Repercussions of gastrointestinal microbiota in postmenopausal osteoporosis

Abstract

Osteoporosis, characterized by diminished bone mass and microarchitectural degeneration, increases fracture risk, particularly in postmenopausal women (postmenopausal osteoporosis), leading to reduced quality of life and increased mortality. Recent research has highlighted the gut-bone axis, showing how the gut microbiota influences bone health through immune modulation, endocrine regulation, and calcium absorption. Dysbiosis, an imbalance in gut microbiota (e.g., decreased Bacteroidetes and increased Firmicutes), has been linked to osteoporosis by enhancing osteoclast activity and bone loss. Probiotics, such as Lactobacillus strains, promise to increase bone density and alter immune responses. Still, challenges remain in translating these findings to human applications due to issues with dosage and tolerability. Future studies will seek to clarify the function of the gut microbiome in bone health, hence opening the path for tailored therapies aimed at gut flora. Emphasizing postmenopausal osteoporosis, this article will investigate how gut microbiota influences calcium absorption, endocrine regulation, immunological modulation, and bone metabolism. The goal is to develop treatments aimed at gut microbiota to enhance patient outcomes and osteoporosis management. A review of existing literature was conducted, focusing on experimental studies and clinical trials that investigate the relationship between gut microbiota and bone health, including immune and endocrine mechanisms. Relevant studies were selected based on their focus on gut microbiota and bone metabolism, and their findings were synthesized to explore the impact of microbiota modulation on bone health outcomes.

Graphical abstract.

Introduction

Osteoporosis (OP) is a condition that affects the skeleton, leading to a loss of bone mass and a breakdown of the bone’s structure. ¹ This makes bones more fragile and increases the risk of fractures. Osteoporotic fractures, especially in the spine, pelvis, or hips, are linked to a lower quality of life due to high rates of morbidity and, in some cases, increased mortality. ² Postmenopausal osteoporosis (PMO) is a metabolic bone disorder caused by a deficiency in estrogen (E2), resulting in decreased bone density and structural changes that make bones more prone to fragility and fractures. ³ OP affects ~10% of the global population, with 30% of postmenopausal women being particularly affected.^4,5 Fragility fractures, which commonly occur in the spine, hip, and femur, are the most frequent consequence of OP. These fractures typically happen in non-traumatic or minimally stressful situations, leading to pain, deformity, disability, and, in some cases, death. In addition to its impact on bone health, PMO has been associated with an increased risk of several oral health issues, including periodontitis, demonstrating the systemic effects of OP on oral health. ⁶

The microbial population in the gastrointestinal tract is called the gut microbiota (GM), which comprises around 100 trillion bacteria. ⁷ The primary phyla present in the human GM include Proteobacteria, Firmicutes, Actinobacteria, and Bacteroidetes, which collectively account for 90% of the gut’s microbial composition. ⁸ These bacteria play a complex role in maintaining intestinal homeostasis. However, this balance can be disrupted by factors such as age or stress, potentially causing the microbiota to return to its original state. ⁹ A study by Wang et al. ¹⁰ examined the diversity of the gut microbiome in individuals with osteopenia, OP, and healthy controls. They found that, compared to the control group, individuals with OP had a higher abundance of Firmicutes and a lower presence of Bacteroidetes. ¹¹

This article will discuss the critical role of the gut microbiome in bone metabolism, with a particular focus on its influence on PMO. It will explore how GM interacts with various factors such as the immune system, endocrine regulation, and calcium absorption, ultimately contributing to bone health. In preparing this review, articles were selected based on their relevance to the role of the gut microbiome in bone metabolism and the development of PMO. In preparing this review, a systematic approach was used to select relevant articles investigating the relationship between GM and PMO. A comprehensive search was conducted in major electronic databases, including PubMed, Google Scholar, and Scopus, for peer-reviewed articles published in the last 10 years. Studies were included if they focused on experimental animal models or clinical trials examining the interaction between GM and bone health in the context of PMO. Only peer-reviewed articles were considered. Articles were excluded if they did not directly investigate the microbiome’s impact on bone metabolism or lacked experimental data. Key findings were extracted from each study, focusing on the types of microbiota, the methodologies for microbiota analysis, like 16S rRNA sequencing, metagenomics, and the outcomes related to bone health.

