Osteopenia and osteoporosis are common diseases worldwide, which are associated with increased fracture risk, especially in the aging population (1–4). Early detection and intervention for individuals with osteopenia or osteoporosis may effectively reduce the socioeconomic burden caused by fractures (1, 4, 5). Bone mineral density (BMD) is used as an important indicator for the diagnosis of osteopenia and osteoporosis, while bone turnover markers (BTMs) are a useful adjunct for therapeutic monitoring (6, 7). Osteocalcin (OC) is one of the traditional BTMs, which reflects skeletal status and bone remodeling (8, 9). Specially synthesized and secreted by osteoblasts, OC is an abundant non-collagenous protein (10). During the process of post-translational modification, 17, 21, and 24 glutamic acid residues of OC undergo carboxylation mediated by vitamin-K-dependent γ-glutamyl carboxylase (GGCX), which leads to a conformational change with stable α-helical portion and greater affinity for calcium and hydroxyapatite (11, 12). According to the degree of carboxylation, OC presents in two forms: fully carboxylated OC (cOC) and undercarboxylated OC (ucOC) lacking carboxylation at one or more sites (13, 14). Of the total amount of OC in circulation, 40%–60% is ucOC.

Recently, Smith et al. (15) revealed the “U shaped” relationship between ucOC and age in Australian adult men; however, there were no age-related studies on ucOC levels in Chinese men or women. In addition, recent advances have indicated that ucOC not only is a nutritional biomarker sensitive to vitamin K status but also functions as a hormonally active isoform in the regulation of glucose homeostasis and energy metabolism (16). Recent studies have shown that ucOC is a useful marker to identify the increased metabolic syndrome risk and a potential therapeutic target for cardiovascular diseases (17–19). Regarding the field of bone homeostasis, ucOC is released into circulation due to its reduced affinity to the bone matrix. Vergnaud et al. (20) found that serum ucOC predicted the risk of hip fracture independent of femoral neck BMD in older women. Natsumi et al. (21) revealed that vitamin K deficiency and high serum ucOC were correlated with poor bone status in women. Furthermore, some studies have demonstrated that higher serum ucOC levels were associated with lower BMD in women (22–24). Yamauchi et al. (25) have found that ucOC levels were positively correlated with urinary type I collagen cross-linked-N-telopeptide (uNTX), but not BMD in postmenopausal women. Most of the research is performed in women, and there have been few reports on correlations of serum ucOC with BMD and BTMs in men. Although a higher ucOC level was found in individuals with osteoporosis, it is needed to further investigate the relationship between the ucOC level and prevalent osteopenia and osteoporosis (26).

Therefore, in this cross-sectional study, we determined how ucOC levels change with gender and age in the Chinese population and defined the associations of ucOC with BMD, BTMs, and the prevalence of osteopenia and osteoporosis in both Chinese men and women.

This was a large cross-sectional study exploring the profile of serum ucOC levels according to gender and age in the Chinese population. First, we found that men had a higher serum ucOC level than women after adjusting for age, BMI, Cr, and 25OHD, which was consistent with previous studies (28–30), but some studies also showed contrary results (31). The difference may partly be attributed to the difference in the hormonal milieu between genders. Foo et al. (28) believed that female sex hormones might have a role in modulating the level of ucOC, while Hiam et al. (30) found that testosterone may mediate the differences in ucOC levels between men and women. The above discrepancies require further study to elucidate the role of sex steroids in bone metabolism and explain these sex differences. Moreover, we found that in both men and women, ucOC levels decreased with age from youth, reaching the bottom values at the age of 40–49 years, and then increased in the subsequent life. The age-related changes in serum ucOC in the current study were consistent with the “U-shaped” relationship between ucOC and age in adult men proposed by Smith et al. (15). Perhaps due to different populations and sample sizes, the age with the lowest ucOC level in our study was 5 years earlier than that in Smith’s study. Similar age-related changes were also reported in other BTMs, like total OC, P1NP, and β-CTX, which may be related to different bone statuses during the life span (32). In youth, the higher ucOC levels might be correlated with beneficial increases in bone formation and suggested that the skeletal system had not yet reached maturity, while in the elderly, it might reflect the imbalance or uncoupling of bone resorption and formation and be correlated with bone loss (33).

