By Evangeline Mantzioris

Credit: Unsplash/CC0 Public Domain

From the first hint of cold winter mornings, supermarkets and

pharmacies begin heavy marketing to coax us to stock up on vitamin C to get through cold and flu season.

But where did that idea come from?

As we'll see, it's a story including a Nobel Prize winner, a book and some claims that don't quite stack up.

Nobel Prize winner writes a book

Linus Pauling was a chemist from the United States who won two Nobel Prizes. One was in 1954 for chemistry, for his research into chemical bonds. The other was in 1962 for peace, for his activism against nuclear weapons testing.

He was not a dietitian or an infectious disease physician. But that didn't stop him from publishing a book on vitamin C and colds in 1970. This book is widely credited with skyrocketing sales of vitamin C at the time, a trend that continues today.

In this book, Pauling said we should consume high doses of vitamin C, around 1,000–2,000 milligrams a day for good health, and even more to prevent a cold.

His recommendations were said to be based initially on one placebo-controlled trial in children at a ski camp in the Swiss Alps. He then analyzed a further four studies conducted after his claims.

But his interpretation of these studies has been widely criticized. There were incorrect mathematical analyses, reliance on four poor-quality trials and an overemphasis on the study in children.

So, do megadoses of vitamin C work?

The short answer is not really. Multiple studies conducted since those Pauling analyzed show that if vitamin C has an effect on colds, it's minimal. Even then, any marginal benefits occur only under certain circumstances, such as if you're an athlete or in the military.

One study, published in 2023, was a meta-analysis that combined the results of 10 placebo-controlled trials. These looked at the effect of vitamin C on the severity and duration of colds. In total, these trials involved 2,736 healthy people (adults, children and athletes) who regularly took at least 1,000mg of vitamin C a day, not just when they had a cold.

Overall, the researchers found people who took vitamin C had a 15% reduction in severe cold symptoms compared with those who didn't take it. This was expressed as the number of days "confined indoors," which the authors acknowledged as a limitation. Vitamin C did not affect how long mild symptoms lasted.

A meta-analysis in 2013 found that taking lower doses (around 200mg a day) did not decrease the number of colds in the general population. However, there were fewer colds among athletes and military personnel.

Similar to the other study, regular vitamin C supplements showed a reduction in symptom severity in children and adults of 8–14%. However, taking vitamin C at the start of a cold had no benefit.

So, from these two meta-analyses, Pauling's original advice is overstated. There is no evidence vitamin C will reduce your risk of catching a cold.

There is some evidence it will reduce how long severe symptoms last, but on average only by about 10%. So if severe symptoms from a cold last for five days, it will reduce their duration by about 12 hours. If the severe symptoms last for 24 hours, it will reduce their duration by about two-and-a-half hours.

Taking vitamin C when symptoms start will have no effect. You need to be taking it regularly, even before you get sick.

So how much vitamin C is too much?

Pauling's recommendation for good health, at around 1,000–2,000mg a day, is around the upper limit of what's now considered safe to consume.

Australia has not set an upper limit for vitamin C due to inconclusive evidence. But guidelines refer to other expert recommendations that adults should not consume more than 1,000mg a day. In the United States, the upper limit for adults is 2,000mg a day.

Pauling's recommendation is also much higher than the recommended daily intake in Australia and other countries. For adults 19 and older in Australia, this is 45mg.

This is what you would find in half an orange, three to four florets of cooked broccoli or one-third of a glass of orange juice.

So what should I do?

Taking high doses of vitamin C (over 1,000mg a day) continually can increase your risk of adverse events such as:

gastrointestinal effects, such as diarrhea, nausea, stomach cramps and bloating more oxalate excretion, which may lead to kidney stones an increase in iron absorption, which may lead to tissue damage in people with undiagnosed hemochromatosis (an inherited disorder in which you absorb too much iron from your food) interactions with radiation therapy, chemotherapy and the cholesterol-lowering drugs statins.

If you feel you would benefit from vitamin C supplements, discuss this with your primary care physician. You should limit the dose to below 1,000mg a day and be mindful of the side effects.

Remember, less than 4% of Australians are deficient in vitamin C.