Multiple human studies show that adults can slightly increase bone density through heavy strength training combined with high-impact exercises. The gain is modest and site-specific, and preventing inactivity is at least as important as training itself. After menopause, oestrogen loss considerably hampers bone building and exercise cannot replace that hormonal effect.
Adults can still build modest bone mass after reaching peak bone mass (around age 25 to 30), but the potential is smaller than at a younger age. Nearly two dozen studies show that progressive strength training can slightly increase bone density in adults. The effect is site-specific: only the bones directly loaded by the exercise become stronger. The intensity and duration of training largely determine whether there is any effect; a handful of studies found little or no difference.
The most effective approach combines heavy resistance training with high-impact exercises such as jumping. That combination produces more bone gain than gentle movement alone. Swimming has no demonstrable positive effect on bone mass, because it is a non-weight-bearing sport in which the skeleton receives almost no mechanical stimulation. In younger adults, slightly larger gains are possible than in older adults, where the effect amounts more to slowing bone loss or modest maintenance.
At least as important as active training is avoiding prolonged inactivity. Bone loss from sitting still is greater than the bone gain that exercise produces. This makes moving doubly worthwhile: on the one hand it builds something up, but it also protects against the loss that inactivity causes.
For women after menopause, an additional complicating factor applies. Oestrogen deficiency causes greatly increased bone breakdown, both in the inner (trabecular) and the outer (cortical) part of the bone. Exercise cannot compensate for this loss: oestrogen has an independent, irreplaceable effect on bone preservation. Anyone who experiences rapid bone loss after menopause cannot address this through movement alone.
Adequate calcium (1000 to 1200 mg per day) and vitamin D (600 to 800 IU per day) combined with increased physical activity can improve bone mass in premenopausal women with low bone density. This has been investigated in limited studies, so the exact size of the effect is not yet precisely known. In people with osteoporosis and a very high fracture risk, prescription anabolic bone medications such as romosozumab and teriparatide exist that can substantially increase bone density, for example plus eleven percent in the spine after twelve months for romosozumab. These are treatments for a specific medical population, not intended for healthy people who want to optimise their bone mass.
Based on multiple human studies and a limited study in premenopausal women. Strength of evidence varies by sub-question: strong for the harmful effect of oestrogen loss and inactivity, moderate for exercise and bone building, limited for calcium/vitamin D in premenopausal women, strong for anabolic medication in osteoporosis patients.