We selected the Web of Science Core Collection (WoSCC) database to conduct a literature search on 01/03/2024. The search terms is as follows: #1: TS = (osteoporosis OR osteopenia OR osteoporotic OR bone loss* OR Low bone mass OR low bone density), #2: TS = (“gut microbiota” OR “intestinal microbiota” OR “fecal microbiota” OR “gastrointestinal microbiota” OR “gut microbiome” OR “intestinal microbiome” OR “fecal micro-biome” OR “gastrointestinal microbiome” OR “intestinal bacteria” OR “gut bacteria” OR “fecal bacteria” OR “gastrointestinal bacteria” OR “intestinal flora” OR “gut flora” OR “fecal flora” OR “gastrointestinal flora” OR “gut microflora” OR “intestinal microflora” OR “fecal microflora” OR “gastrointestinal microflora”), final = #1 AND #2. LA = (English), and the type of documents was set to “articles” and “review.”. The publication period was specified as 01/01/2014 to 30/03/2024. Following the initial retrieval, we screened the titles and abstracts to confirm the eligibility of the articles based on predefined inclusion and exclusion criteria. The flowchart of the screening process is shown in Figure 1.

Leveraging the capabilities of VOSviewer (version 1.6.18), a bibliometric analysis software of substantial renown (13), we facilitated the generation of visualizations representing cooperative, co-citation, and co-occurrence networks. The analyses conducted in this study utilizing VOSviewer encompassed co-occurrence analysis of keywords, nations, journals and co-cited journals, authors, and co-cited authors, as well as institutions. In the label view used for these visualizations, the colors of the nodes represent different clusters or groups of items (such as institutions, authors, or journals) that are more closely related to each other within the same cluster than to those in other clusters (14).

We also engaged CiteSpace (version 6.1. R1), an alternative software for bibliometric analysis and visualization (15), devised by Professor Chen Meichao of Drexel University. With the assistance of CiteSpace, we established dual map overlay visualizations of journals pertinent to this inquiry and pinpointed references and keywords exhibiting high citation bursts.

We used the R package “Bibliometrix” (version 3.2.1)¹ to illustrate the annual publication volume trends in this research field and the publication output of various countries, which not only showcases the academic output of these countries but also elucidates the state of international collaboration among them. Subsequently, we created trend graphs of cumulative publication volumes for the top 10 institutions and journals, which further revealed the influence of major institutions and journals within the field. Moreover, we conducted a detailed analysis of trend topics using the “Bibliometrix” package.

Full records and cited references of the retrieved articles were downloaded from the WoSCC database and saved as.txt format for below analysis.

Considering the yearly increase in publication numbers, the entire period can be divided into two phases: Phase I (2014–2019), and Phase II (2020–2024). As shown in Figure 2, the number of publications in Phase I was relatively low, with an average annual publication count of about 20.6, representing the initial stage of research on gut microbiota- osteoporosis. Entering Phase II, the number of publications began to increase significantly, with an average annual publication count of 81, marking a substantial rise compared to Phase I. This trend demonstrates the growing recognition among scholars of the significance of gut microbiota in osteoporosis.

A total of 881 institutions, and 61 nations have contributed to this collective body of literature. As depicted in the geographical network map of Figure 3A, the top 10 contributors hail from diverse regions encompassing Asia, Europe, and North America. Table 1 discloses that a predominant portion of publications originates from China (253) and USA (111), collectively accounting for a commanding 68.8% of the total global publications. Hot on their heels are South Korea (N = 28, 5.29%), Italy (N = 27, 5.10%), and Japan (N = 27, 5.10%).

Further, Table 1 accentuates that the publications from China boast the highest cumulative citation frequency (3,883), followed distantly by those from USA (3,537), Italy (769), and Japan (757). In Figure 3B, lines are utilized to represent the frequency of international academic collaboration, with the node’s size signifying each country’s publication tally. The visual representation in Figure 3B underscores a thriving international research collaboration landscape, with China, the United States, and Japan engaging in dynamic cooperative efforts. A close collaborative synergy between South Korea and Canada is also discernible.

Although China leads in publication volume by a significant margin compared to other countries, the proportion of its multi-country publications (MCP) is relatively low in comparison to its domestic research output, as shown in Figure 3C. This suggests that in this field of research, there is a notable lack of academic collaboration between the China and other countries.

