Does vitamin C keep you healthy?

Photo of citrus fruits (PantherMedia / Carmen Rother)

Taking vitamin C every day to try to prevent colds won't protect most people from colds. It only slightly shortens the amount of time that they're ill. Starting to take vitamin C once you already have cold symptoms won't have any effect on your cold.

You need a certain amount of vitamin C to stay healthy and well, and most people get enough in their daily diet. Vitamin C (ascorbic acid) can be found in fruits and vegetables, and citrus fruits and berries have especially high levels of vitamin C. Medical conditions that are caused by vitamin C deficiency, such as scurvy, are practically non-existent in countries like Germany.

Despite this, many people take vitamin C supplements every day in order to prevent a number of different illnesses, particularly common colds. Some of these products have more than one gram of vitamin C, which is more than ten times the recommended daily amount. Because the body can't store vitamin C, the excess vitamins are usually flushed out of the body in urine within a few hours, so they end up in the toilet. The German Institute for Risk Assessment (Bundesinstitut für Risikobewertung, BfR) considers 100 milligrams of vitamin C per day in your diet to be enough.

Researchers from the Cochrane Collaboration – an international network of researchers – looked into the question of whether taking large doses of vitamin C can protect against colds or relieve the symptoms. To find out, they analyzed studies comparing vitamin C with a product that didn't contain any active ingredients (a placebo).

29 studies, involving more than 11,000 children and adults, tested whether the regular use of supplements containing at least 200 mg of Vitamin C prevented colds. Most of the studies tested a dose of 1,000 or more milligrams of vitamin C per day. Some of the participants took the vitamin C over a period of several years.

Research results

The studies showed that it wasn't possible to prevent colds by taking vitamin C every day over a longer period of time. But doing so did shorten the amount of time that people were ill by about 10 percent. In other words, a cold that would have lasted ten days was over in nine. The cold symptoms were also a bit milder in people who always took vitamin C. It didn’t shorten the length of colds in men and women who started taking it only after they became ill.

Some of the studies looked at whether vitamin C can prevent colds in people exposed to short periods of very strenuous physical activity, often in connection with extremely cold temperatures. Examples include marathon runners or soldiers doing winter exercises in a mountainous region. In these studies, the participants started taking vitamin C two to three weeks before the very strenuous activities as a means of preventing colds. It was found that doing so reduced their risk of developing a cold by about half.

Taking very high doses of vitamin C regularly may cause diarrhea, but study participants who took vitamin C didn’t report having more side effects than those who used a placebo.