Gut microbiota

The human intestinal tract comprises about 100 trillion microorganisms, including bacteria, viruses, fungi, and archaea. The microbial composition primarily consists of thick-walled bacteria such as Actinomycetes, Proteus, Bacteroides, and microalgae. Although these bacteria have a far greater variety of genetic characteristics than humans, their significance in preserving bodily health has received less attention. ¹² Human intestinal symbiotic bacteria support the digestion and absorption of food, secrete microbial metabolites, and maintain mucosal barrier function. ¹³ A particularly important aspect of the GM’s influence on health is the gut-brain axis. GM affects the production of hormones such as growth hormones and sex hormones, both of which are essential for maintaining bone density. When the GM becomes imbalanced (dysbiosis), it can disrupt the regulation of these hormones, leading to reduced bone strength and an increased risk of fractures.^13,14 This connection between microbial imbalance and hormonal regulation is critical because hormones directly influence bone metabolism.

Actinobacteria, Firmicutes, Proteobacteria, and Bacteroidetes are the major bacterial groups of the intestine. Different bacterial groups within the same phylum can perform a range of functions. For example, butyrate-producing bacteria such as Butyricicoccus and Faecalibacterium are part of the Firmicutes family Ruminococcaceae. These butyrate-producing bacteria have shown potential as probiotics for modifying the microbiota in gastrointestinal disorders like inflammatory bowel disease. ¹⁵ Similarly, Ruminococcus gnavus, a well-known member of the Ruminococcaceae family, has been associated with conditions like endocarditis, liver abscesses, and infections following prosthetic hip joint replacement. ¹⁶ Recent research has highlighted the influence of the GM on bone metabolism. Evidence suggests that interactions between bone cells and the immune system are influenced via GM. ¹⁷ The human GM consists of symbiotic, commensal, and pathogenic microorganisms, and dysbiosis, or disruption of the microbiota, is linked to lower bone density. This reduction in bone density is a key factor contributing to OP, which in turn increases the risk of fractures. ¹⁸

Gut microbiome and bone metabolism

Gut microbiome homeostasis is critical for human health. The latest research shows that the gut microbiome can affect normal bone metabolism, leading to metabolic and inflammatory bone disorders. ¹⁹ Germ-free (GF) mice exhibited higher trabecular volume bone mineral density (BMD) and improved histomorphological indices in the trabecular bone compared to conventionally raised mice (CONV-R). However, after re-colonization with the GM, GF mice exhibited a decrease in cortical cross-sectional area and trabecular BMD, suggesting that the GM affects bone mass. ²⁰ Re-colonization of microbes in mice led to the initial critical drop in the mass of bone; however, stable bone growth occurred, creating a new equilibrium in bone mass. ²¹ Moreover, immature GM colonized GF mice from donors with nutritional statuses or distinctive ages, indicating that GM affects bone morphology differently based on age and nutrition. ²² Research has shown that immunodeficiency and utilization of antibiotics during growth stages can disrupt the microbiota, leading to compromised bone biomechanical characteristics in mice, further emphasizing the significant role of GM in regulating bone health. ¹⁹

In addition, adult offspring exposed to low-dose penicillin (LDP) post weaning, or those receiving LDP from their mothers during pregnancy, displayed altered bone mineral content and BMD, indicating the lasting impact of early-life antibiotic exposure on bone health. ²³ The GM is also linked to the development of metabolic and inflammatory bone disorders, such as OP, osteoarthritis, and auto-inflammatory osteomyelitis. For example, the prevalence of specific gut bacteria, such as Methanobrevibacter sp. and Lactobacillus sp., was significantly associated with the prediction of osteoarthritis in rats, as measured by the modified Mankin score.^24,25 A trial conducted in postmenopausal women evaluated the impact of probiotic supplementation on BMD. The study found that women who received probiotics showed a significant improvement in BMD compared to the placebo group, especially in the lumbar spine (LS) and hip regions. This study demonstrates that probiotics can positively influence bone health by modulating GM composition and its interaction with the endocrine system. ²⁶

Mechanisms in the modulation of bone metabolism by GM

The exact mechanisms by which gut microbes influence bone metabolism remain largely unclear. However, various pathways have been proposed, including the immune system, calcium absorption, and endocrine regulation, as potential ways in which the GM may impact bone metabolism. The GM regulates bone metabolism through multiple mechanisms (Figure 1).