We employed VOSviewer to conduct a visual analysis of the 881 institutions incorporated in this study, as portrayed in Figure 4A. Figure 4A offers a graphical delineation of the inter-institutional collaboration network, accentuating the strong ties that exist among a diverse array of institutions. As suggested by Figure 4A’s visualization, considerable collaboration is apparent between Shanghai Jiao Tong University and Zhejiang University, as well as between Southern Medical University and Central South University. However, it is pivotal to note that these collaborations principally take place within each institution’s home country, revealing a striking paucity of vibrant academic collaborations between institutions from disparate nations.

Figure 4B, alongside Table 1, ranks the top institutions by their contributions to the literature. Shanghai Jiao Tong University leads the cadre with 37 publications, closely followed by Southern Medical University and Central South University with 33 and 32 publications, respectively. Notably, the disparity in publication volume across these institutions does not emerge as significant.

Figure 4B showcases a promising incline in the annual publication output from the top 14 institutions in recent years. Nanjing University of Chinese Medicine blazed the trail by being the first to contribute to this field, but its cumulative document count appears to have plateaued starting from 2017. Conversely, although Shanghai Jiao Tong University joined the field later, it has witnessed a swift escalation in publication output beginning from 2019.

Harnessing the capabilities of VOSviewer, we curated a visual representation of journals and co-cited journals within this research realm. Our dataset includes a total of 246 journals, and we incorporated the top 33 journals with a minimum publication count of 4 (Figure 5A); the size of the nodes represents the publication volume of each journal. Figure 5B unfurls a network map of co-cited journals, featuring those commanding a minimum of 25 citations. As explicated in Figure 5B, 312 co-cited journals were displayed, reflecting the aggregate link strength. The five most frequently co-cited journals, exhibiting the most formidable total link strength (TLS), comprised: J Bone Miner Res (TLS = 85,399), Nutrients (TLS = 58,480), Nature (TLS = 57,100), PLoS one (TLS = 55,021), and Bone (TLS = 54,481) (refer to Table 2).

Local citations, deduced from the reference list, afford insight into their localized impact, whereas total citations mirror wider interest across various disciplines. Within this ranking, J Bone Miner Res commandeered the list with 1,096 citations, followed by PLoS One with 727 citations, and Nature with 725 citations (as illustrated in Table 2). This clearly indicates a high proportion of high-caliber publications within these journals. Clearly, these journals are high-quality international publications that provide support for gut microbiota-osteoporosis research.

Figure 5C sketches the annual outputs of the top 10 journals spanning from 2014 to 2024. The publication volume in Nutrients has experienced a steep ascent in recent years. Conversely, the publication growth in Journal of Agricultural and Food Chemistry has been relatively placid. Table 2 catalogs the top 10 most productive and co-cited journals incorporated in this inquiry. Nutrients (impact factor = 6.71, 2024) surfaced as the preeminent publisher, boasting 29 publications. Further, there were 16 publications in Frontiers in Endocrinology (IF = 6.05, 2024), 12 publications in Frontiers in Cellular and Infection Microbiology (IF = 6.07, 2024) and Calcified Tissue International (IF = 4.20, 2023). Eight of the top 10 journals fell under the Q1 JCR region.

In CiteSpace, the dual-map overlay technique provides researchers with a macroscopic view of the cross-disciplinary interactions and citation relationships among scholarly articles. This visualization is composed of two parts: one represents the disciplines of the citing articles, and the other represents the disciplines of the cited articles, each displayed on the left and right maps, respectively. The labels on these maps identify the journals or research areas involved. The paths from citing to cited articles reveal the flow of knowledge between disciplines. The color and thickness of these paths indicate the citation intensity and the time frame, assisting researchers in quickly identifying academic trends and impacts over different periods. Thus, the dual-map overlay not only serves as a bridge for communication between various disciplines but also highlights key areas in academic dissemination, providing a powerful tool for exploring trends, predicting hot research topics, and fostering potential interdisciplinary collaborations (16, 17).

Utilizing CiteSpace, we crafted a dual map overlay of journals pertaining to the role of gut microbiota in osteoporosis, as illustrated in Figure 5D. Clusters residing on the left of the orange line designate citing journals, whereas the cluster to the right of the orange trajectory signifies co-cited journals. The principal path (the yellow, orange, and one green paths) reveals that articles emanating from the realms of molecular/biology/genetics are primarily cited by researchers engaged in veterinary/animal/science journals, molecular/biology/immunology journals and medicine/medical/clinical journals. Furthermore, the other green path indicates that articles originating from the spheres of health/nursing and medicine are mainly cited by researchers involved in medicine and clinical journals. The outcomes from the dual-map overlay of journals may suggest that the current gut microbiota – osteoporosis research is zeroed in on molecular medical and clinical aspects.

