The claim comes from researchers at the University of Bern in Switzerland who conducted experiments on rats.

They said that 1g of onion a day can help prevent the process that causes the condition - resorption, where calcium seeps from the bones making them brittle.

And 500mg mixed with garlic, lettuce, tomato, cucumber, rocket and parsley had a similar result.

Rats fared well

On a diet of onions the rats developed stronger bones and suffered less resorption, and the onions' effect on bones could be seen within 12 hours, the scientists said.

Writing to the journal Nature, Dr Roman Muhlbauer and Dr Feng Li said: "Our results indicate that several common vegetables in the human diet alter bone metabolism in the rat.

The disease causes fractures

"If this also happens in humans, then including an appropriate amount of these vegetables in the daily diet could be an effective and inexpensive way to decrease the incidence of osteoporosis."

Osteoporosis can cause the spine to bend and makes bones more likely to break. In the UK, 52,000 women suffer hip fractures each year as a result of the disease.

Estimates put the cost of the disease to NHS somewhere between �600m and �1bn, although it is difficult to quantify as sufferers will often need special care as well as immediate treatment for fractures.

Specialists fear costs could rocket as the more people live longer and eat less calcium, which builds bones.

Continuing studies

The next stage for the researchers is to identify which compounds in the onions has the positive effect, and to see if it will work in humans.

"We hope we have opened up a new field and that this will be a possibility to prevent osteoporosis provided it works in humans and that humans can eat sufficient amounts of this stuff," Dr Muhlbauer said.

Earlier this month, doctors at London's St George's Hospital linked eating too much salt with the disease.