Subsequently, we explored the associations of ucOC with BMD and BTMs in the Chinese population. First, we found that a higher ucOC level was correlated with lower BMD at the lumbar spine, femoral neck, and total hip in both men and women. Ito et al. (34) and Vergnaud et al. (20) found that ucOC was significantly and negatively correlated with femoral neck BMD, while Yamauchi et al. demonstrated that it was only negatively correlated with lumbar BMD (25). However, Liu et al. (24) considered that the increase of ucOC might be related to the changes in bone quality rather than mineral density, as they found no associations between ucOC and BMD. Our findings were similar to the results of Szulc et al. (23) and Emaus et al. (35) that elderly women with abnormally high serum ucOC have lower BMD values at all sites. Cummings et al. (36) found that a 26% decrease in femoral neck BMD of patients with abnormal levels of ucOC corresponds to a five- to sevenfold increase in hip fracture from the prospective data. Although the reason for the strong association between ucOC and BMD is not clear, the phenomenon might indicate that increased serum ucOC reflected intrinsic abnormalities of the bone matrix leading to bone loss and increased fragility. Additionally, we showed that a higher ucOC level was correlated with higher P1NP and β-CTX levels in both men and women, while Yamauchi et al. (25) demonstrated a strong and positive correlation between ucOC and uNTX in postmenopausal women. Although detailed mechanisms have not been clarified, one hypothesis may partly explain the positive correlation. When bone turnover is accelerated, markers including P1NP, β-CTX, and total OC are excessively synthesized, and greater quantities of vitamin K may be required. The relative insufficiency of vitamin K and increased OC are likely to cause increased ucOC levels (25). Since vitamin D is one of the main drivers of bone mineralization, we further evaluated the correlation of ucOC with BMD and BTMs in the vitamin D-sufficient subjects. In this study, we found that a higher ucOC level was still associated with a lower BMD and higher levels of P1NP and β-CTX.

Moreover, we found that a higher level of ucOC was associated with a higher risk of osteopenia or osteoporosis in both men and women, after adjusting for age, BMI, Cr, and 25OHD. Horiuchi et al. (26) revealed that serum ucOC level in individuals with osteopenia or osteoporosis was significantly higher than that in individuals with normal BMD, and Vergnaud et al. (20) considered that a higher ucOC level was a predictor of hip fracture in older women. However, currently, the relationship between ucOC levels and the risk of prevalent osteopenia or osteoporosis remained unclear. A hypothesis may partly explain the role of ucOC in the skeleton; that is, an abnormally high level of ucOC can reflect the accelerated bone turnover and cause bone loss and decreased BMD, which in turn leads to osteopenia or osteoporosis and serious fractures. Further randomized controlled studies or prospective studies are needed to reveal the specific causality and mechanism.

There were also some limitations in our study. First, some important variables were lacking, such as vitamin K, which acted as a coenzyme of carboxylase and catalyzed the carboxylation of OC. Our study did not evaluate the effect of vitamin K. Second, we used the commercial ELISA kit to measure the level of ucOC in serum, which was not considered the gold standard. Therefore, the serum ucOC level in this study could not reflect the levels measured by other technologies. Third, this is a cross-sectional study; thus, the proven causality between the ucOC, and BMD, BTMs, and prevalence of osteopenia and osteoporosis was limited. Therefore, further studies with a large sample size, wide age span, appropriate gender distribution, and unified measurement are still needed.

In conclusion, we first revealed the profile of the serum ucOC level according to gender and age in the Chinese population and demonstrated the associations of ucOC with BMD, BTMs, and the risk of prevalent osteopenia or osteoporosis. Our findings may provide a clue to elucidate the function of ucOC in bone metabolism.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by the ethics committee of the Human Research of the Shanghai Jiao Tong University Affiliated Sixth People’s Hospital. The patients/participants provided their written informed consent to participate in this study.

Study design: YX and ZZ. Study conduct: YX, LL, and WH. Data analysis and interpretation: YX and LS. Manuscript draft: YX and LS. Manuscript content revision: WH, ZZ, and LL, YX and LS contributed equally to this work. All authors contributed to the article and approved the submitted version.

This study was supported by the National Key R&D Program of China (2018YFA0800801), Shanghai Municipal Key Clinical Specialty, Key Specialty Construction Project of Pudong Health and Family Planning Commission of Shanghai (PWZzk2017-29), Outstanding Leaders Training Program of Pudong Health Bureau of Shanghai (PWRI2018-02), and Science Foundation of Shanghai Jiao Tong University Affiliated Sixth People’s Hospital (lygl202101).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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