The nutrient appears to have modest prevention power.
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Vitamin C is often touted as a natural cold remedy. The nutrient is featured in supplements promising to boost the immune system. Nobel laureate Dr. Linus Pauling famously claimed that taking large doses of vitamin C helps thwart a cold. Is there something to these claims? "The data show that vitamin C is only marginally beneficial when it comes to the common cold," says Dr. Bruce Bistrian, chief of clinical nutrition at Harvard-affiliated Beth Israel Deaconess Medical Center.
About vitamin C
Our bodies don't make vitamin C, but we need it for immune function, bone structure, iron absorption, and healthy skin. We get vitamin C from our diet, usually in citrus fruits, strawberries, green vegetables, and tomatoes. The Recommended Dietary Allowance for men is 90 milligrams (mg) per day, and for women, it's 75 mg per day.
The impact on colds
The most convincing evidence to date comes from a 2013 review of 29 randomized trials with more than 11,000 participants. Researchers found that among extremely active people—such as marathon runners, skiers, and Army troops doing heavy exercise in subarctic conditions—taking at least 200 mg of vitamin C every day appeared to cut the risk of getting a cold in half. But for the general population, taking daily vitamin C did not reduce the risk of getting a cold.
More encouraging: taking at least 200 mg of vitamin C per day did appear to reduce the duration of cold symptoms by an average of 8% in adults and 14% in children, which translated to about one less day of illness. "That could be important for some people, since the common cold causes 23 million lost days of work each year," says Dr. Bistrian.
What you should do
If you want the benefits of vitamin C, you'll need to consume it every day, and not just at the start of cold symptoms.
Should you take a supplement? "It's better to get vitamin C from food, because you also get other important nutrients. Eat the recommended five servings of fruits and vegetables per day for general health, and you'll get enough vitamin C," advises Dr. Bistrian.
What about claims that massive doses of vitamin C can help prevent a cold? Some studies have suggested there may be a benefit, but they required doses of 8,000 mg per day.
At doses above 400 mg, vitamin C is excreted in the urine. A daily dose of 2,000 mg or more can cause nausea, diarrhea, abdominal pain, and it may interfere with tests for blood sugar.