In the exploration of gut microbiota in osteoporosis, 3,166 researchers participated. The top 10 contributors collectively produced 79 publications, representing approximately 14.9% of the total output within this field (Table 3). Ohlsson C and Hernandez CJ emerged as the most productive authors, with 9 publications (Table 3). The H-index, a metric designed to quantify a scholar’s impact who has authored H papers each garnering at least H citations, was employed to appraise the influence of the scientific investigations. As evidenced in Table 3, Li YJ distinguished himself as the author boasting the highest H-index, followed by Hernandez CJ et al. Anchored in the understanding of the brain-gut-bone axis and its effects on bone metabolism, Li YJ’s highest impact factor paper (published in Crit Rev Food Sci Nutr, IF = 11.20) probes into the potential influence of probiotics and prebiotics on osteoporosis (18). This extensive exploration touches on various facets, including the regulation of gut metabolites, the integrity of the intestinal epithelial barrier, and the critical roles played by neuromodulation, immune regulation, and endocrine regulation. By doing so, the article shines a light on a promising and innovative approach toward the prevention and treatment strategies of osteoporosis in the future.

VOSviewer provides a visualization of the interconnections among authors, as exhibited in Figure 6A. There exists a profound collaboration between Xiao HH and Wong MS as well as Chan CO. Similarly, a dynamic partnership is observed between Yao, Xin-Sheng and Yao, Zhi-Hong. Co-citation analysis scrutinizes the association among items based on their co-citation frequency. Deploying VOSviewer, a totality of 153 authors, each with a minimum citation count of 20, were evaluated, as delineated in Figure 6B.

As expounded in Table 3, Li YJ emerges as the most frequently co-cited author (co-citation = 205), succeeded by Ohlsson C (co-citation = 197), and Yan J (co-citation = 180). Out of the 17,853 co-cited authors, six scholars received more than 100 co-citations. Ohlsson C’s most cited article reviews the impact of the gut microbiota on bone mass (19). The research indicates that the gut microbiota regulates bone mass by modulating the host immune system. Moreover, dietary and environmental factors can influence the composition of the gut microbiota, thus affecting bone metabolism. The authors put forward the idea that the gut microbiota could serve as a novel therapeutic target for treating osteoporosis. This groundbreaking work is the first to propose that the gut microbiota could be an innovative approach toward managing osteoporosis and preventing fractures.

The gut microbiota profoundly and subtly impacts host bone health (23), both directly [for example, through microbe-associated molecular patterns (MAMP) or pathogen-associated molecular patterns (PAMP)] and indirectly (for instance, via metabolite production), by modulating innate pro-inflammatory or anti-inflammatory reactions (24).

Eubiosis, colloquially known as a “healthy microbiota,” is recognized as a crucial factor in preserving the physiological and metabolic integrity of the organism. Traditionally, eubiosis is conceived as a harmonious balance within the gut microbiome ecosystem, characterized by a predominance of beneficial bacterial species (25). These beneficial microbiota consortia reinforce intestinal epithelial barrier integrity through direct, cooperative mechanisms. Furthermore, they shape the host’s immune landscape indirectly by generating a variety of essential metabolites.

Studies indicate that SCFAs are the most significant metabolites known to positively affect Treg cells, particularly butyrate, which enhance Treg cell differentiation and function (26). This is achieved by promoting the expression of relevant genes and by inhibiting the activity of histone deacetylases. Moreover, indole-3-acetic acid (IAA) and indole-3-propionic acid (IPA), metabolites produced by the gut microbiota from tryptophan, can promote the differentiation of Treg cells as well, but through activating the aryl hydrocarbon receptor (AhR) (27). Consequently, an increase in Treg cells triggers the release of anti-inflammatory cytokines and reduces inflammatory signaling pathways, such as NF-κB, thereby enhancing intestinal immune tolerance and maintaining an anti-inflammatory milieu (28).

Contrary to eubiosis, “dysbiosis” denotes alterations in the composition and function of the primary microbial communities, linked to the emergence of various diseases (25).