Figure 1.

Gut microbiota uses several mechanisms to regulate bone metabolism.

Correlation with the immune system

Bone metabolism is affected by the gut microbiome via the immune system. Recently, the involvement of the immune system in bone metabolism has been recognized in research, resulting in osteoimmunology, in which the factors of immunity playing a role in bone remodeling are highlighted. The RANKL-RANKOPG axis, as well as the immunoreceptor tyrosine-based activation motif signaling route, plays a crucial role in maintaining healthy bone turnover and is involved in bone disorders related to immune-mediated bone metabolism. ²⁶ Recent research demonstrated that the GM could interact with the immune cells and impact the host. ²⁷ A study found that GF mice exhibited distinct immunological profiles compared to CONV-R mice, including reduced levels of pro-inflammatory cytokines, fewer CD4+ T cells, and decreased osteoclast precursor cells in the bone marrow. These differences may explain why GF mice tend to have better bone mass than their CONV-R counterparts. ²⁰ In mice, interleukin-17 (IL-17) and interferon-gamma, produced by intestinal segmented filamentous bacteria, are involved in the differentiation of osteoclasts and osteoblasts, respectively. ²⁸ These findings suggest that the GM influences the host’s immune system, thereby regulating bone metabolism. The gut microbiome plays a critical role in modulating lymphocyte activation in response to fluctuations in sex hormones. Similar results have been observed in animal models treated with leuprolide and gonadotropin-releasing hormone (GnRH) agonists that induce menopause. ²⁸ Preliminary studies in osteopenic and osteoporotic patients have shown significant alterations in the GM composition, suggesting a strong correlation between microbial diversity, the immune system, and bone metabolism. This cross-talk is crucial for maintaining bone health, with immune cells such as Th17 and Treg cells playing central roles. Th17 cells, which promote inflammation, are linked to increased bone resorption, while Treg cells, which suppress inflammation, help prevent excessive bone loss. ¹⁷ The balance between these two cell types is vital for regulating bone turnover, as they influence osteoclastogenesis and osteoblastogenesis. Figure 1 highlights the significant role of GM in regulating hormonal pathways. GM interacts with gonadal steroids, including E2 and testosterone, and affects serotonin production, both of which are important for maintaining bone density.^17,28

Correlation with the endocrine system

The gut microbiome plays a critical role in regulating bone metabolism through the endocrine system. Hormones, alongside the immune system, are key regulators of bone metabolism. Insulin-like growth factor-1 (IGF-1), which acts as a paracrine or autocrine growth factor, plays a key role in bone cell interactions by encouraging the differentiation and proliferation of osteoblasts, osteoclasts, and chondrocytes. ²⁹ Both growth hormone and parathyroid hormone contribute to regulating bone metabolism through the IGF-1 signaling pathway. ²⁹ Intermittent administration of parathyroid hormone has been shown to boost local IGF-1 production, activating this pathway to support bone growth. ³⁰ Additionally, growth hormone influences the growth plate, either directly or via IGF-1, to stimulate cartilage formation and facilitate longitudinal bone growth. ³¹ Additionally, gonadal hormones, such as E2 and androgens, modulate bone metabolism by influencing bone turnover and mass. ³² The gut microbiome has gained recognition as an “endocrine organ” because of its ability to interact with the hypothalamic-pituitary-adrenal (HPA) axis and regulate hormones like cortisol, E2, and testosterone, which in turn affect bone health. ³³ Research has shown that colonization of the GM in GF mice can significantly increase serum IGF-1 levels, leading to improved bone mass and normalized bone development. ²¹ This highlights the crucial role of microbial regulation in endocrine signaling pathways for maintaining bone health. ³³

