The amounts of retinol in fortified foods and vitamin supplements may need to be reassessed

Diane Feskanich, epidemiologist

Beta carotene is converted by the body to vitamin A as required.

Women specifically taking a vitamin A supplement had a 40% greater risk of hip fracture than women not taking the supplement.

Researchers analysed dietary questionnaires from more than 70,000 postmenopausal women - all nurses from 34 to 77 years old - over an 18-year period.

They found 603 women had hip fractures after only mild or moderate trauma.

Re-assessment

One theory is that too much vitamin A inhibits the ability of vitamin D to help the body absorb calcium, said the report's lead author epidemiologist Dr Diane Feskanich.

She said previous studies suggest vitamin A affects cells that work in bone remodelling - the breakdown and building of bone.

She said: "There is a biologic reason to support what we're observing, but we don't know what those reasons are.

"The amounts of retinol in fortified foods and vitamin supplements may need to be reassessed since these add significantly to total retinol consumption.

"If you're taking a multivitamin and consuming fortified milk and cereal... after a while, there are just too many sources."

An adult needs about 600-700 micrograms of vitamin A daily to maintain healthy skin, hair and immune system.

Multivitamin tablets typically contain about 1,500 micrograms.

'Speculative' research

A National Osteoporosis Society spokesman said: "It is important to note that the study is an observational one and thus can only be used to speculate concerning dietary/bone health associations and encourage further research in the area.

"Interestingly, there is a high incidence of osteoporosis in both men and women from Scandinavian countries, where sunlight exposure is limited and intake of vitamin A is particularly high.

"Previous work has shown a link between high dietary intake of vitamin A and the development of osteoporosis.

"Furthermore, vitamin A is known to antagonise the action of vitamin D."

Researchers cautioned that people should not stop taking multivitamins, which help lower the risk of other diseases.