Dysbiosis in the gut microbiota disrupts the “functional balance” between pro-inflammatory and anti-inflammatory microbes, altering intestinal immunity and biasing the immune system toward a pro-inflammatory response (Figure 9). This alteration affects the differentiation of naive CD4+ T cells. In particular, specific bacteria, including segmented filamentous bacteria (SFB), Bifidobacterium adolescentis, and Eggerthella lenta, are known to augment Th17 cells (29). This leads to an increase in the secretion of pro-inflammatory cytokines, such as IL-17 and TNF-α from Th17 cells, further promoting inflammatory responses and bone resorption (30). Conversely, certain bacterial species, including Lactobacillus rhamnosus GG (LGG), L. reuteri, and Bifidobacterium breve, can positively impact the abundance and function of Treg cells (29). However, the reduction of these beneficial bacterial species can inhibit the differentiation of regulatory T cells (Tregs), leading to a decrease in the secretion of anti-inflammatory cytokines derived from Tregs, such as IL-4, IL-10, and TGF-β. This ultimately promotes osteoclastogenesis and inhibits bone formation. The dysbiosis-induced alteration in the Th17/Treg balance reduces the host’s immunosuppressive capacity, which exacerbates the inflammatory state of the intestinal microenvironment and directly impacts bone metabolism.

As mentioned above, dysbiosis can also be defined as alterations in the composition of the primary microbial communities, typically characterized by a reduction in beneficial bacteria and/or an increase in harmful bacteria. This reduction in beneficial microbes can not only directly compromise the integrity of connections between intestinal epithelial cells, diminishing the thickness and quality of the gut mucosal mucus layer, thus reducing barrier function and enhancing intestinal permeability, a condition commonly referred to as “leaky gut” (31). Furthermore, the proliferation of pathogenic bacteria and their toxins (such as lipopolysaccharide, LPS) can stimulate intestinal mucosal immune cells: dendritic cells (DCs) in the intestine can capture and process antigens, activating T cells, and promoting the differentiation of Th17 cells (32). Macrophages then trigger inflammatory responses through pattern recognition receptors (such as Toll-like receptors), resulting in the production of excessive pro-inflammatory cytokines, such as TNF-α, IL-6, and IL-1β (33). These inflammatory factors, secreted by intestinal mucosal immune cells in response to pathogens and toxins, not only increase local intestinal inflammation but can also directly damage intestinal epithelial cells, leading to reduced expression of tight junction proteins (such as occludin and claudin), thereby further increasing intestinal permeability. Consequently, bacteria, toxins, and other foreign antigens can more easily enter the bloodstream, leading to a systemic inflammatory response.

The impact of inflammation on bone metabolism is well recognized (34, 35). Chronic systemic inflammation, through the activation of immune cells such as macrophages and T cells, leads to the production of inflammatory cytokines (TNF-α, IL-1, and IL-6) that directly influence bone metabolism. Specifically, TNF-α and IL-1 are known to promote the differentiation and maturation of osteoclast precursors, leading to increased bone resorption (36, 37). Additionally, IL-6 facilitates osteoclastogenesis by upregulating RANKL expression and drives bone immune reactions via the JAK/STAT signaling pathway, thus accelerating bone resorption (38). Moreover, IL-17 boosts osteoclast activity by fostering RANKL expression, thereby influencing bone resorption, and it may indirectly affect osteoblast function by inhibiting the Wnt/β-catenin signaling pathway (39). Furthermore, the inflammatory milieu amplifies the interaction between RANKL (a key osteoclast differentiation factor) and its receptor RANK, further enhancing osteoclast formation and activity (40).

Overall, the inflammation initiated by gut microbiota dysbiosis and the resulting immune disorder play a pivotal role in osteoporosis progression. Inflammatory responses triggered by dysbiosis lead immune cells, such as macrophages and dendritic cells, to release an array of inflammatory mediators, directly impacting bone metabolism through key pathways such as the NF-κB signaling pathway and the RANKL/OPG/RANK system, among others. Additionally, the inflammatory state from dysbiosis disturbs the dynamic equilibrium between Th17 and Tregs, not only facilitating osteoclast differentiation but also suppressing bone formation, thus laying the groundwork for osteoporosis development. These mechanisms highlight the critical importance of understanding the interplay between gut microbiota and the host immune system, suggesting that regulating this intricate network might provide novel prevention and treatment options for osteoporosis.

Among current drug treatments for osteoporosis, a range of options such as bisphosphonates, SERMs, PTH and its analogs, RANKL inhibitors, and Sclerostin inhibitors have been confirmed to effectively increase bone density, decelerate bone loss, and lower fracture risks (41, 42). Although these drugs offer viable treatments for osteoporosis, their application is faced with several limitations. These limitations range from potential safety issues with long-term use, such as jaw osteonecrosis and atypical femur fractures associated with bisphosphonates, to significant patient-to-patient variability in treatment outcomes. Additionally, the high costs of developing new drugs and an inadequate understanding of complex disease mechanisms present further challenges. Furthermore, existing treatments mainly target a single aspect of the pathological process and often fail to address the multifactorial etiology of osteoporosis, highlighting the need for more comprehensive treatment strategies.