Recent clinical trials also suggest that the gut microbiome may influence bone health through hormonal pathways in humans. A clinical trial examining the impact of probiotic supplementation in postmenopausal women found that the treatment resulted in improved BMD, particularly in areas such as the LS and hip. The study indicated that probiotics may modulate GM composition, which in turn could regulate the HPA axis and hormonal balance, suggesting a direct influence of gut bacteria on endocrine function related to bone metabolism. Additionally, human studies have suggested that the human colon microbiome may biotransform polycyclic aromatic hydrocarbons, a naturally occurring pollutant, into substances with estrogenic activity, potentially influencing bone metabolism and hormonal signaling. ³³

Influence of calcium absorption

The GM plays a key role in calcium absorption, which is essential for the metabolism of bones. Calcium, the mineral that makes up most of the bone tissue, is vital for bone growth, while vitamin D helps facilitate calcium absorption. A deficiency in either vitamin D or calcium can lead to conditions like OP.^34–36 Clinical research has shown that higher calcium intake can reduce bone resorption in teenage girls. For example, one study found that a daily intake of 47.4 mmol of calcium, compared to the recommended 22.5 mmol, resulted in reduced bone resorption. ³⁷ Research also suggests that variations in calcium metabolism across different ethnic groups are linked to bone health and the risk of OP. These variations are influenced by factors such as dietary calcium intake, renal calcium excretion, and other relevant factors. ³⁷ A diet low in calcium can lead to faster bone turnover, weaker bone structure, and poor trabecular microarchitecture in areas like the femur, femoral neck, hard palate, mandible, and spine. ³⁸ Calcium absorption occurs through two main pathways: passive paracellular diffusion and active transcellular transport. The active transcellular pathway depends on ion pumps and is influenced by the level of 1,25-dihydroxy vitamin D (the active form of vitamin D). ³⁹

Proteins involved in the transcellular pathway include calbindin D9k, which facilitates transport of intracellular calcium transport, PMCA1b, which helps release calcium to bloodstream, and TRPV6, CaT1, and ECaC2, which assist in calcium absorption from the gut lumen. Both passive absorption and active vitamin D-dependent transport work together to ensure proper calcium intake. ³⁹ Research shows that calbindin D9k, the key protein for calcium transport, is found in both the cecum and large intestine of rats, indicating a similar mechanism may occur in humans. ⁴⁰ The interaction between vitamin D and the GM plays a significant role in calcium absorption. High doses of oral vitamin D3 have been shown to alter the upper gastrointestinal microbiota, significantly reducing opportunistic bacteria like Pseudomonas, Escherichia coli, and Shigella. ⁴¹ Vitamin D also helps maintain the integrity of the intestinal mucosal barrier through its receptors, further illustrating the complex relationship between vitamin D, gut health, and calcium absorption.

This ultimately impacts the immunogenicity of intestinal cells as well as the makeup of gut microbes, ⁴² as shown in Figure 2. The figure shows that microbiota influences intestinal lumen pH and calcium transport, thereby regulating BMD. Specifically, short-chain fatty acids (SCFAs) produced by gut bacteria influence calcium absorption in the intestines through transcellular transport mechanisms. ³⁹ Clinical studies have shown that vitamin D and calcium metabolism are affected by the GM, which helps modulate how efficiently calcium is absorbed and utilized for bone development. ⁴³

Figure 2.

Gut microbiota serve various functions within the body. They can trigger an inflammatory response by interacting with antigen-presenting cells, contribute to maintaining the integrity of epithelial cells, and influence the differentiation of naive T cells (modified from D’Amelio and Sassi ⁴³ ).

