Vitamin D alone does not demonstrably reduce fracture risk. Only in combination with calcium do large studies show a modest but significant reduction, particularly in older adults and people with a deficiency. For the average healthy adult without a deficiency, the benefit of supplementation is unclear.
Vitamin D alone, without additional calcium, does not demonstrably reduce the risk of bone fractures. A large meta-analysis (PMID 31860103) showed that the risk of all fractures with vitamin D supplementation barely differed from placebo (RR 1.06), and there was no significant effect for hip fractures either (RR 1.14). Vitamin D alone also has virtually no effect on bone density: only one specific measurement at the top of the thigh bone showed a small benefit, but not for the hip as a whole (PMID 24119980). The researchers concluded that widespread use of vitamin D to prevent bone loss in people without a demonstrable deficiency does not appear to be appropriate.
The combination of vitamin D and calcium is a different story. Two large meta-analyses do show a protective effect here. One (PMID 31860103, nearly 50,000 participants across 6 randomised trials) found a 6% lower risk of all fractures and 16% fewer hip fractures. The other (PMID 26510847, nearly 31,000 participants across 8 studies) found as much as 15% fewer fractures overall and 30% fewer hip fractures. These figures relate to doses of roughly 400-800 IU of vitamin D per day combined with 1000-1200 mg of calcium per day.
In population research (observational, meaning not an experiment) we see that people with higher vitamin D levels in their blood sustain fewer bone fractures: for every 10 ng/mL higher level there is a 7% lower risk of all fractures and a 20% lower risk of a hip fracture (PMID 31860103). However, this does not prove a causal relationship: people with higher levels may also lead healthier lives in other respects.
Vitamin D supplementation is most useful for people with a demonstrable vitamin D deficiency or an elevated fracture risk, such as the elderly and residents of nursing homes. In these groups, the combination of calcium and vitamin D appears to be the most effective (PMID 38892706, PMID 32972636). In people without a deficiency and without an elevated risk, the benefit of supplementation is therefore far less clear.
Guidelines for osteoporosis treatment position vitamin D as a supportive measure within a broader package: alongside specific medications, adequate calcium, fall prevention, and physical exercise (PMID 35478046, PMID 28761958). Vitamin D alone is explicitly not a substitute for proven drug treatment when osteoporosis is present.
Two large meta-analyses of RCTs form the core (PMID 31860103 and 26510847, together ~80,000 participants). Supplemented by a meta-analysis on bone density (PMID 24119980) and guideline publications (PMID 35478046, 28761958, 38892706, 32972636). The strength of evidence for the distinction between vitamin D alone versus vitamin D + calcium is strong; for subgroup effects in the elderly and institutionalised patients it is moderate.