Fig. 1.

A schematic diagram for the mechanism of gut microbiome in the development of osteoporosis. The gut microbiome is mainly influenced by diet, antibiotics, and probiotics. The gut microbiome exerts a notable influence on regulating bone mass via a variety of mechanisms including: 1) affecting beneficial bacteria, increasing oestrogen bioavailability, and then regulating bone mass with the aid of prebiotics; 2) increasing the expression of inflammatory cytokine responses by the immune system; 3) producing metabolites of gut microbiome such as short-chain fatty acids; 4) changing intestinal permeability and increasing the promoting effect of vitamin D on bone mineral absorption; and 5) impacting the gut-brain axis and the level of endocrines. 5-HT, 5-hydroxytryptamine; ALP, alkaline phosphatase; DC, dendritic cell; FMT, fecal microbiota transplantation; GR1^-, granulocytes 1; IGF-I/II, insulin like growth factor-I/II; IL-17, interleukin-17; LPS, lipopolysaccharide; NDO, non-digestible oligosaccharide; NF-κB, nuclear factor kappa-B; OC, osteocalcin; OPG, osteoprotegerin; RANKL, receptor activator of NF-κB ligand; Runx2, runt-related transcription factor 2; SCFA, short-chain fatty acid; TNF-α, tumour necrosis factor-α.