## 1 Introduction  
Osteoporosis is one type of metabolic bone disease characterized by low bone mass, impaired bone microarchitecture, and bone fragility and susceptibility to fracture. Osteoporosis typically presents in postmenopausal women, and the cause is reduced ovarian function, which results in decreased estrogen. Thus, hormone replacement therapy (HRT) has generally been used to prevent and treat menopausal symptoms, including osteoporosis [1]. However, many studies have shown that HRT has some serious side effects, such as increased risk of breast and endometrial cancers and cardiovascular diseases [2]. Therefore, compared with modern available therapeutic approaches, alternative therapies, such as health foods containing natural products, have attracted interest in the treatment of menopausal symptoms, including osteoporosis development [3, 4].  
Saururus chinensis (SC) has been effectively applied as a traditional medicine in Korea and China for edema, jaundice, gonorrhea, and several inflammatory diseases. SC extract (SCE) has been shown to exert anti-inflammatory, antioxidant, anti-asthmatic, antihypertensive, antiangiogenic, and anti- atopic effects [5, 6, 7]. Recently, our research showed the anti-osteoporotic activity of SCE in ovariectomized rats and that this activity is mediated by the inhibitory effect on bone resorption and osteoclast formation of the phytochemicals isolated from SC, such as sauchinone, (-)-saucerneol, and saurolactam [8, 9, 10].  
Various studies have been conducted to analyze the effect of natural products in the treatment of osteoporosis. For example, an extract of _Sinomenii Caulis_ was shown to partially alleviate osteoporosis by inhibiting pro-inflammatory cytokines and activating osteoporosis-related pathways in a zebrafish osteoporosis model [11]. In another study, isoflavone-enriched soybean leaves attenuated osteoporosis by suppressing osteoclasts via its anti-inflammatory activity [12]. Kukoamine B from _Lycii Radicis Cortex_ was shown to regulate markers associated with osteoblast differentiation and bone mineral density in MC3T3-E1 cells, as well as improve OVX-induced bone mineral density loss and impaired bone structural properties in OVX mice [13].  
Metabolomics is an emerging technique for quantitative and qualitative analysis in the field of systematic biology, and it explores the metabolic mechanism of the entire organism by detecting the changes in small endogenous metabolites with subsequent data analysis via multivariate statistical approaches [14]. Not only has its use been dramatically increased to identify potential biomarkers for disease diagnosis, it is also used to characterize natural products [15; 16; 17]. It is also a powerful tool to evaluate the therapeutic effects of herbal medicines and dissect their underlying mechanisms [18; 19]. The technique has been applied to study numerous metabolic disorders, but few have investigated its use in osteoporosis [20; 21; 22; 23; 24]. Compared with other instrumental analysis tools that have been used for metabolite profiling [25], ultra-performance liquid chromatography/mass spectrometry (UPLC/MS) is considered more promising in metabolomics research owing to its high resolution and accuracy and good coverage of metabolites [26; 27; 28].  
In this study, UPLC/ESI-Q-TOF MS was applied to obtain a holistic picture of metabolic changes in ovariectomized (OVX) models of estrogen deficiency-induced osteoporosis. Biomarkers of interest were identified via multivariate statistical analysis and metabolomics databases. Further, the metabolic pathways associated with these biomarkers were explored to further elucidate the mechanism underlying the bone-protective effect of SCE.