High doses of vitamin D increase the number of falls and fractures in older women, say Australian researchers, who warn against using the vitamin in this way.
Professor Geoff Nicholson, of Barwon Health and the University of Melbourne in Geelong, and colleagues investigated the impact of a single annual high-dose of oral vitamin D on older women.
"We expected to find a reduction in falls and fractures but we found exactly the opposite," says Nicholson, whose findings appear today in the Journal of the American Medical Association.
Vitamin D is needed in order for the body to absorb calcium, necessary for healthy bones.
It also plays a role in keeping muscles strong, especially those around the limb girdles.
The Sun is a major source of the vitamin, says Nicholson, but indoor lifestyles and the advent of anti-skin cancer campaigns have contributed to low vitamin D levels.
"A good proportion of the population, particularly women who are older, have lower vitamin D levels than are considered healthy," says Nicholson.
Low vitamin D leads to osteoporosis and weak muscles, increasing the risk of falls and fractures.
Vitamin D supplements
Nicholson says evidence to support the use of vitamin D supplementation (typically 700 to 1000 international units of vitamin D per day), both with and without calcium, has been mixed, with some studies showing no effect and others showing it reduces fractures and falls.
He says one limitation of studies is that people often stop taking their daily low dose vitamin D.
"These things may work. Some studies show they do. Some show they don't. But compliance is a big problem," says Nicholson.
One way around this is to give a high dose of vitamin D less frequently.
In this latest study, Nicholson and colleagues tested the high-dose strategy in 2256 women aged 70 years or older, considered to be at high risk of fracture.
The women were given 500,000 international units of cholecalciferol (a form of vitamin D), or placebo, orally once a year for three to five years.
The findings surprised the researchers.
During the randomised controlled trial there were 5404 falls, with women in the high-dose vitamin D group having 15% more falls than the placebo group.
The vitamin D group had 171 fractures versus 135 in the placebo group, with 26% more fractures for participants in the vitamin D group. They also had a 31% higher incidence of falls in the first three months after taking the vitamin.
Nicholson says the actual increase in numbers of fractures and falls was small, but the results need to be considered in the context of high-dose treatment "becoming trendy".
"The absolute increase in risk is quite small," he says.
"[But] if you did it in every woman in Western societies over 70 you're talking about many many fractures."
Nicholson says scientists don't yet understand how vitamin D could be having this effect at high doses.
"It's very complicated and at this stage we're not in a position to explain it," he says.
Nicholson says the findings are supported by a more recent study from the UK and it will be important for further research to understand more about the impact of vitamin D on the body.
The funding for the study came from the National Health and Medical Research Council.
A spokesperson for the Royal Australian College of General Practitioners (RACGP), Dr Vicki Kotsirilos describes the study as "an excellent well performed study".
"The College position does not support the use of very high doses of vitamin D for the prevention of falls and fractures in elderly women," says Kotsirilos, founder of the Australasian Integrative Medicine Association (AIMA) and chair of the RACGP-AIMA joint working party.
She says there are some studies that support the value of vitamin D from sunshine and/or supplementation in lower doses.
Kotsirilos adds, lifestyle factors including diet, exercise and getting adequate sunlight play an important role in the prevention of osteoporosis, falls and fractures.