Vitamin D pills' effect on healthy bones queried
Healthy adults do not need to take vitamin D supplements, suggests a study in The Lancet which found they had no beneficial effect on bone density, a sign of osteoporosis.
But experts say many other factors could be at play and people should not stop taking supplements.
University of Auckland researchers analysed 23 studies involving more than 4,000 healthy people.
The UK government recommends children and over-65s take a daily supplement.
The New Zealand research team conducted a meta-analysis of all randomised trials examining the effects of vitamin D supplementation on bone mineral density in healthy adults up to July 2012.
The supplements were taken for an average of two years by the study participants.
Bone mineral density is a measure of bone strength and measures the amount of bone mineral present at different sites in the body. It is often seen as an indicator for the risk of osteoporosis, which can lead to an increased risk of fracture.
The trials took place in a number of different countries including the UK, the US, Australia, Holland, Finland and Norway.
Although the results did not identify any benefits for people who took vitamin D, they did find a small but statistically significant increase in bone density at the neck of the femur near the hip joint.
According to the authors, this effect is unlikely to be clinically significant.
Free up resources
Prof Ian Reid, lead study author, from the University of Auckland, said the findings showed that healthy adults did not need to take vitamin D supplements.
"Our data suggest that the targeting of low-dose vitamin D supplements only to individuals who are likely to be deficient could free up substantial resources that could be better used elsewhere in healthcare."
Writing about the study in The Lancet, Clifford J Rosen from the Maine Medical Research Institute agrees that science's understanding of vitamin D supports the findings for healthy adults, but not for everyone.
"Supplementation to prevent osteoporosis in healthy adults is not warranted. However, maintenance of vitamin D stores in the elderly combined with sufficient dietary calcium intake remains an effective approach for prevention of hip fractures."
The Department of Health currently recommends that a daily supplement of vitamin D of 10 micrograms (0.01mg) should be taken by pregnant and breastfeeding women and people over 65, while babies aged six months to five years should take vitamin drops containing 7 to 8.5 micrograms (0.007-0.0085mg) per day.
Dr Laura Tripkovic, research fellow in the department of nutritional sciences at the University of Surrey, said the study was important but very specific.
"I'm not surprised they didn't find any evidence of the effects of vitamin D on bone density because there are so many other factors involved in osteoporosis, like genes, diet and environment.
"To pin it all on vitamin D... it's difficult to do that."
Dr Tripkovic said it was no good taking vitamin D supplements if people didn't also maintain a healthy, balanced diet containing calcium and take plenty of exercise.
She said most healthy people should be able to absorb enough vitamin D naturally, through sunshine and diet.
"But if people are worried about their vitamin D levels then a multi-vitamin tablet would do. If you have bone pain and muscle aches then you should go and see your GP and discuss it."
We get most of our vitamin D from sunlight on our skin, but it is also found in certain foods like oily fish, eggs and breakfast cereals.
However, taking too much vitamin D in the form of supplements can be harmful because calcium can build up and damage the kidneys.
Experts advise taking no more than 25 micrograms (0.025mg) a day.
The UK guidance is currently being reviewed.