In large population studies, people who eat lots of fruits and vegetables high in vitamin C appear to have a reduced risk for various types of cancer, including cancers of the mouth and lung. But it's not clear that these benefits come specifically from vitamin C. The Age-Related Eye Disease Study found that a daily supplement containing 500 mg of vitamin C can help slow the progress of macular degeneration, an age-related eye disease that causes vision loss. But we don't know what benefits, if any, are provided by vitamin C in particular.
Many people take vitamin C supplements in unnecessarily high doses to prevent or treat various conditions for which its effectiveness is unproved. The vitamin C supplements have variously been said for staving off sunburn and improving the appearance of wrinkles. None of these benefits have been confirmed in scientific studies.
Perhaps the best-known and most widely promoted use of high-dose(高剂量)vitamin C is to prevent or treat the common cold. Most of the evidence shows that high-dose vitamin C will, at most, shorten a cold by one day. In ordinary circumstances, vitamin C has no preventive value, although a few studies have reported a 50% reduced risk of developing colds among people taking vitamin C in extreme circumstances-for example, skiers, marathon runners, and soldiers working in extreme conditions.
The recommended vitamin C dosage per day for healthy women is 75 mg per day. For adults, the highest safe daily intake is likely to be 2, 000 mg per day. Vitamin C is water-soluble, so any amount more than you need will not be stored in the body. It's safe in almost any amount from foods, and supplements in recommended amounts are also regarded as safe for most people. In some people, high doses-more than, say, 2,000 or 3,000 mg per day-can cause a variety of symptoms. People with kidney conditions should take no more than 1,000 mg a day.
