Ageing gut microbiome may contribute to bone loss through the immune system

The gut microbiome is the community of micro-organisms living in the intestines. Its composition shifts with age: bacteria that trigger inflammatory responses become more prevalent, while species that produce beneficial metabolites decline. This pattern is called dysbiosis. Researchers reviewed what is currently known about how gut microbiome dysbiosis may drive bone loss through the immune system.

The immune system as the link

Bone is continuously built by osteoblasts and broken down by osteoclasts. Osteoporosis arises when the balance tips toward greater breakdown. The researchers describe how an aged gut microbiome influences immune cells in ways that amplify signals activating osteoclasts. Immune cells including T cells, B cells and macrophages undergo age-related changes in behaviour that disrupt the balance between bone-protective and bone-destructive signals.

A key part of this involves the ratio between two molecules: RANKL, which activates osteoclasts, and OPG, which inhibits them. Chronic low-grade inflammation, driven by a disrupted microbiome, shifts this balance toward greater RANKL activity. The bone marrow plays a central role here: it is not only a site of bone remodelling but also a niche where immune cells develop and mature.

Associations, not yet proof

The authors are clear that this is a review of existing evidence, not new experimental data. Many of the described links are based on animal models or observational studies in humans. Which mechanisms are most important, and whether microbiome interventions can meaningfully protect bone health, remains an open question. The research field is moving quickly.

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Search terms to explore further: gut microbiome dysbiosis inflammaging, RANKL OPG bone remodelling immune system, immunosenescence osteoporosis bone marrow