Given the crucial role of gut microbiome dysbiosis and resultant inflammation in the pathogenesis of osteoporosis, focusing research on the gut microbiome as a treatment target for osteoporosis is not only a response to the limitations of current treatments but also a logical extension of a comprehensive understanding of osteoporosis’s complex pathophysiological mechanisms. Keywords with the strongest citation bursts, such as “fecal microbiota transplantation,” “fermented milk,” “protein,” “bioavailability,” “nf kappa b,” highlight that regulating the gut microbiome through methods like fecal microbiota transplantation or specific dietary patterns to control inflammation represents a promising future research direction for enhancing bone health and preventing or treating osteoporosis (Figure 10).

Diet has a significant impact on bone health throughout the entire lifespan and is a major determinant of the types and proportions of microbes in the host organism. Thus, when assessing the impact of changes in the gut microbiome on bone health, diet should be considered an important confounding factor (43).

Diet directly affects the gut microbiome, modifying its composition or metabolic outputs, potentially contributing to disease progression or maintaining bodily balance (44). A high-fiber diet rich in fruits, vegetables, legumes, and whole grains provides various polysaccharides and oligosaccharides indigestible by human enzymes. These dietary fibers act as prebiotics, specifically nourishing beneficial gut bacteria and promoting the production of SCFAs (such as butyrate, propionates, and acetates) (45). Fermented foods such as yogurt, kefir, sauerkraut, and kimchi are rich in live microorganisms that contribute to the increased diversity of the gut microbiota. By consuming fermented foods, one can increase the abundance of beneficial bacteria, improve gut barrier function, and have anti-inflammatory effects (46). The Mediterranean diet, enriched with vegetables, fruits, nuts, seeds, legumes, whole grains, fish, and olive oil, is associated with increased microbial diversity and the promotion of beneficial bacterial growth. This diet, rich in monounsaturated fats, polyphenols, and fiber, not only positively affects the gut microbiome but also enhances the production of SCFAs, thereby improving gut barrier function, and reducing inflammation as common benefits (47).

High-protein diets, especially those rich in animal proteins, alter the gut microbiome by increasing the abundance of bacteria capable of fermenting protein. Consequently, this could lead to the production of potentially harmful by-products, including ammonia, amines, and sulfides (48). Western diets, marked by high fat, high sugar, and low fiber, lead to a gut microbiota composition inclined toward increased inflammation levels and decreased diversity (49). Such dietary patterns commonly result in increased intestinal permeability (“leaky gut”) and systemic inflammation.

In recent years, fecal microbiota transplantation (FMT) has been widely used to treat a variety of diseases (50), including Crohn’s disease, metabolic syndrome (51), diabetes (52), and neurological disorders (53). Remarkably, FMT has also demonstrated considerable potential in treating osteoporosis (54). Unlike individual or mixed bacteria, FMT maximally retains the original diversity and quantity of active microbial communities, thereby enabling a quicker reinstatement of gut microbiota stability in osteoporosis patients (54). In a study by Zhang et al., transplanting the gut microbiota of children into ovariectomized (OVX) mice effectively prevented bone loss caused by ovariectomy and increased the bone strength of the mice. Moreover, 16S rRNA gene sequencing revealed that transplanting the fecal microbiota of children reversed the OVX-induced reduction in Akkermansia abundance, while direct supplementation with Akkermansia could prevent bone loss in OVX mice (54).

Yet, the choice of FMT donors, the presence of numerous harmful bacteria in the transplants, and the long-term outcomes of the treatment approach continue to cast doubt on its clinical safety (55, 56). Future research should focus on identifying specific microbial strains or consortia that have the most significant impact on bone metabolism, exploring the optimal timing and frequency of FMT for the best bone health outcomes, and understanding individual variations in response to FMT. Clinical trials that monitor the gut microbiota composition, bone density, and bone metabolism markers before and after FMT are essential to ascertain the effectiveness of this method in preventing or treating osteoporosis.

Research on various interventions targeting the gut microbiota is still in its initial stages, and we emphasize the necessity for future studies, particularly the need for more high-quality, large-scale, long-term clinical and mechanistic studies to validate the efficacy and safety of gut microbiota intervention measures.