Osteosarcopenia, characterized by the coexistence of osteopenia/osteoporosis and sarcopenia, represents a significant health concern in geriatrics, with an increased risk of falls and fractures. The enterokine fibroblast growth factor 19 (FGF19) was recently shown to prevent muscle weakness in preclinical models.

This study investigated the therapeutic potential of FGF19 in mitigating bone and muscle deterioration in aged male mice. Twenty-one-month-old C57BL/6 male mice received daily injections of human recombinant FGF19 (0.1 mg/kg) for 21 days.

Histological and functional analyses revealed a shift toward larger muscle fibers in FGF19-treated mice as well as an increased muscle strength, without affecting muscle mass. In parallel, X-ray microtomography showed that FGF19 had no overt negative impact on bone, with a range of modest, site-specific, and opposing effects.

In the distal femur metaphysis FGF19, it reduced cortical thickness, but significantly increased bone cross-sectional area, with an overall increased polar moment of inertia, a geometrical parameter linked to favorable mechanical properties. It also elevated cortical bone porosity in the same region.

There were no significant effects on trabecular bone or cortical bone parameters in the proximal femur side at the lesser trochanter level nor at the femoral midshaft or in the tibia. In the L2 vertebra, cortical porosity decreased.

Histomorphometry of trabecular bone and analysis of transcriptional output of selected genes in femurs revealed only minor changes in bone cellular activities and gene expression after three weeks of treatment. In conclusion, FGF19 treatment increased muscle strength in aged male mice, without negatively impacting aging bone.