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Taking vitamin D and calcium supplements reduces the risk of hip fractures by about one sixth, but taking vitamin D alone does not, according to a new study from the Nuffield Department of Population Health (NDPH) at the University of Oxford.

The research was led by Research Fellow Dr Pang Yao and Robert Clarke, Professor of Epidemiology and Public Health Medicine, at NDPH. The researchers combined findings from 11 observational studies of vitamin D blood levels and the associated risk of fracture with more than 39,000 participants, 6 trials of combined vitamin D and calcium treatment with more than 49,000 participants, and 11 trials of vitamin D supplements alone with 34,000 participants. The research has been published in JAMA Network Open.

The observational studies showed that low blood levels of vitamin D are associated with higher risks of fracture, but the trial results have been inconclusive. The trials of vitamin D plus calcium showed a small reduction in the risk of fractures. In people over the age of 80 years, and those living in institutions however, the effects were greater, with the risk of hip fracture lowered by almost one-third. While the trials of vitamin D alone did not show a reduction in the risk of fracture, these trials included some large studies of very high annual doses of vitamin D which increased the risk of fracture, and few of the trials tested doses of vitamin D high enough to raise the blood levels of vitamin D sufficiently to reduce the fracture risk.

Professor Clarke said: 'Vitamin D supplements are widely taken and believed to have beneficial effects in older people. Vitamin D reduces the risk of hip fracture by one sixth when administered together with calcium supplements, but the available evidence suggests that when administered alone it has no protective effects on fracture risk.'

Co-author Jane Armitage, Professor of Clinical Trials and Epidemiology at NDPH said: ‘Previous trials of vitamin D alone have been constrained by their differing methods and doses. Very high annual doses of vitamin D may actually be harmful, while no large trials have tested high enough daily doses of vitamin D to show plausible effects on the risk of fracture.’

The researchers advocate the need for further large trials assessing the separate and combined effects of vitamin D and calcium in older people at high risk of fracture, before providing definitive recommendations on vitamin D and calcium supplements for bone health.