Q: Whatever happened to the idea that vitamin C can boost your immune system and prevent colds?

A: Vitamin C, also known as ascorbic acid, was promoted as a health supplement for decades. It is perhaps best known for its one-time reputation for preventing and treating the common cold. This idea was heavily promoted in the 1970s by one of the 20th century’s most celebrated biochemists, Nobel laureate Linus Pauling. (But he did not win the Nobel Prize for his theories about vitamin C.)

Vitamin C is crucial for making collagen, the substance that lends structural support to tendons, ligaments, bones and blood vessels. Vitamin C is also a potent antioxidant. As the cells in our body do their jobs, they produce a group of harmful chemical waste products called “free radicals.” These free radicals are also created by environmental factors such as tobacco smoke, ultraviolet rays and air pollution. Antioxidants can chemically combine with free radicals, rendering them harmless.

Unfortunately, despite the hype, vitamin C does not appear to prevent colds, and its ability to shorten the duration of colds is minimal. A recent summary of 29 studies involving more than 10,000 people confirmed the lack of benefit of regular vitamin C supplementation.

There also is little evidence that vitamin C supplements prevent cancer, heart disease or cataracts.

The recommended daily allowance for men is 90 milligrams per day. Women should get 75 mg per day. If you are a smoker, add 35 mg per day to your recommended amount. Taking vitamin C supplements in amounts far higher than the RDA offers no proven health benefits, but it probably won’t hurt.

Most people meet the RDA for vitamin C through their diets. Citrus fruits are rich in vitamin C. Sweet red pepper is another excellent, but less well known, source of vitamin C.

Just because vitamin C doesn’t appear to boost your immune system doesn’t mean there are no “natural” immune system boosters. Indeed, if you want to strengthen your immune system and boost your ability to fight off colds, try exercise. Regular moderate aerobic exercise can bolster your protection from common respiratory infections. Sleep is also important. People who get enough sleep are less prone to upper respiratory infections.

People who eat diets high in fruits and vegetables also appear to be less likely to catch colds. (Though it’s less certain that the diet alone is responsible.) Some studies also found increased susceptibility to colds with increasing levels of stress.

In short, while regular use of vitamin C is unlikely to protect you against colds, there are other things you can do to protect yourself and strengthen your immune system.

DR. ANTHONY KOMAROFF is a physician and professor at Harvard Medical School. Go to his website to send questions and get additional information: www.askdoctork.com.