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Dr. Jonathon Maguire of St. Michael's Hospital is shown in this undated handout image.

HO/THE CANADIAN PRESS

Giving children high daily doses of vitamin D instead of the standard recommended amount doesn't appear to reduce the number of times they come down with wintertime sniffles, a study suggests.

For the last 30 years, vitamin D has been thought to play a role in preventing or reducing the number of colds and bouts of flu children experience over the fall and winter.

But Toronto researchers found children who received the standard daily dose of 400 international units (IUs) and those given 2,000 IUs per day both had an average of almost two upper respiratory infections during the cold-weather months.

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"What we found was there was no difference in the number of viral infections that children got, regardless of which group they were in – the high-dose group or the regular-dose group," said Dr. Jonathon Maguire, a pediatrician at St. Michael's Hospital.

"I think that we just busted a myth here, that giving more vitamin D doesn't seem to protect against viral infections."

To conduct the study, published Tuesday in the Journal of the American Medical Association, researchers enrolled about 700 children aged one to five, whose parents were to administer one of two daily oral doses of vitamin D. Half of the kids were randomly selected to receive 400 IUs, while the other half were to get 2,000 IUs.

Maguire said data from laboratory experiments had suggested that vitamin D might help produce certain proteins that can protect the respiratory system from viral infections.

"So we hypothesized that if we gave children a high dose of vitamin D, we might promote the production of these proteins and protect them from getting wintertime colds," he said.

"But what this paper is showing is that giving more has no benefit in terms of viral infections."

Among the group of children who received 400 IUs of vitamin D drops daily, each developed 1.91 colds on average, compared to 1.97 each in the group of kids given 2,000 IUs – a difference that Maguire called not statistically significant.

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"We asked parents every time the child got a cold to take a (swab) of the nasal secretions and send them back to us," he said. "We ran that sample on a fancy machine that told us exactly what virus they had. So we knew whether they had an adenovirus or a coronavirus or enterovirus or influenza.

"There really was no difference between the groups, they were almost identical," Maguire explained, adding that the number of flu cases in the high-dose group was slightly lower, but not enough to be clinically meaningful.

The researchers confirmed the children had received their assigned dosages through tests that measured the level of vitamin D in their blood, both at the start and end of the study. As well, parents were asked to return their used vials of vitamin D drops, and close to 100 per cent had been depleted, Maguire said.

Dr. Michael Allan, a family physician and director of evidence-based medicine at the University of Alberta, said the study "answered an important question because there is a very strong belief system right now that more vitamin D equals better vitamin D."

"It's kind of this pervasive idea around vitamins ... we know a little will make us healthy and keep us healthy, so if we take more we'll be very healthy," he said from Edmonton.

"And it's a kind of fallacy."

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Allan, who was not involved in the research, said the authors acknowledged in their paper that they didn't compare a group of children taking 400 IUs of vitamin D versus a group getting a placebo, or dummy, preparation to see if receiving the standard dose meant kids would get fewer wintertime colds and cases of flu.

"But the question now occurs to me – and I think to many people – is that for kids in Canada, will giving the standard dose add anything over placebo?" he said.

"Now we've got this question answered in children that we don't need these high doses, the question is what dose do we need – if any?"

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