The prevalence of the metabolic bone disease among men with T2DM in this study of 20.3% is lower compared to previous studies done by Xia et al, which showed a prevalence of 65.7% [12] and Chen et al, with a figure of 59.1% [13]. This is most likely because while the current study had a predominantly Malay cohort, both previous studies were conducted within the Chinese population, which had been reported to have a higher risk of osteoporosis compared to other ethnicities [14]. Furthermore, the mean age of the two studies was higher compared to our study population, which was 78 and 68 years old, respectively, whilst our study population's median age was 64 years of age. Notably, we included those above the age of 50 years to emphasise the importance of early screening for metabolic bone disease among this high-risk population.
Previous research has shown that T2DM is an independent risk factor for fractures, which is associated with elevated fracture risk at a higher BMD level compared to the normal population [21]. Thus, fracture risks in T2DM patients cannot be explained by BMD results alone, and therefore necessitates the use of bone turnover markers as surrogate biomarkers to determine more accurate fracture risks in this population. Bone resorption markers include hydroxyproline; pyridinium crosslinks, CTX and tartrate-resistant acid phosphatase (TRAP) [22]. Some bone formation markers include procollagen type 1 N propeptides (P1NP), osteocalcin and ALP [22]. In this study, we used CTX as the bone resorptive marker and bsALP as the bone formation marker. This is about the International Osteoporosis Foundation, which recommended that the serum CTX be used as the reference marker for bone resorption due to its specificity to bone and performance in clinical studies [23]. BsALP is specifically looking at bone isoform and is a good bone formation marker in CKD patients as it is unaffected by renal function [24]. There is a statistically significant difference in CTX levels between T2DM men with metabolic bone disease and without the disease. Twenty per cent (20%) of participants with the metabolic bone disease had high CTX levels of > 0.6 ng/mL, compared to 5.1% of those without the condition with a p-value of 0.016. This would suggest that men with T2DM experience more bone resorption. However, in this study, we were unable to demonstrate a significant difference in bsALP between men with T2DM, with and without the metabolic bone disease. This is most likely due to the overall low prevalence of metabolic bone disease within the cohort.
Parathyroid hormone (PTH) itself is not a bone turnover marker, but its release is regulated by low serum calcium concentration. PTH has both bone formation and bone resorption action. Net bone mass is depending on level and exposure to PTH. High and prolonged exposure to PTH will result in increased osteoclastic activity and subsequent bone resorption. Higher iPTH is, therefore, associated with higher bone turnover activities [25–27]. This is reflected in our study, which observed that those with a higher iPTH of ≥ 6.9 pmol/L had a 3-times higher odd ratio of developing the metabolic bone disease [OR 3.056 (1.141, 8.187)]. This finding is consistent with previous studies by Agrawal et al which showed high iPTH is associated with low BMD [28]. In the current study 27 subjects had high iPTH levels > 6.9 pmol/L, of these 44.4% had deficient vitamin D and 33.3% had insufficient vitamin D. It is widely known that there is an inverse relationship between vitamin D levels and iPTH levels [29] as vitamin D deficiency can result in secondary hyperparathyroidism. Although the prevalence of vitamin D deficiency/ insufficiency in this study was high (68.9%), there is no association between osteoporosis and vitamin D insufficiency in our study population, which was similarly reported by a previous study by Ahmed et al [30].
Interestingly, the use of DPP4i in men with T2DM was shown to be protective against metabolic bone disease. This seems to be consistent with a recent meta-analysis of 28 randomized clinical trials, which concluded that the use of DPP4i reduces the risk of fractures when compared with placebo [31]. However, this effect is still controversial in contrast to other meta-analyses by Fu et al [32] and Mamza et al [33] which showed no differences in fracture risks of patients taking DPP4i. Yang et al [34] suggested that DPP4is may promote bone formation and reduces bone resorption through DPP4 substrates and DPP4-related energy metabolism, which is mainly seen in sitagliptin and not with other DPP4-Is. This is consistent with our observation in our study as the majority (70.1%, n = 40) of our patients were prescribed sitagliptin, meanwhile, the rest were on vildagliptin (7%, n = 4) and linagliptin (22.8%, n = 13). Although the current study was not primarily aimed at looking at pharmacotherapy effects on the outcomes measured it was nonetheless interesting to postulate a potential benefit of this particular drug.
