By the way, doctor: What's the right amount of vitamin C for me?

Q. I'm 79 years old and have been taking 3,000 mg of vitamin C a day for years. I'm now uneasy about taking this amount and plan to cut back to 1,000 mg daily. Is this the right vitamin C dose? Will my body be startled by the abrupt change?

A. Cutting back on daily vitamin C is a wise decision. You've been taking much more vitamin C than you need.

Vitamin C, or ascorbic acid, plays a role in many vital functions, including metabolism and immunity. We can't make vitamin C in the body, so we need to get it from dietary sources. It's found in high concentrations in many fresh fruits and vegetables (see the chart); it's also available in most multivitamins and as a single-ingredient supplement.

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, esophagus, stomach, colon, and lung. But it's not clear that these benefits come specifically from vitamin C. The Age-Related Eye Disease Study found that a combination of vitamins, including vitamin C, helped slow the progression of macular degeneration, an age-related eye disease that causes vision loss. But we don't know what benefits, if any, are conferred 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 touted for preventing cardiovascular disease and osteoarthritis, staving off sunburn, and improving the appearance of wrinkles. None of these purported benefits have been verified in scientific studies.

Perhaps the best-known and most widely promoted use of high-dose vitamin C is to prevent or treat the common cold. The bulk of the evidence shows that high-dose vitamin C will, at most, shorten a cold by one day. In ordinary circumstances, vitamin C appears to have 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 sub-Arctic conditions.

The recommended vitamin C dosage per day for healthy women is 75 mg per day and for men it's 90 mg per day. For adults, the tolerable upper intake level (UL) — the highest daily intake likely to pose no risks — is 2,000 mg per day.

Vitamin C is water-soluble, so any excess is usually excreted in the urine rather than 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, very high doses can cause symptoms such as diarrhea, nausea, and heartburn. 

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