Bone health: calcium, vitamin D, and what actually prevents a fracture

Osteoporotic fracture is the outcome that matters, and it depends on two things: bone strength and whether the person falls. Both halves are treatable, and the supplements sold for bones address only the first half weakly.

Calcium and vitamin D. Calcium and vitamin D supplementation has a modest effect on bone mineral density in people who are deficient, and the evidence for reducing fracture is inconsistent — some trials and pooled analyses show a small reduction, others show none. The clearest signal is in institutionalised or deficient populations, which is where supplementation is best justified. More is not better: calcium intakes well above the recommended amount have been associated with harm, including kidney stones and possibly cardiovascular events, and the recommended intake is a target rather than a floor to exceed.

Vitamin K2 has small trials showing bone effects and it is mechanistically plausible (it is a cofactor for osteocalcin carboxylation). The fracture-outcome evidence is not established. It also interacts with warfarin — K2 is vitamin K, and it directly opposes warfarin's mechanism.

Protein. Adequate protein intake matters for bone and muscle, and older adults commonly under-eat it. This is a food-level intervention with better evidence than most supplements.

Load and strength. Weight-bearing and resistance exercise increase bone density at the loaded sites, and this has randomised trial evidence. It is the intervention with the best bone evidence and it is not a supplement.

Falls prevention — the other half. Balance and strength training, a medication review (sedatives and antihypertensives that cause dizziness are a common cause of falls), vision check, home hazard review, and adequate vitamin D. A hip fracture is usually the result of a fall. Preventing the fall is at least as important as the density.

Osteoporosis drugs. For someone with a fragility fracture or confirmed osteoporosis, bisphosphonates and other drugs reduce fracture risk substantially. A supplement is not a substitute, and the decision belongs to a clinician with a DEXA result.

Vitamin D dosing. The recommended intake for adults is around 600-800 IU daily, rising to 800 IU for older adults, with higher doses for documented deficiency under medical supervision. Very high intermittent dosing has been associated with increased falls and fractures in trials — another case of the U-curve.