Should You Take Vitamin C When You Get Sick?

Good morning!

Vitamin C is probably the supplement most people feel confident about. It's been around forever, it's cheap, and aside from the other health benefits, there's a general understanding that it helps with colds. What most people don't know is that this reputation traces back almost entirely to one man, a Nobel Prize winner whose ideas have shaped supplement purchasing habits for over 50 years.

This week, we're unpacking what vitamin C actually does, what the research says it doesn't do, and where it turns out to be more interesting than anyone expected.

Key Takeaways

• Vitamin C doesn't prevent colds for most people, but taking it regularly may slightly reduce symptom severity; starting it after symptoms appear doesn't help.

• Most people can meet their vitamin C needs through food, while regularly taking high doses can cause side effects including diarrhea, nausea, cramping, and potentially kidney stones.

• Vitamin C's proven roles go well beyond immunity: it helps build collagen for healthy skin, blood vessels, bones, and connective tissue, improves absorption of plant-based iron, and supports normal immune function.

CORE

Popularized by a Nobel laureate

Linus Pauling won two Nobel Prizes, one in 1954 for chemistry, and one in 1962 for peace, for his activism against nuclear weapons testing. He was, by any measure, one of the most accomplished scientists of the 20th century. 

In 1970, he published a book claiming that high doses of vitamin C, around 1,000 to 2,000mg per day, could prevent colds and dramatically improve general health. Keep in mind, he was also not a dietitian or an infectious disease physician. The book sold widely, and vitamin C supplement sales skyrocketed (a trend that continued largely uninterrupted for the next five decades).

His original claims were based on one placebo-controlled trial in children at a Swiss ski camp, plus four additional studies he analyzed afterwards. His interpretation of those studies has since been widely criticized due to incorrect mathematical analyses, reliance on poor-quality trials, and over-emphasis on the ski camp study in children.

This matters not because Pauling was a bad scientist, he wasn't, but because it illustrates how credibility in one domain can shape public health behavior in another, and how a confident recommendation built on shaky early data can become an article of faith long after the evidence has moved on.

Vitamin C Against Colds

The findings are fairly consistent across the large meta-analyses conducted since Pauling's work, and they tell a more nuanced story than "vitamin C works" or "vitamin C doesn't work."

A 2023 meta-analysis (an analysis that combines the results of many studies on the same topic to get a clearer, more reliable overall answer) combining ten placebo-controlled trials involving 2,736 people, adults, children, and athletes, who regularly took at least 1,000mg of vitamin C per day found a 15% reduction in severe cold symptoms compared to those who didn't take it. This was measured as fewer days confined indoors. Vitamin C did not affect how long mild symptoms lasted. 

A 2013 meta-analysis found that taking lower doses around 200mg per day did not decrease the number of colds in the general population, though athletes and military personnel showed some benefit. Both meta-analyses found a reduction in severity of symptoms in children and adults of roughly 8-14% for regular supplementation. 

What that 10% reduction in severity means in practical terms is, if severe cold symptoms typically last five days, regular vitamin C supplementation reduces that by about 12 hours. If severe symptoms last 24 hours, the reduction is about two and a half hours. That's the actual magnitude of the benefit, real, but modest, and only available to people who were already taking it before they got sick.

So the consistent finding across all the research is that taking vitamin C at the onset of symptoms has no measurable benefit. The supplementation/increased consumption needs to already be in place.

Can You Have Too Much Vitamin C?

You’re probably not following Pauling’s recommendation of 1,000-2,000 mg per day. The U.S. upper limit for adults is 2,000 mg daily, with the recommended daily intake for adults sitting at 90mg.

That’s roughly an orange’s worth. While some estimates suggest around 7% of Americans are deficient in this vitamin, meeting your daily needs isn’t exactly difficult.

The issue is what happens when you regularly take high doses. Sustained intake above 1,000 mg per day can cause gastrointestinal problems like diarrhea, nausea, and cramping. It can also increase oxalate excretion, potentially contributing to kidney stones. This to say, it should come as no surprise that too much of anything can cause problems.

What Vitamin C Does

Vitamin C is genuinely essential, and the real case for making sure you get enough of it has almost nothing to do with colds.

Vitamin C is required for collagen synthesis, which is the process by which the body builds and repairs connective tissue in skin, tendons, blood vessels, and bone. This is why scurvy, the disease of severe vitamin C deficiency, produces symptoms like wounds that won't heal, bleeding gums, and skin that bruises at the slightest touch. The collagen framework literally starts to break down, and the body can't maintain structural tissue without it. 

It also significantly enhances the absorption of non-heme iron (the type of iron found in plant foods) which is inherently less bioavailable than the iron in meat. For anyone eating a primarily plant-based diet, this interaction is far more practically meaningful than anything the cold research has to offer.

On the immune side, vitamin C does play a genuine role, just not the one most people assume. It supports the function and production of white blood cells, particularly neutrophils and lymphocytes, and helps maintain the skin barrier as a first line of immune defense.

ENDNOTE

Final Thoughts

The bottom line? For most people eating a varied diet, vitamin C deficiency isn’t a meaningful concern. Fruits, vegetables, and other whole foods provide plenty, and the daily target is modest enough that an orange can cover it.

And if you’re taking vitamin C specifically to avoid a cold, the evidence is much less exciting than the reputation. It won’t stop you from getting sick, may only slightly reduce the severity or duration of a cold when taken consistently, and is unlikely to do much if you wait until symptoms appear to start taking it.

That doesn’t make vitamin C useless. Far from it. It plays important roles throughout the body. But perhaps the biggest takeaway is that you don’t need megadoses to get those benefits.

Until next week!

Adrian Macdonald | Team Dietitian | The 8% Newsletter Author