The Institute of Medicine's current recommended dietary intake for vitamin C is 90 milligrams for men and 75 milligrams for women (add an extra 35 milligrams for smokers). The upper limit for vitamin C is set at 2 grams per day.
There's no good evidence that megadoses of vitamin C improve health. As the evidence continues to unfold, 200 to 300 milligrams of vitamin C a day appears to be a good target. This is easy to hit with a good diet and a standard multivitamin.
Food sources: Excellent food sources of vitamin C are citrus fruits or citrus juices, berries, green and red peppers, tomatoes, broccoli, and spinach. Many breakfast cereals are also fortified with vitamin C.
Vitamin C has been in the public eye for a long time. Even before its discovery in 1932, nutrition experts recognized that something in citrus fruits could prevent scurvy, a disease that killed as many as two million sailors between 1500 and 1800. (1) In the 1970s, Chemistry and Peace Nobel laureate Linus Pauling promoted daily megadoses of vitamin C (the amount in 12 to 24 oranges) as a way to prevent colds and some chronic diseases.
There's no question that vitamin C plays a role in controlling infections. It's also a powerful antioxidant that can neutralize harmful free radicals, and it helps make collagen, a tissue needed for healthy bones, teeth, gums, and blood vessels. (2) The question is, do you need lots of vitamin C to keep you healthy?
No. Vitamin C's cold-fighting potential certainly hasn't panned out. Small trials suggest that the amount of vitamin C in a typical multivitamin taken at the start of a cold might ease symptoms, but for the average person, there's no evidence that megadoses make a difference, or that they prevent colds. (3) Studies of vitamin C supplements and heart disease, cancer, and eye diseases such as cataract and macular degeneration also show no clear patterns.
References
1. Carpenter KJ. The history of scurvy and vitamin C. Cambridge: Cambridge University Press, 1986.
2. Institute of Medicine. Dietary reference intakes for vitamin C, vitamin E, selenium, and carotenoids. Washington, DC: National Academy Press, 2000.
3. Douglas RM, Hemila H, Chalker E, Treacy B. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev. 2007:CD000980.