Taking vitamin D or calcium does not help prevent repeat fractures in elderly people, a study suggests.
Osteoporosis increases a person's risk of bone fractures
Researchers at Aberdeen and York universities looked at people who had already had a fracture due to osteoporosis - thinning of the bones.
Many people take vitamin D and calcium to try to protect their bones, but the study found those taking supplements did not go on to have fewer fractures.
Osteoporosis campaigners said taking supplements would not cause any harm.
However, they advised elderly people concerned about their bone health to eat a healthy balanced diet instead.
Vitamin D can be obtained from foods including fatty fish like salmon, and milk products.
University of Aberdeen researchers randomly assigned the 5,300 people over 70 taking part in the study, recruited from 21 hospitals across the UK, either to take a dummy pill, a daily supplement of vitamin D, calcium or both supplements together.
They were then followed up for between 24 and 62 months.
A total of 698 people had experienced a fracture, the Lancet study showed.
However the incidence of fracture did not differ between the groups.
A second study, published in the British Medical Journal, looked at 3,314 women aged 70 and over and at high risk of hip fracture.
Half were given calcium and vitamin D tablets to take daily, the rest just received a leaflet on diet and prevention of falls.
All women were monitored for an average of two years.
Fracture rates in the two groups were not significantly different.
The studies only examined the effect of taking vitamin D and/or calcium on people who had already had a fracture, but were taking no other medication.
They did not look at people taking bisphosphonates, drugs which stop further loss of bone mass, or people living in care homes.
Previous studies have suggested both these groups do benefit from taking vitamin D and calcium.
Professor Adrian Grant, who led the research, said older, frailer people were more likely to have vitamin D deficiency and therefore to benefit from supplements.
He said: "Our trial indicates that routine supplementation with calcium and vitamin D, either alone or in combination, is not effective in the prevention of further fractures.
"And it has clarified that it's looking at the more appropriate use of bone active drugs [bisphosphonates] that should be the approach."
But Professor Grant said people should not be worried if they take the supplements.
"Vitamin D and calcium are not harmful. There are no adverse effects, although people who take calcium can find the tablets very hard to chew and swallow and may experience stomach upsets and indigestion."
However Professor Philip Sambrook of the University of Sydney, writing in the Lancet, warned the Aberdeen study found only two thirds of people were still taking their pills after two years, and patients in this study were younger than in other research which had shown a benefit from taking vitamin D.
Jackie Parrington, deputy chief executive of the National Osteoporosis Society said: "This study shows us that, if you are an older person, the benefits of calcium and vitamin D as a treatment option to prevent further fractures are uncertain.
"It helps to do is increase our knowledge about what are the most effective types of drug treatments to give to people at certain ages."
She added: "Older people who think they are at risk of breaking a bone because of osteoporosis should consider eating a healthy, well mixed diet to ensure they get the full range of vitamins and minerals.
"We would encourage everyone to get vital nutrients from food rather than buying supplements. They should also discuss their risks with their GP."