Loading up on vitamin D is unlikely to prevent the common cold this winter, a new study from New Zealand suggests.
Despite past research suggesting the sunshine vitamin may help clear bacteria and otherwise boost immune health, Kiwis taking monthly megadoses of vitamin D got just as many colds as those who were given vitamin-free placebo pills.
"In the population we studied, we can be very confident that it has no effect on prevention or severity (of colds)," said lead author Dr. David Murdoch, from the University of Otago in Christchurch.
That population included adults who were healthy to begin with, so Murdoch and his colleagues can't say whether vitamin D may help ward off colds in kids or people who are very deficient in the vitamin.
For the new study, they randomly assigned 322 adults either to receive a monthly dose of vitamin D - starting at 200,000 international units for the first couple of months, then dropping to 100,000 IU - or to get a placebo.
Over the next year and a half, people taking vitamin D had an average of 3.7 colds and other respiratory infections, versus 3.8 in the comparison group - a difference so small it could have been due to chance. And there was no difference in how long colds lasted or in their severity, the researchers reported Tuesday in the Journal of the American Medical Association.
"There's been quite a lot of hype around vitamin D for infection and all sorts of things," Murdoch told Reuters Health. But the only convincing evidence so far has linked extra vitamin D to better bone health, he said.
"A lot of people are making a lot of money out of promoting these substances, but… there's not a lot to back it up," Murdoch added.
Vitamin D supplements can be bought over the counter for anywhere from $2 to $3 or $20 to $30 for a month's supply. Foods rich in the vitamin include salmon, swordfish and eggs, as well as fortified milk, yogurt and orange juice.
Vitamin D is also manufactured in the skin in response to sunlight, but away from the equator, winter sun is too weak to supply enough of the vitamin to meet all the body's needs.
Participants in the new study started with an average vitamin D level of 29 nanograms per milliliter and in those who got the D supplements, levels rose to 48 ng/mL or more for the duration of the study.
The US National Institutes of Health considers a level of 20 ng/mL and above adequate for overall health, with vitamin D deficiency occurring at less than 12 ng/mL.
"If you have a good diet and definitely if you're taking vitamin D, taking more is not going to help," said Dr. Jeffrey Linder from Brigham and Women's Hospital in Boston, who wrote a commentary accompanying the new study.
He said some research has suggested vitamin D helps support the immune system of kids in the developing world who are deficient. But the New Zealand participants, he told Reuters Health, are likely to be similar to Americans who are looking for a way to ward off the common cold.
Dr. Adit Ginde from the University of Colorado School of Medicine in Aurora pointed out that over 90 per cent of the participants were European, and minorities tend to have lower levels of vitamin D.
"For those that just want to try something, (a vitamin D supplement) is a relatively safe intervention," said Ginde, who has studied vitamin D and respiratory infections but wasn't involved in the new research.
"But I don't think at this point we can say it's proven to work," he told Reuters Health.
"Colds, they're so common and they affect so many different systems in our body, that I'm pretty sceptical that we're going to come up with the cure for the common cold," Linder said.
"Good standard advice about staying healthy, eating a well-balanced diet - that's the cure for the common cold, or as good as you're going to get."