The results of this retrospective, longitudinal study showed that low serum osteocalcin levels at baseline were significantly related to worse metabolic profiles and independently related to an increased risk of MetS incidence in Korean men. After adjusting for conventional risk factors for MetS, such as PBF, BMI, and smoking status, as well as factors considered to be criteria for MetS, the serum osteocalcin level remained an independent protective factor against MetS. Therefore, a low serum osteocalcin level may be an indicator of increased risk of MetS in Korean men.
Experimental animal studies have showed that osteocalcin regulates glucose and fat metabolism. Lee et al. [6] showed that osteocalcin promotes the proliferation of pancreatic β cells, while increasing insulin and adiponectin secretion in adipocytes via osteocalcin-targeted KO mice. The KO mice exhibited higher blood glucose levels, greater fat pad mass, and higher serum TG levels than did the wild-type mice [6]. Consistent with the findings in mouse models, our study showed higher TG levels, PBF, and serum glucose levels in participants with low serum osteocalcin levels. FPG and HbA1c levels decreased progressively from the lower to upper quartiles of serum osteocalcin levels, consistent with previous findings in humans [11]. However, no differences were observed in HOMA-IR, which approximates insulin resistance, across the osteocalcin quartiles; this finding was similar to that in some previous studies [22, 23] and different from that in others studies [9, 24, 25]. Insulin resistance is a fundamental cause of diabetes and MetS. However, the relationship between osteocalcin levels and the incidence of diabetes is controversial. Studies showed that a low serum osteocalcin level was independently related to an increased risk of incident type 2 diabetes mellitus in Japanese postmenopausal women [26] and the Chinese population [27]. In a nested case-control study, low serum osteocalcin levels were associated with an increased risk of incident diabetes during a mean 5-year follow-up period [28]. An inverse association between the osteocalcin level and the change in FPG was found in a prospective analysis by Pittas et al. [29]. On the other hand, Hwang et al. [30] found that the serum osteocalcin level was not associated with the development of type 2 diabetes after a mean of 8.4 years in a retrospective cohort study, despite the presence of a positive association between serum osteocalcin and favorable metabolic parameters at baseline. Another prospective research showed that lower serum osteocalcin levels did not predict future development of diabetes [31]. Taken together, these results suggest that osteocalcin may not play a major role in the development of diabetes in humans.
The G protein-coupled receptor class C, group 6, subtype A (GPRC6A) is known as a receptor for osteocalcin and transduces the signal through the phospholipase C-inositol 1,4,5-trisphosphate-calcium2+ pathway and adenylyl cyclase-cyclic adenosine monophosphate-protein kinase A pathway to promote the secretion of insulin and adiponectin and modulate insulin sensitivity [32]. However, recently, Jørgensen et al. found that full locus GPRC6A KO mice presented normal glucose metabolism that did not differ from that exhibited by wild-type mice; this was not consistent with the finding in previous KO mice models known as GPRC6A exon II KO and exon IV KO [33]. Furthermore, it was found that insulin signaling in murine osteoblasts enhanced the conversion of osteocalcin into the active, undercarboxylated form and regulated glucose homeostasis by signaling through GPRC6A in mice [34–36]. However, in another study, no significant changes were observed between mice with postnatal deletion of the insulin receptor in osteoprogenitor cells and control mice, although undercarboxylated osteocalcin levels were significantly decreased in the mutant mice [37]. Therefore, more studies are needed to clarify the molecular mechanism of action and functions of osteocalcin.
Considerable controversy has arisen over several findings that mouse models may not satisfactorily represent the human osteocalcin physiology [38]. The reasons for the variable metabolic phenotypes in terms of glucose and energy metabolism in different mouse models remain unclear [37]. However, the involvement of serum osteocalcin in MetS has been largely studied through cross-sectional research in humans [10, 11, 24, 25, 39–41], which showed that serum osteocalcin levels were negatively associated with MetS. Although serum osteocalcin levels are affected by age, sex, menopausal status, and ethnicity [40], Saleem et al. [41] found an inverse association between serum osteocalcin and the presence of MetS in black and non-Hispanic white individuals. Yang et al. [39] investigated postmenopausal women in China and found similar results, which were also consistent with those in some studies involving Korean men and postmenopausal women [24] and older Caucasian men [10, 25]. Furthermore, a meta-analysis demonstrated significantly lower circulating levels of serum osteocalcin in participants with MetS than in those without MetS in 12 cross-sectional studies [11].
The benefits of exercise in terms of muscle energy metabolism and insulin sensitivity have been well documented [42]. Increases in undercarboxylated osteocalcin levels following aerobic exercise have been observed in animal models [43] and humans [44]. Recently, Rahimi et al. [45] found that exercise-induced elevation of undercarboxylated osteocalcin was a potential cause of increased adiponectin, which has insulin-sensitizing, anti-atherogenic and anti-inflammatory properties. Meanwhile, osteocalcin is synthesized in the presence of vitamin K and undergoes post-translational carboxylation in a vitamin K-dependent manner [46]. Although clinical trials of vitamin K supplementation have yielded mixed results [47], several studies have shown that vitamin K supplementation increased carboxylated osteocalcin [48] and decreased undercarboxylated osteocalcin, thus improving insulin sensitivity [49]. Although there is controversy regarding the form of osteocalcin that is functionally active in humans [27, 28, 49], exercise and vitamin K may act as regulators of glucose metabolism by influencing serum osteocalcin.
This retrospective follow-up study had some limitations. First, the participants were volunteers for a health check-up. The prevalence of MetS in our study was lower than that in the general population [3] because our participants might be particularly concerned about their health. Another limitation is women were excluded from in this study because female serum osteocalcin levels are affected by menopausal status (i.e., premenopause, perimenopause, and postmenopause) [21, 50]. In addition, our questionnaire did not permit obtaining detailed information about menstrual pattern changed that could have otherwise determined menopause status [51]. Furthermore, because this study only included men recruited from a single-center, the representative capacity of the results may be limited. Second, we did not consider dietary or exercise habits, which are well-known effective factors for the development or resolution of MetS. Third, whether or not undercarboxylated osteocalcin is the active form in humans remains unclear [27, 28, 52, 53]. Undercarboxylated osteocalcin alone has a biological effect in mice [6]. However, we did not differentiate serum osteocalcin with respect to the ɣ-carboxylation status and measured the total osteocalcin levels alone. However, to minimize the possibility of this effect, we excluded men who took vitamin K antagonists or multivitamin supplements including vitamin K, as carboxylation is a vitamin K-dependent process [6, 46]. Fourth, although we excluded participants who used medication for osteoporosis at baseline and/or during the follow-up period, we could not consider other bone conditions such as Paget’s disease and fracture. If these diseases are considered, the results may be different. Fifth, because the follow-up period was relatively short, we could not confirm the long-term effects of serum osteocalcin levels. Nevertheless, to our knowledge, this is one of the few studies investigating the association between serum osteocalcin and the development of incident MetS in a relatively large sample.