Why ageing bone marrow turns to fat, and a PEMF study in mice

Bone or fat?

Bone is not finished once it is built. Osteoblasts lay down new bone and osteoclasts remove old bone throughout life. One source of new osteoblasts is the mesenchymal stem cells in the marrow, which can become either osteoblasts or fat cells. You can explore the skeleton in our body atlas.

A 2004 study in Aging Cell by Moerman and colleagues reported that with age, fewer marrow stem cells commit to the osteoblast lineage and more to the fat lineage. Bone-building genes such as Runx2 were expressed less, while PPAR-γ2, which drives fat-cell development, was expressed more.

People show a similar pattern. In 2001 Danish researchers analysed hip-bone biopsies from 53 healthy adults aged 30 to 100 and 26 people with osteoporosis. With age, the share of marrow taken up by fat rose and the share of trabecular bone fell, and the two were inversely correlated. People with osteoporosis had more marrow fat than age-matched controls. This is a correlation, and the authors noted the mechanism was still unknown.

Sensory nerves inside bone

Bone contains nerves as well as blood vessels. A 2013 mouse study in Nature by Fukuda and colleagues showed that a protein made by nerve cells, semaphorin 3A (Sema3A), is needed to maintain bone mass. Mice lacking Sema3A only in neurons had fewer sensory nerves in their bone and lower bone mass, and removing sensory nerves in normal mice also reduced bone mass.

In mice, then, sensory nerves are not just pain wiring; they also send signals that take part in building bone.

What the 2025 PEMF study did

PEMF (pulsed electromagnetic fields) uses a magnetic field switched on and off in short bursts to induce weak currents in tissue; the principle is covered in PEMF and whole-body vibration. In a 2025 study in Nature Communications, Wang and colleagues exposed young (4-month) and old (20-month) male mice to PEMF at 8Hz and 3.8mT for one hour a day over four weeks.

• In old mice, trabecular bone volume fraction and trabecular thickness in the thigh bone increased, along with a marker of bone formation. Young mice showed no such change.
• Marrow stem cells expressed fewer fat-differentiation genes, and bone-resorbing osteoclasts were unaffected.
• In mice whose sensory nerves were genetically removed, PEMF no longer increased bone formation.
• Sensory neurons exposed to PEMF secreted more Sema3A, and blocking its receptor Nrp1 on the stem cells cancelled the effect.

The core finding is that in this mouse model a physical stimulus from outside the body was linked, via sensory nerves, to the direction stem cells take.

What this study does not say

The limits are clear. Only male mice were studied, with key measurements on about five animals per group. One frequency and intensity was tested for four weeks, and young mice did not respond. The authors themselves note that the amount of new bone seen in mice may differ in human patients. The study does not address females, other settings, or bone density and fracture risk in people.

Human PEMF research has mostly looked at fractures that fail to heal. A 2011 Cochrane review of four randomised trials (125 people) concluded that electromagnetic stimulation may offer some benefit, but the evidence was inconclusive and insufficient to guide practice.

If you are worried about your bones

• If osteoporosis or fractures concern you, start with a bone density test and a doctor’s assessment; diagnosis and treatment belong with your doctor.
• If you have a pacemaker, implantable defibrillator, cochlear implant or other implanted electronic device, talk to a doctor before using any PEMF device.
• If you are pregnant or being treated for a condition, check with your doctor first.
• PEMF does not replace diagnosis or treatment.

BAHIOLOGY and Wave Motion

BAHIOLOGY’s Wave Motion delivers PEMF with adjustable frequency and intensity together with low-frequency waves and vibration; BAHIOLOGY researches and develops it, built together with its investor and manufacturing partner Hue Light Co., Ltd. You can see it in four technologies. This article explains research findings; it does not promise any particular effect.