How quickly do you lose bone mass when you stop exercising?

An exact timeline, such as 'you lose this many percent of bone mass per month when you stop exercising', does not appear in the available studies. What the studies do make clear is that prolonged inactivity, such as bed rest or immobilisation, demonstrably leads to bone mass loss. How quickly that happens specifically after stopping exercise in everyday life cannot be determined from this research.

What is known is that physical inactivity is considered a primary cause of osteoporosis, the condition in which bones become more brittle and break more easily. Physical activity, and particularly strength training and weight-bearing exercises, is needed to maintain bone mineral density. Walking alone has only a limited effect in this regard.

Inactivity affects bones and muscles simultaneously. Muscle loss (sarcopenia) and bone mass loss reinforce each other: fewer muscles means less mechanical load on the bone, which accelerates bone loss. Among men aged 60 to 64, around 14% already suffer from this combination; among men over 75, that figure rises to nearly 60%. This illustrates how quickly the situation can deteriorate as you grow older and become less active.

If you want to protect your bones after a period of reduced activity, progressive strength training is the intervention with the strongest evidence base. This means performing resistance exercises regularly and gradually increasing the load, so that the bone receives a stimulus to become stronger or at least not to weaken further.