Q. I'm 79 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 dose? Will my body be startled by the abrupt change?

A. Cutting back 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 it 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.

Food (serving size) Vitamin C (mg)

Guava (1 medium) 165

Strawberries (1 cup) 98

Cantaloupe (¼ medium) 95

Papaya (1 medium) 95

Bell pepper, red, raw (½ cup) 95

Orange juice (¾ cup) 60

Kale (1 cup, cooked) 53

Broccoli (½ cup, cooked) 50

Bell pepper, green, raw (½ cup) 45

Tomato juice (1 cup) 45

Mango (1 medium) 30

Lemon juice (½ cup) 30

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 daily supplement containing 500 milligrams (mg) of vitamin C, 400 international units (IU) of vitamin E, 15 mg of beta carotene, 80 mg of zinc, and 2 mg of copper can help 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 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 to three days. In ordinary circumstances, it appears to have no preventive value, although a few studies have reported a 50 percent 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 daily intake of vitamin C for healthy women is 75 mg per day (120 mg per day for women who are pregnant or breastfeeding). 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, high doses -- more than, say, 2,000 or 3,000 mg per day -- can cause symptoms such as diarrhea, nausea, heartburn, gastritis, fatigue, flushing, headache, and insomnia. People with chronic liver or kidney conditions, gout, or a history of calcium-oxalate kidney stones should take no more than 1,000 mg a day.

Although it doesn't sound as if you've had any side effects, taking 3,000 mg of vitamin C per day is a waste of your money. You should decrease your intake, but do it gradually. Prolonged use of high doses speeds up the metabolism of vitamin C, so cutting the dose abruptly may lower the level in your body below normal. Go slowly to allow your metabolism to readjust. Try reducing your intake by 500 mg each week until you reach 1,000 mg a day.

Remember that the best sources of vitamin C are fresh fruits and vegetables. Keep them covered and refrigerated and cook them on low heat to prevent deterioration of the vitamin. -- Celeste Robb-Nicholson, M.D., Editor in Chief, Harvard Women's Health Watch

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Copyright © 2010 Celeste Robb-Nicholson, M.D. - Harvard Health Letters