Consequences of GM via the gut-brain axis

Recent research has highlighted the significant role the gut microbiome plays in producing hormones like serotonin (5-HT), which have a profound impact on the neurological system. One key finding is that the 5-HT signaling pathway is a vital factor in stimulating and maintaining bone formation. In a study by Sjögren et al., ²⁰ mice with lower levels of serotonin exhibited an increase in trabecular bone volume and tissue volume. This suggests that the gut microbiome might influence bone health by modulating metabolic hormones.^44,45

Another study, which utilized a unique mouse genetic background, observed differences in bone health based on the presence of gut flora. CONV-R mice, with normal GM, showed more cortical and trabecular bone in their femurs compared to 8-week-old male GF mice with a BALB/c genetic background. ⁴⁶ Additionally, research has found that mono-colonizing mice with a strain of Lactobacillus plantarum promoted bone growth. Interestingly, a study by Li et al. ⁴⁷ revealed that the microbiota’s presence did not negatively impact bone health. It was also noted that serum growth hormone levels were significantly higher in CONV-R or mono-colonized mice compared to their GF counterparts, indicating that microbial colonization may influence the hypothalamic-pituitary axis. In this instance, it appears that GM influences neuroendocrine activity to enhance bone health, ⁴⁸ as elaborated in Figure 3. The figure shows that the gut-brain axis plays a pivotal role in regulating bone metabolism. The GM influences the central nervous system, which in turn regulates the HPA axis, thereby affecting hormonal balance and bone health. Disruptions in the GM lead to neurotransmitter imbalances, which can impact both gut motility and bone remodeling. The interaction between gut microbes and the brain also affects intestinal permeability, which influences systemic inflammation and bone resorption. ⁴⁵

Figure 3.

Gut microbiota interaction with the brain.

Protagonist of gut microbiome on bone functioning

The gut microbiome plays a significant role in maintaining the mechanical function of bones. As mentioned earlier, the health of bones, including their strength and structure, is closely influenced by the gut bacteria. Changes in the microbiome can affect everything from the distribution and composition of bone material to its microstructure and overall mass, ultimately determining the bone’s ability to withstand mechanical stress. ⁴⁹ Interestingly, it has long been observed that low doses of antibiotics can accelerate animal growth by altering the GM, which in turn improves calorie absorption. Additionally, the gut microbiome produces a range of important vitamins like niacin (B3), thiamine (B1), pantothenic acid (B5), biotin (B7), folate, vitamin K, pyridoxal phosphate, tetrahydrofolate, and cobalamin (B12), all of which contribute to overall body health. ⁵⁰ These vitamins are absorbed by the gut and distributed through the bloodstream, and recent studies suggest they play a crucial role in maintaining the integrity of the bone matrix.

Apart from its dietary origin, vitamin K, produced by the GM, has been long associated with bone strength. Several randomized controlled trials have highlighted that low levels of vitamin K status are significantly linked to a higher risk of fractures, reinforcing its importance in bone health. ⁵¹ These findings corroborate the notion of impaired bone tissue associated with deficient vitamin K levels. Vitamin K plays a pivotal role in regulating the abundance of proteins, such as osteocalcin, within the bone matrix. Osteocalcin, a major non-collagenous protein in bone, relies on vitamin K for its carboxylation, facilitating its proper binding to bone minerals during bone development. ⁵²

Liaison of PMO with intestinal microbiota

According to reports, about 1.5 million fractures happen annually in patients suffering from OP in the United States. The majority of the cases involve women over 50 years of age with PMO. Skeletal instability and microarchitectural degeneration are symptoms of the condition. The elevated danger of postmenopausal fractures is associated with low BMD and qualitative alterations in microarchitecture, according to the conceptual definition of OP. ⁵³ Whether OP is determined by fracture frequency or low BMD (a T-score; a standard deviation measure that compares an individual’s BMD to the average BMD of a healthy young adult population is 2.5 or below), the prevalence of the condition differs. ⁵⁴ The reduced bone density which is a common sign of OP and dysregulated gut flora are somehow related. ⁹

Recent data from the United States (2005–2010) shows that 10.3% of Americans over the age of 50 are affected by OP, which impacts over 10 million individuals. Additionally, 43.9% of people over 50 have poor bone mass.^9,55 The composition of the gut microbiome plays a crucial role in maintaining bone health. The many factors that influence bone health are closely linked to the GM. Research by Xu et al. ⁵⁶ found that the immune system is connected to bone loss in PMO, with the GM playing a key regulatory role. Another study showed that the gut microbiome is involved in regulating bone mass, potentially influencing bone strength by modulating immunity, which in turn affects osteoclast activity. ⁵⁷ Furthermore, additional research has shown that gut flora significantly impacts the development of OP.^10,58

