THE safety of high-dose vitamin D to treat osteoporosis has been called into question after Melbourne researchers found the treatment was linked to an increased rate of falls and fractures.

The results of the six-year clinical trial, published in the Journal of the American Medical Association yesterday, showed that women who received a single, annual high dose of vitamin D had a 26-per-cent-higher rate of fractures and a 15-per-cent-higher rate of falls compared with those on a placebo.

The study's co-author, Geoff Nicholson, the head of the University of Melbourne's department of clinical and biomedical sciences based at Barwon Health, said researchers were expecting to find the opposite - a reduction in falls and fractures in the high-dose vitamin D group, in the order of about 25 per cent.
''It was unexpected. We had every reason to believe [high-dose vitamin D] would prevent falls and fractures,'' he said.

The study involved more than 2000 women aged 70 and over, who measured falls in a diary. Older women are prone to low vitamin D levels, particularly in winter due to limited exposure to the sun.
The vitamin is important for bone and muscle strength.

Professor Nicholson said falls and fractures mostly occurred in the first three months after the vitamin D was given, when levels were highest.

He said it was unclear why high levels of vitamin D would create a risk of falls and fractures, but the study had disproved that it was a safe treatment for osteoporosis.

''The [vitamin D] levels we got up to would not have, up until now, been considered harmful,'' he said.

Professor Nicholson said it was important that people receiving low-dose vitamin D - of up to 1000 international units per day - continued with their treatment unless otherwise instructed by their doctor.