Vitamin D supplements will not protect Australians from catching colds, flus and other respiratory infections, new research has found.
The findings come after a five-year clinical trial, led by Brisbane's QIMR Berghofer Medical Research Institute, involving 16,000 Australians aged between 60 and 84.
The trial was the largest of its kind to study the relationship between vitamin supplements and respiratory infections.
Each participant was asked to take a capsule once a month for five years half were given a vitamin D supplement, while the other half were given a placebo or sugar tablet.
At the end of every year, participants completed a health survey detailing any major illnesses or medical events.
Some 2,600 participants were also asked to keep a daily diary during winter to record general observations of their health, particularly if they had cold or flu symptoms.
Lead researcher, Professor Rachel Neale said while the findings showed taking vitamin D did not stop people from getting respiratory tract infections, there was some evidence that it reduced the length and severity of infection.
"The key message is that if you are not vitamin D deficient, taking vitamin D is unlikely to stop you from getting a head cold or the flu," Professor Neale said.
"It may reduce the length of it a little bit, but not enough to really warrant taking vitamin D if you're not already vitamin D deficient."
Professor Neale said the findings had not surprised her.
"There's been quite a lot of controversy over vitamin D and how much of it we need in our bloodstream," she said.
"The findings of this, which suggest that more is not better, would indicate that we shouldn't just go out and routinely supplement the whole Australian population.
"It does tell us that being aware of avoiding vitamin D deficiency is important.
The data will be used to inform public policy and clinical care.
Participant Erica French was eager to join the clinical trial after being given conflicting advice about vitamin D consumption.
"I was hearing a lot of different information," she said.
"First of all I've got my own doctor telling me, 'no more sun Erica & hat, sunscreen, cover up, keep an umbrella over yourself'.
"Then on the other hand ... the other information you read is that you've got to have some sun exposure every day to get your vitamin D up, so I'm thinking, 'I don't get this, which is the way I'm supposed to go?'"
She said she remembered being "sick-free" for five years during the trial.
"I can remember saying to my doctor that I thought I was really healthy," Ms French said.
"I think it's good to know that we don't have to be putting vitamin D in our cereal or our milk or whatever else that we are as a nation mostly getting enough, and that by taking extra we're maybe doing other things, but not necessarily helping with respiratory tract infections.
"I am not sure that it's fully answered all of my questions & but it has helped."
The Institute's five-year trial concluded just before the coronavirus pandemic took hold across the world, killing nearly 2 million people to date.
Despite this, Professor Neale said the results were still relevant in a time when immune systems have never needed to be stronger.
"We didn't measure coronavirus & but we can kind of extrapolate," she said.
"Our finding of a reduced length and severity of infection, albeit a small reduction, indicates that perhaps vitamin D does influence the immune system in people.
"And that means that in people who are vitamin D deficient, if we were to treat their deficiency, that might have some benefit for their immune system and arguably some benefit for the coronavirus."