SCFAs and PMO

Calcium is the primary mineral in human bones, and the body typically relies on dietary sources to obtain it. Most of the calcium from food is absorbed in the upper part of the small intestine before passing through the intestinal wall. ⁵⁸ SCFAs, which are produced by gut bacteria, play a significant role in regulating how the microbiome influences calcium absorption.^59,60 Studies have shown that probiotics can help convert dietary fiber into SCFAs, and increasing intake of probiotics or prebiotics can boost SCFA secretion from the digestive tract.^61,62 Additionally, research suggests that butyric acid may help regulate immune responses by influencing dendritic cells, which promote osteoblast differentiation and prevent osteoclast formation in bone marrow cells. ⁵⁸

When comparing the GM in postmenopausal osteopenia and control groups, the osteopenia group showed higher bacterial community richness, which was associated with increased diversity compared to the control group. In contrast, the PMO group had much lower diversity. Moreover, LS BMD was positively correlated with observed species and Shannon indices, but no statistical association was found between E2 levels and microbial diversity measures.^63,64 To better understand the differences in microbiota composition, a genus-based prevalence analysis was conducted to identify distinct enterotypes within the three groups.

The collective samples formed from those of three unique enterotype clusters. ⁶⁵ The most abundant species in enterotype 2 was Prevotella 9, followed by Klebsiella, Escherichia/Shigella, and Phascolarctobacterium in enterotype 1, and Bacteroides in enterotype 3. However, Fisher’s exact test showed no statistically significant variations in the percentage distribution of the various enterotypes among the three groups. ⁶⁶ The majority of the available information on the connection between intestinal microbiota and PMO comes from animal studies. Most typically utilized Pathogen Modeling Program (PMP), a “model animal” refers to a non-human species used to mimic certain aspects of human disease or biological processes for research purposes, animal models are rats that have been surgically or medically treated. Ovariectomy (OVX) is the most common surgical procedure utilized to develop PMO in mice. The bilateral OVX has been utilized to effectively establish the grisly condition of PMO in the lumbar vertebra, distal femur, and proximal tibia following the Food and Drug Administration’s requirements for the preclinical and clinical assessment of PMO treatment. ⁶⁷ GnRH agonists have been extensively used to study PMO in rats.^47,56 When administered at high doses over a prolonged period to rats, particularly those kept in a GF environment, GnRH agonists reduce the release of endogenous gonadotropins, GnRH, and E2. ⁶⁸ The bone loss induced by GnRH agonists is reversible. It has found that long-term treatment with GnRH agonists led to a decrease in bone mass, density, and turnover in Sprague Dawley rats, though some of these changes could be partially reversed after stopping treatment.^56,68 E2 deprivation, whether through OVX or GnRH agonist treatment in mice, increases bone resorption and turnover while reducing BMD and volume in long bones and lumbar vertebrae, closely resembling the symptoms seen in postmenopausal patients. ⁵⁶ Figure 4 specifically illustrates the effects of E2 deficiency (such as in PMO) on the GM and bone health. Following OVX, low E2 levels lead to increased gut permeability, allowing microbial products to enter the bloodstream and stimulate the release of pro-inflammatory cytokines like TNF+ and IL-17. ⁵⁶

Figure 4.

Following estrogen loss due to OVX in mice, gut permeability is increased, leading to the release of TNF+ and Th17 cells from the gut. These activated T cells migrate to the bone, where they promote osteoclast-mediated resorption of trabecular bone. Additionally, the elevated release of intestinal microbiota during OVX may contribute to the increased production of TNF+ and Th17 cells in peripheral lymphoid organs, such as lymph nodes. These immune cells can then travel to the bone, enhancing spongy bone resorption (modified from Xu et al. ⁵⁶ ).

OVX: ovariectomy.

