD differs between ethnicities(30). Using our reference values, the prevalence of osteoporosis was found to be 9.3% and 0.7% for post-menopausal women and men above 50 years of age, respectively. Comparatively, the prevalence of osteoporosis is estimated to be about 2-10 times higher at 17.3%, 20.4%, and 18.9% for women and 5.6%, 7.0%, and 5.6% for men, when using reference data from Hologic, NHANES, and China, respectively. Comparison between osteoporosis prevalence derived using reference data from our study, Hologic, and other countries, the percentage differences ranges between 60.5-163.6%.
Similar discrepancy was reported in another study in South East Asia. In a study of 653 Vietnamese men and women, prevalence of osteoporosis was 29% in women and 10% in men, with the local reference values. However, the prevalence were much higher - 44% in women and 30% in men, when using the DXA-provided reference values(15). In a large Chinese study, the young adult BMD reference value was found to be 4-5% lower compared to the US reference values(19). This resulted in an artificial 2-fold increase in the estimated prevalence of osteoporosis - from 13.3% to 26.7% in women, and from 5.9% to 11.1% in men(19).
The LS BMD of women from this study is higher in all age group, except for the 50-59 age group, when compared to Goh et al.(12). Furthermore, Goh(12) reported peak LS BMD occurring at 40-49 age group compared to 31-40 age group in our study. The difference is likely due to cohort effect, for the previous study was conducted about 16 years ago. In this study, a steep BMD decline occurs in women aged 41-50 for both LS and WB BMD. This is best explained by the decrease in oestrogen at menopause, as onset of menopause occur between 36-59 years of age in Singaporean women(31). Similar trends in LS BMD of women - with the decline of BMD in the same age range, were reported in studies of other ethnic groups(15, 19, 21). Our study showed that Singaporean women have higher LS BMD in the younger age group (21-50yrs) compared to the Caucasians. This may partly be attributed to an earlier onset of menarche in Singaporean women (12 years of age) compared to the Caucasians (13 years of age)(32, 33). The YAM value for lumbar spine in women was similar to the NHANES and China values. However, the different SD values explained the difference in prevalence. As the WHO definition of diagnosis of osteoporosis depends on the SD value, a small change in SD value may cause notable variation in T-score, resulting in significant effect on the number of individuals meeting the WHO diagnosis criteria(20). From the regression model, the relationship between BMD and age in women was best fitted with a polynomial equation of the third degree (Figure 1), which is consistent with other studies(15, 19). According to the functional relationship, our data shows Singaporean women reaches predicted peak LS BMD at 31 years old - close to Caucasian (30-39 years old)(34, 35) and Chinese women (34 years old)(19).
Age group at which peak LS BMD occur in men was similar with the previous study, however the peak BMD value was higher in our study (1.006±0.115g.cm-2 vs 1.049±0.119g.cm-2)(36). LS BMD across all age group was also found to be lower compared to this study(36). These differences maybe due to better nutrition of the current cohort effect compared to the previous cohort from 18 years ago. The lower peak BMD reported, correspond with the low BMD in the current study 31-40 age group (Table 3). The WB BMD and LS BMD of men in our study do not share similar trend as the women. There is a steep decline in the 31-40 age group, and then increasing trend in older age groups. Similar trends had been reported for WB (17) and LS BMD(35) elsewhere. Singaporean men have a lower LS BMD in the 21-50 years age group but higher in older age groups compared to the Caucasians. Compared to Caucasians, Singaporean men aged 66 years and older have a higher WB BMD. The relationship between LS BMD with age was best fitted with a positive linear regression equation, and the relationship between WB BMD and age was best fitted with a polynomial equation of the second degree (Figure 1). Despite the apparent increasing trend in BMD in the older age group, post-hoc analysis revealed no significant difference in BMD within age groups. Similar trend was also reported in the NHANES studies, regardless of race and ethnicity(35). YAM was found to be lower in Singapore men compared to NHANES and China. Sedentary lifestyle (78%) in Singaporean adolescent may explain the lower YAM BMD(37). The relationship between age and BMD in older men could be confounded by cohort effect, with the older generation having a healthier lifestyle(38). However, such relationship has been reported in other studies (20, 39, 40). For older men, BMD have been found to increase with age(41). The apparent increase in BMD have been attributed to degenerative-changes of the LS, such as osteophytes, aortic calcification, vertebral compression fracture, scoliosis, and osteoarthritis(42). The difference in rate of spinal BMD diminution was found to be 1% and 10% per decade in men and women, respectively(43). However, other studies reported that lumbar spine BMD increases at a rate of 1.5-3.5% per decade in men over 60 years old(44-46). The compound effect of older men physiology and the cohort effect likely explained the lack of significant change in BMD across age in our study.
