Bone mineral density

Bone mineral density (BMD) is the standard way to quantify skeletal strength, typically measured by DEXA scan at the lumbar spine and hip. It sits at the intersection of two concerns that matter for longevity: how reliably it predicts fractures, and whether improving it prevents them. Those two questions have different answers.

BMD as a fracture predictor

Each one standard-deviation drop in BMD roughly doubles your population-level fracture risk. A 1996 meta-analysis by Marshall and colleagues found that site-matched prediction is strongest: lower spine BMD raised vertebral fracture risk about 2.3-fold, and lower hip BMD raised hip fracture risk about 2.6-fold [1]. The same analysis carries an important caveat: BMD predicts risk across populations but cannot tell you whether a specific individual will fracture. It is a risk stratifier, not a crystal ball.

Hip fracture in particular is a longevity event, not an orthopedic one. A 2010 meta-analysis found relative hazard of death in the first months after hip fracture was roughly 6-fold in women and 8-fold in men, with an 8-18% excess mortality persisting through year one at age 80 [2]. That downstream toll is why skeletal health belongs in any serious conversation about healthspan.

Interventions that raise BMD, and whether fractures follow

Two antiresorptive drug classes have cleared the highest bar: demonstrating reduced fracture rates in large randomized trials, not BMD changes.

Denosumab (the FREEDOM trial) cut vertebral fracture risk by 68% and hip fracture risk by 40% [3]. Alendronate (the FIT trial) reduced vertebral fractures by 47-55% and hip fractures by 51% [4]. These findings are concordant across drug classes, which strengthens confidence in the signal.

High-load resistance and impact training also moves the BMD needle. The LIFTMOR trial found that a supervised program of heavy lifting and jumping produced a lumbar spine BMD gain of roughly 4% [5]. That is a real, meaningful change in the marker.

The fracture endpoint, though, is a different story. A Cochrane review of exercise and bone density in postmenopausal women found genuine BMD improvements alongside a non-significant fracture odds ratio of 0.61 (95% CI 0.23-1.64), a result the authors attributed to the trials being underpowered rather than to a null effect [6]. The direction is encouraging; the evidence is not yet conclusive.

Vitamin D supplementation, at least in adults with already-adequate levels, adds another data point in the same direction. The VITAL/LeBoff trial found no significant fracture reduction from vitamin D supplementation in a replete population [7].

The surrogate trap

The pattern across these interventions is worth sitting with. Antiresorptive drugs raise BMD and reduce fractures. Exercise raises BMD but has not been shown to reduce fractures in adequately powered trials. Vitamin D can raise levels but didn't move fracture rates in replete adults. The mechanistic logic, "BMD up, therefore fractures down," is only as reliable as the evidence behind each specific intervention that produces the BMD gain.

This is a clean example of what researchers call surrogate-versus-outcome discipline. A biomarker improvement is a hypothesis about a clinical outcome, not proof of it. For antiresorptive therapy, that hypothesis has been tested and confirmed. For exercise, the hypothesis remains plausible but unproven at the fracture endpoint.

Where the evidence is contested

Exercise and fracture prevention. The Cochrane review [6] reported a 39% relative reduction in fracture odds with exercise, which sounds clinically meaningful, but the confidence interval spans from a 77% reduction to a 64% increase in risk. Proponents argue the trials were too small and too short to detect a real benefit that the BMD data suggest should exist. Skeptics counter that BMD gains from exercise may reflect changes in bone geometry or density distribution that DEXA doesn't capture cleanly, or conversely, that BMD gains alone don't translate to reduced fracture risk through the same pathways as antiresorptive drugs. Neither side denies that exercise has value for fall prevention and muscle strength, which are independent fracture risk factors, but the direct BMD-to-fracture pathway remains unestablished for exercise specifically.

References

1. Marshall 1996, BMJ — BMD predicts fracture (RR ~1.5/SD; ~2.3–2.6 site-matched) · PubMed 8634613
2. Haentjens 2010, Ann Intern Med — hip fracture → large excess mortality · PubMed 20231569
3. Cummings 2009 FREEDOM, NEJM — denosumab: vertebral −68%, hip −40% · PubMed 19671655
4. Black 1996 FIT, Lancet — alendronate: vertebral −47–55%, hip −51% · PubMed 8950879
5. Watson et al. 2018 J Bone Miner Res — LIFTMOR trial: HiRIT in postmenopausal women with osteopenia/osteoporosis; lumbar spine BMD ↑ ~4% · PubMed 28975661
6. Howe 2011, Cochrane — exercise raises BMD but fracture benefit not significant · PubMed 21735380
7. LeBoff et al. 2022 (VITAL), NEJM — vitamin D & fractures · PubMed 35939577