People who take a vitamin D supplement are less likely to have respiratory infections such as influenza or the common cold, a large study has found.
The international study, including New Zealand researchers, analysed individual patient data from more than 11,000 people in 25 trials.
It found that the rate of respiratory infections was 2 percentage points lower among those taking vitamin D than among those not taking the supplements - 40 per cent, compared with 42 per cent.
The effect was greater in those identified by blood tests as being quite deficient of the vitamin.
"Our study reports a major new indication for vitamin D supplementation: the prevention of acute respiratory tract infection," the researchers, including Professor David Murdoch, of Otago University at Christchurch, and Associate Professor Cameron Grant, of Auckland University, write in the British Medical Journal (BMJ).
"Overall there was a benefit, and even if you've got enough [in your blood] there might be a small benefit," Murdoch told the Herald.
He said that because of the diversity of the trials, it wasn't possible to make recommendations about the right dose, but the study had found those taking smaller and more frequent doses, such as daily or weekly, rather than larger, monthly doses, had a reduced infection rate.
Grant has previously reported an Auckland University trial that found that giving a vitamin D supplement during pregnancy and to the babies reduced the children's rate of dust-mite allergy and possibly their rate of asthma.
He recommended all pregnant women and young infants take a vitamin D supplement during winter and spring, when it is hardest for the body to make enough vitamin D from sunlight hitting the skin.
Vitamin D is also in some foods, including canned salmon and tuna, eggs, lean meat and dairy products. Some countries fortify foods with vitamin D.
More than half of New Zealand newborns have low levels of vitamin D. Deficiency of the vitamin can cause the weak-bone disease rickets.
But the new study hasn't resolved the question of whether everyone should take a vitamin D supplement.
Associate Professor Mark Bolland, of Auckland University, and a colleague say in a BMJ editorial, "It seems unlikely that the general population would consider a 2 per cent [percentage point] absolute risk reduction sufficient justification to take supplements."
They note that the new findings come just three months after they concluded that vitamin D supplements "should not be taken by adults to prevent non-musculoskeletal disease".
"Should these [new] results change clinical practice? Probably not."
They were too diverse and not sufficiently applicable to the whole population.
"We think that they should be viewed as hypothesis-generating only, requiring confirmation in well designed, adequately powered randomised, controlled trials."