Ah, autumn: Time for leaf-peeping, pumpkin spice lattes, turtlenecks... and the sniffles.
Fall and winter are peak seasons for colds. And almost everybody from Aunt Gladys to the cashier at the grocery store wants to tell you about their magic bullet for fighting wintertime bugs. One of the most popular word-of-mouth cold remedies is to dose up on vitamin C.
But does boosting your vitamin C intake do anything to prevent or shorten colds?
Some studies suggest taking vitamin C has a modest effect on the common cold, but don't expect miracles, one expert says.
"It's fair to say that vitamin C supplementation both shortens duration of cold and offers some protection against colds, though it's not very dramatic," said Stephen Lawson, a researcher at the Linus Pauling Institute at Oregon State University, who studies micronutrients. [Myth or Truth: 7 Ancient Health Ideas Explained]
Vitamin C gained its reputation as an almost miraculous substance after two-time Nobel Prize winner Linus Pauling first touted the benefits of the micronutrient in a series of books, with titles such as "Vitamin C and the Common Cold," (W.H. Freeman, 1976) and "How to Live Longer and Feel Better" (W.H. Freeman, 1976).
Pauling suggested that taking large doses of vitamin C could do everything from fight cancer to prevent heart disease. But many of Pauling's more dramatic claims were not borne out in research done since then, and by the time he died in 1994, some in the medical establishment had labeled him a quack.
Today, the Institute of Medicine recommends that adult males get 90 milligrams of vitamin C daily in their diet, women get 75 mg daily and children consume between 15 and 75 mg daily, depending on their age and sex. Lawson's institution, which was founded by Pauling, recommends taking 400 milligrams of vitamin C a day to promote optimal health.
But whether vitamin C is a reasonable way to treat the common cold remains a more open question. The American Association of Family Physicians says the vitamin is unlikely to shorten the duration or severity of colds, while the American Academy of Nurse Practitioners suggests it could be effective at reducing the duration of a cold, but not its severity.
A 2013 review of dozens of studies found that vitamin C reduces the duration of colds by 8 percent in adults, and 14 percent in children. Considering that the average cold lasts about a week, that reduction translates to about one less day of symptoms.
For people in the general population, vitamin C didn't ward off colds, the researchers also found. But among people under intense physical stress -- such as marathon runners, skiers and soldiers exercising in the Arctic -- those who took vitamin C supplements were about half as likely to get colds as those who did not, the researchers found. Most of the studies that the researchers reviewed were double-blinded, meaning that neither the participants nor the doctors who ran the study were told which participants were getting a sugar pill and which were taking the vitamin supplement.
The protocols of the studies in the review involved different amounts of vitamin C, with a minimum daily dose of 200 milligrams, and the vitamin was taken for different durations. Some studies also looked at continuous use of the drug at moderate levels, not the "mega-dosing" that has become popular among natural-remedy proponents.
But Lawson said that studies haven't looked at whether mega-dosing works for colds. "I don't think that there's sufficient evidence or understanding of the molecular mechanisms to really establish a dose-response relationship," Lawson told Live Science.
In any case, recent results from lab tests by one independent testing company, LabDoor, which were not peer-reviewed, found that for most vitamin C supplements, the true dosage of vitamin C in the pills matches closely with the dosage listed on the label.
Lawson noted that the source of the vitamin doesn't matter. "The body doesn't distinguish between vitamin C in a tablet, and in oranges or other fruit or vegetables," Lawson said.
As for the flu, there are no modern studies suggesting that the vitamin can help prevent the flu, though research from the 1940s and 1950s hints at a protective effect, Lawson said.
Though it's not clear how vitamin C might ward off the common cold, one possibility is that it activates white blood cells in some way, Lawson said.
Another possible mechanism, which researchers investigated in a 2008 study in the Journal of Electroanalytical Chemistry, is that vitamin C may both reduce and boost production of compounds called reactive oxygen species, which are harmful chemical byproducts of metabolism that can damage DNA and tissue. That finding suggests it's possible that the vitamin reduces inflammation, Lawson said.
A small study published in the Journal of the American College of Nutrition found that vitamin C may dampen the inflammatory response caused by the chemical histamine. This antihistamine effect could reduce the symptoms of a cold, such as runny noses, that occur when the body ramps up its inflammatory response, Lawson said.
Taking a daily vitamin C pill at normal amounts is unlikely to cause serious side effects, Lawson said.
It would be hard to "overdose" on oral vitamin C, Lawson said. The molecules that line the gut that move the vitamin from the digestive tract into the bloodstream tightly regulate how much vitamin C makes it into the blood stream. And, in addition, vitamin C is water-soluble, meaning the kidneys can flush any extra vitamin C out of the body.
However, people taking very high doses (more than 2,000 mg) may be prone to diarrhea or stomach cramps, and men who have had kidney stones in the past that are high in a chemical called oxalate should avoid supplementing with vitamin C, as the drug may enhance the formation of those types of stones, Lawson said.
In short, vitamin C could have some modest benefit for people seeking to avoid colds, but it's no miracle cure. There are other measures more likely to have a noticeable effect on colds, such as regular hand-washing and getting enough dietary zinc, according to a 2014 study in the Canadian Medical Association Journal.