Osteoporosis in men is frequently caused by secondary factors such as hypogonadism and hyperthyroidism. In this study, we screened all patients with thyroid function tests and total testosterone levels to exclude secondary causes of osteoporosis, of which one patient was excluded from the study due to newly diagnosed thyrotoxicosis. Fink et al reported that bone loss is accelerated with low total testosterone levels [35]. In our study, 88.5% of the participant had normal testosterone levels and testosterone levels were not statically different between those with metabolic bone disease and those with normal BMD. This could positively indicate that our study subjects do not have evidence of hypogonadism and that those with the condition had been adequately identified and treated.
Osteoporosis is a known complication of advanced liver disease e.g. chronic cholestasis and primary biliary cholangitis [36]. It has been proposed that bilirubin diminished osteoblast differentiation and mineralization [37] leading to low bone formation and it also increased osteoclast genesis and increased osteoclast viability as bilirubin increased the expression of osteoclastogenic related mRNAs leading to an increase in bone resorption [38]. Both mechanisms contribute to the development of osteoporosis. However, the association between bilirubin levels and osteoporosis has not been well established, with conflicting results. Yan [32] and Wu et al [33] have examined the connection between bilirubin and metabolic bone disease in T2DM patients and have found that raised bilirubin levels lower the incidence of osteoporosis. In contrast, Zhao et al showed that bilirubin levels did not affect BMD or the incidence of fractures based on Mendelian randomization (MR) study [39]. However, to the best of our knowledge, there has been no previous report demonstrating a negative correlation between total bilirubin ad BMD, although there has been proposed pathophysiology that bilirubin might affect bone metabolism leading to an increased risk of metabolic bone disease [37, 38]. Our study observed that an increase of 1 mmol/L of total bilirubin showed an increase in the odds of developing metabolic bone disease by 1.1 times [OR: 1.138 (95% CI: 1.058, 1.223), P-value < 0.001]. This could be further studied in a bigger sample looking specifically into the population with higher bilirubin levels such as chronic liver disease.
Many previous studies had shown strong relationships between age, BMI and the duration of diabetes mellitus and the development of osteoporosis. However, we did not see a significant result for these variables, which was most likely due to the small sample size. Independent sample T-test identified weight, BMI, corrected calcium level and CTX as significant risk factors for metabolic bone disease. However, this was no longer demonstrated with the multivariate analyses. Future studies with a larger sample size and the addition of a control group may be beneficial.
In the current study, the prevalence of metabolic bone disease was higher when BMD was assessed at the neck of the femur compared total femur or total spine (20.3% vs 10.1% total hip and 6.1% total spine). The DEXA scan result, men with T2DM and metabolic bone disease notably showed the lowest value of femoral neck t-score (-1.82 ± 0.75) compared to the total hip t-score (-0.73 ± 0.84) and total spine t-score (-0.24 ± 0.03). Which is comparable to Chen's report [13]. According to Ann et al, T2DM individuals exhibit a higher loss of BMD at the femoral neck [40]. T2DM individuals have deteriorated cortical microarchitecture [41]. All this explain increased hip fracture risk in T2DM individual.
This was a cross-sectional study, thus limiting the causal relationship. Furthermore, the convenient sampling in a single-centre study made the data limited and could not be representative of the general population. The sample size of this study was relatively small, which was mainly attributed to the Covid-19 pandemic and the movement control restrictions. Future studies could include a control group to determine the effects of T2DM on BMD compared with a normal population without T2DM. Due to logistic reasons, the DEXA scan was performed by 2 different trained radiographers but could have led to some technical variations in the tests. Therefore, further prospective, long-term, multicentre observational data, particularly on trabecular bone mass, could perhaps provide more valuable insights into this increasingly important and debilitating disease in the future.
In conclusion, our study showed that about one in 5 men with T2DM have metabolic bone disease, with its associated factors including higher total bilirubin, iPTH and use of DPP4i. This study underscores the importance of screening for metabolic bone disease in this population to allow early detection, initiation of treatment and other preventive measures.