PMO and intestinal microbiota

The effects of probiotics on bone density can also shed light on the role of gut bacteria in bone metabolism. Studies in mouse models, including both male and ovariectomized female mice (used to simulate E2-deficient postmenopausal conditions), have examined this relationship. For instance, Lactobacillus strains have been shown to enhance bone density while also modulating the GM.^69–71 Specifically, Lactobacillus reuteri has been administered to ovariectomized mice to protect against bone loss, potentially by reducing Trap 5 expression and the activation of NF-κB ligand, both of which are markers of osteoclast activation, leading to decreased osteoclastogenesis. ⁷¹ In earlier research by Sjögren et al., ²⁰ it was shown that mice raised in GF environments had increased trabecular bone mass compared to controls. When colonized with microbiota from conventionally bred mice, the bone phenotype was restored. Furthermore, lower levels of CD4+ T cells and TNF-α in the bone marrow of GF mice were linked to a reduction in osteoclast precursors and an increase in bone mass. This suggests that commensal GM may reduce bone mass by promoting bone resorption and inhibiting bone formation. ⁷²

The clinical trial investigates the effect of a combination of three Lactobacillus strains on postmenopausal bone loss. The trial, conducted on early postmenopausal women, demonstrates that the probiotic treatment notably reduces LS BMD loss compared to the placebo group. The Lactobacillus-treated group showed no significant bone loss in the LS over the 12-month trial period, in contrast to the placebo group, which experienced a marked decrease. This evidence indicates the potential effectiveness of probiotic treatment in protecting against postmenopausal bone loss in women. ⁷³ One study investigated the effects of L. reuteri ATCCPTA 6475 (L. reuteri 6475) on bone loss in older women with low BMD. In a 12-month double-blind, placebo-controlled trial, the supplement was found to significantly reduce overall bone loss in the L. reuteri 6475 group compared to the placebo, as indicated by both intention-to-treat and per-protocol analyses. These findings suggest that L. reuteri 6475 has the potential to prevent age-related bone loss and OP. ⁷⁴ Additionally, further research demonstrated that bone loss induced by OVX in mice could be partially prevented by administering tetracycline, a short-term antibiotic treatment that selectively suppresses certain intestinal bacteria, ultimately leading to increased bone mass in the mice.^75,76 Hence, antibiotic therapy altered the microbiome, which in turn affected bone mass. Antibiotics not only destroy the microbiota but also change its makeup, lowering the variety of microbial species found in the gut. The intestine bacterial load’s diversity and number are likely to have a role in the microbiota’s ability to regulate bone mass. ⁷⁷ Stunted bones, shorter bones, and increased cortical bone density development were seen when GF mice were colonized with feces samples from impoverished kids with immature microbiota. This suggests that an immature microbiota may have bone anabolic effects. ⁷⁸

Role of genetics in PMO

OP candidate genes can be broadly categorized based on their role in the metabolism of osteoclastic and osteoblastic cell structures, as well as their involvement in mineral and collagen regulation and hormonal pathways, particularly sex hormones. Various gene polymorphisms have been investigated for their association with BMD and the risk of fractures.^79,80 Among these, a few genes have been extensively reviewed, including the vitamin D receptor (VDR), ⁸¹ estrogen receptors (ESR1 and ESR2), collagen type I alpha 1 chain (COL1A1), transforming growth factor-beta (TGF-β), and low-density lipoprotein receptor-related protein. Additionally, genes such as IL-6, methylenetetrahydrofolate reductase, and aromatase (CYP19) have also been studied in depth. ⁷⁹ Most research indicates a connection between these genes and BMD, particularly fractures, which are more commonly observed in women, less often in men, and rarely in both sexes. ⁸² However, narrative reviews have reported inconsistent findings on the relationship between VDR, COL1A1, TGF-β, and fractures. ⁷⁹

PMO therapy by GM

In a healthy state, the gut microbiome, intestinal epithelial barrier, and immune system are interconnected to prevent intestinal infections and maintain musculoskeletal equilibrium. ⁸³ However, when the balance of the GM is disrupted, intestinal bacteria can pass through the epithelial barrier, initiating an immune response that promotes osteoclastic activity and contributes to ongoing bone loss, which is a characteristic feature of PMO. ⁵⁶

