Which supplements are proven to help strengthen bones?

The combination of calcium and vitamin D is the best supported by evidence. A meta-analysis of 8 randomised trials with nearly 31,000 participants found the risk of total fractures fell by 15%, and the risk of hip fractures by as much as 30%. A separate systematic review in postmenopausal women confirmed that bone density increased at multiple sites. The two substances appear to enhance each other: the effects of calcium alone or vitamin D alone are considerably more modest.

Vitamin D as a standalone supplement, without calcium, has virtually no measurable effect on bone density in people without a vitamin D deficiency. A systematic review of 70 statistical comparisons found only 6 significantly positive results, and even then the benefit was limited to a small advantage at the femoral neck. If you have no demonstrated deficiency, vitamin D alone is not a good reason to supplement for your bones.

Vitamin K2 shows improvements in lumbar spine bone density in several randomised trials in postmenopausal women, and fracture risk fell significantly when one outlier study was excluded. The evidence is positive but not yet fully consistent: which studies you include makes a difference to the outcome. On top of an existing osteoporosis treatment with bisphosphonates, vitamin K added nothing to bone density, as shown by a smaller study of 105 women.

Calcium as a standalone supplement is not recommended for fracture prevention in the general postmenopausal population. In people with a demonstrably low intake or poor absorption it may still be useful. Excessive calcium supplementation carries risks, including possible cardiovascular problems. The same applies to other bone supplements: more is not automatically better. For magnesium and phosphorus, the evidence for supplementation in bone health is limited and context-dependent.