Natural bioactive compounds in estrogen deficiency-induced osteoporosis: Classification and an integrated gut microbiota-metabolite-immune-bone framework

Table 3 Evidence strength, limitations, and future directions for natural bioactive compounds targeting the gut-bone axis

Evidence category	What is currently supported	Representative examples	Main limitations	Recommended future direction	Translational value
Human intervention and observational evidence	Gut microbiota, metabolites, phytoestrogens, and selected natural products are associated with BMD or bone turnover markers	8-prenylnaringenin trial, PMOP gut microbiota/metabolite studies, breast metabolomics, menopause microbiota studies	Small sample size, short duration, heterogeneous baseline microbiota, limited fracture endpoints	Microbiome-stratified RCTs with BMD, bone turnover, bone quality, and fracture- related outcomes	Limited and heterogeneous human evidence; clinical efficacy remains unestablished
Strong causal animal evidence	Microbiota can mediate or transfer osteoprotective effects	DOP-FMT model, OVX- induced intestinal T cell trafficking, defined bacteria/metabolite studies	Mainly rodent-based; translational relevance to human menopause remains uncertain	Combine FMT, antibiotics, gnotobiotic models, metabolite rescue, and receptor blockade	Mechanistic relevance in experimental models; human translation remains uncertain
Probiotic and defined strain evidence	Specific strains can improve bone loss through immune and barrier regulation	LGG, Lactobacillus acidophilus, Lactobacillus sakei, Parabacteroides distasonis, Clostridium sporogenes	Strain specificity, colonization stability, dose, duration, and long-term safety unclear	Define strain-level metabolic outputs and host-response phenotypes	Potential for clinical investigation; strain-specific efficacy and safety require validation
Natural compound + OVX + 16S/metabolomics evidence	Many natural compounds improve bone parameters and reshape microbiota	Puerarin, quinoa, Moringa oleifera, ginkgolide B, taraxasterol, flavan-3-ols	Often correlation-based; limited causal validation	Add antibiotic depletion, FMT, metabolite supplementation, and pathway inhibition	Preliminary-to-moderate preclinical support; causal validation is needed
Complex herbal formulation and network pharmacology evidence	Multi-component interventions may regulate microbiota, metabolites, endocrine signals, and bone pathways	Roucongrong Pill, traditional Chinese medicine formulas, Jiaru fuling	Active components and key causal nodes are difficult to identify; docking does not prove mechanism	Use component deconstruction, key metabolite validation, and modular causal testing	Exploratory potential; causal attribution and active-component validation remain limited
miRNA and extracellular vesicle evidence	miRNAs may mediate microbiota-host and BMSC-bone communication	Fecal miRNA regulation of microbiota, BMSC exosomal miRNAs, circulating bone-related miRNAs	Few studies directly connect natural compounds, microbiota, EV-miRNAs, and bone outcomes	Integrate fecal/serum/BMSC EV-miRNA sequencing with metabolomics and bone phenotyping	Emerging research direction; currently hypothesis-generating

RCT: Randomized controlled trial; BMD: Bone mineral density; OVX: Ovariectomy; FMT: Fecal microbiota transplantation.