Probiotics, as beneficial bacteria, can play a crucial role in restoring this balance. By modulating the gut microbiome, probiotics may strengthen the intestinal barrier, reduce systemic inflammation, and support immune function, potentially mitigating bone loss in PMO. For instance, in the treatment of diet-induced metabolic disorders, Berberine has been shown to selectively enhance the abundance of butyrate-producing bacteria, such as Allobaculum, Bacteroides, Butyricoccus, and Blautia. These bacteria, known for their anti-inflammatory effects, may offer therapeutic benefits by reducing systemic endotoxins and inflammatory cytokines in OVX-periodontitis rat models. ⁸⁴

In addition to Berberine, probiotics like Lactobacillus and Bifidobacterium species are associated with enhancing gut barrier integrity and immune regulation, particularly the modulation of Th17 cell differentiation and IL-17A production. IL-17A, a pro-inflammatory cytokine produced mainly by CD4+ Th17 cells, has been implicated in the pathophysiology of metabolic bone disorders, including PMO. In cases of E2 deprivation, IL-17A levels increase in the gut and bone marrow, disrupting intestinal barrier function and promoting bone resorption. ⁴⁷ Probiotics have been shown to lower IL-17A production by enhancing gut integrity and reducing the systemic circulation of endotoxins, thus potentially reducing bone loss in PMO.

Furthermore, IL-17A neutralization combined with probiotics has been shown to support bone remodeling by mitigating the failure of bone regeneration in the absence of E2. ⁸⁵ Studies also suggest that probiotics can help reduce local IL-17A levels in gums or gingival crevicular fluid, which is often elevated in individuals with chronic periodontitis, indicating a pathogenic role of IL-17A in inflammatory bone loss.^84,86,87 Probiotic supplementation, along with therapies like Berberine, may help manage IL-17A levels, suppress osteoclastogenesis, and promote bone health by modulating the Th17/IL-17 response and improving GM balance.

Limitations

While this review provides valuable insights into the role of GM in bone metabolism, several limitations should be acknowledged. First, the majority of studies included in this review are based on animal models, particularly GF mice and OVX rats, which may not fully replicate the complexities of human bone metabolism or microbiota. Furthermore, the variability in experimental designs, including differences in animal species, microbiota composition, and methodologies for assessing bone health, introduces potential biases. Additionally, many studies focus on specific bacterial species or interventions, which limits the generalizability of the findings. There is also a lack of long-term clinical trials in humans that directly link microbiota interventions to improved bone health. As such, further research, including well-designed human clinical trials, is required to validate the findings and determine the practical implications of GM-based therapies for bone health.

Future scenario

Future research in the field of OP aims to clarify the complex relationship between bone health and GM. The goal of research is to learn more about the processes behind the effects of gut microorganisms on bone metabolism. In elderly women, a correlation was found between the gut microbiome and OP. Innovative treatment approaches that target the GM, such as personalized therapies, have potential benefits in controlling and preventing PMO. Furthermore, continuing studies into omics and high-throughput sequencing technologies provide a more precise view of the intricate microbial diversity linked to OP, opening the door to precision treatment catered to unique microbial profiles. Furthermore, investigating the interplay between bone cells and the immune system, as clarified through animal models, is essential for formulating novel approaches that utilize the gut-bone axis for medicinal objectives. These discoveries might lead to a new era in OP management as we move forward by directing us toward more specialized, efficient, and individualized care.

Conclusion

Recent research shows that the GM has a significant impact on bone health and should be factored into OP treatment. A healthy microbiome helps maintain balanced bone metabolism, while an imbalance can increase osteoclast activity, leading to bone loss. The interaction between gut bacteria and bone cells, like osteoclasts and osteoblasts, plays a crucial role in this process. While probiotics show promise as a complementary treatment, most studies have been on animals, and more work is needed to determine safe and effective doses for humans. Although these findings strongly suggest the gut microbiome’s influence on bone metabolism, it’s important to approach the implications for humans with caution. Clinical trials in humans are necessary to confirm the safety, effectiveness, and potential of microbiota-targeted therapies, like probiotics, for treating PMO. More research is needed to fill the gaps in human data and explore how microbiota-based treatments could be integrated into OP care.