Ethnicity has been established as a determinant of BMD and the risk of osteoporosis(30). While majority of BMD research have compared the local reference values to the Caucasians, it is important to note that variance in BMD values was also reported among different ethnic groups in Asia. For example, the age-specific BMD in Chinese was reported to be lower compared to the Japanese and Koreans(20). Differences was also found among different ethnic group in a country based on geographical locations, possibly due to differences in diet, lifestyle and body size(20). As a country in South East Asia, while predominantly ethnic Chinese, diet, lifestyle and culture of Singaporean differs from those of people groups in China. Additionally, there are also ethnic variation within Singapore. Singaporean Chinese women has a 40% higher hip fracture rates compared to local Malay and 90% higher than Indian women(2). Post-hoc secondary analysis of covariance our data showed that there were no significant differences in LS BMD among the three major ethnic group for both men (p=0.692) and women (p=0.802). This result is similar to previous study(12). However, significant differences was reported in femoral neck BMD among Singapore women(12).Though the BMD differences between different ethnicity in Singapore has not been investigated, various influencing risk factors for low BMD, such as obesity, vitamin D deficiency, smoking, alcohol consumption and genetic heterogeneity, have been studied. Ethnic-specific genetic variants and risk factors associated with low BMD warrant future research (2).
Lifestyle is also an established determinant of BMD. As this is a cross-sectional study, inter-generation lifestyle differences may have a cohort effect on the mean BMD of each age group. Singapore’s economic development is unique. Over three decades, it has progressed from a labour-intensive industry to a predominantly modern-day service industry. In 2016, 36.5% of Singaporean adults are reported to be physically inactive, and 69.7% adolescent boys and 83.1% adolescent girls between 11-17 years of age are physically inactive(47). It has been reported that 10% increase in peak bone mass in children will reduce the risk of osteoporotic fracture by 50% during adult life(48). Low physical activity in adolescents may increase risk of low bone mass and osteoporosis in middle to older ages.
To our knowledge, this is the first prevalence study of osteoporotic conditions in Singapore using population-based BMD reference values. While study sample size may not be large, it is randomly selected a priori from a nationally and ethnically representative residential population. Therefore, this population-based dataset adds to the much needed local and South East Asian BMD reference database. Individuals with clinical conditions deemed to interfere with bone metabolism were also excluded from the data analysis. Ideally, peak bone mineral density should be estimated from a longitudinal study, following up participants from the age of 5 to 40, but such study is not feasible. The DXA lumbar spine BMD was extracted from the whole-body scan, which may affect the accuracy of the BMD obtained. An important limitation is that other skeletal sites, such as femoral neck, Ward’s triangle, trochanter, and total hip, were not available. The study was part of a larger study on body composition and physical performance and so only a whole body DXA scan using standard protocol was performed (where the regions of interest are head, arm, forearm, leg, ribs, thoracic spine, lumbar spine and pelvis). Site-specific scans of femoral neck and hip region was not performed. Another limitation is the exclusion of subjects with fractures or surgeries that affected physical function. Fractures at hip or femoral neck are known to be more devastating, and are important osteoporosis signs. Therefore, further local population-based study on these areas will be needed.