Many Americans take that much, or more, in hopes of preventing colds and reaping the widely celebrated antioxidant benefits of vitamin C. Antioxidants, which block cellular and molecular damage caused by the highly reactive molecules called free radicals, are believed to protect against heart disease, cancer, eye disorders like cataracts and macular degeneration, and other chronic health problems.
But the British researchers, chemical pathologists at the University of Leicester, found in a six-week study of 30 healthy men and women that a daily 500-milligram supplement of vitamin C had pro-oxidant as well as antioxidant effects on the genetic material DNA. The researchers found that at the 500-milligram level, vitamin C promoted genetic damage by free radicals to a part of the DNA, the adenine bases, that had not previously been measured in studies of the vitamin's oxidative properties.
The
finding, published in the current issue of the British journal Nature,
corroborates warnings that have been issued for decades by an American
physician, Dr. Victor Herbert, professor of medicine at the Mount Sinai
School of Medicine in New York. Dr. Herbert has shown, primarily through
laboratory studies, that vitamin C supplements promote the generation
of free radicals from iron in the body.
''The vitamin C in supplements mobilizes harmless ferric iron stored in the body and converts it to harmful ferrous iron, which induces damage to the heart and other organs,'' Dr. Herbert said in an interview.
''Unlike the vitamin C naturally present in foods like orange juice, vitamin C as a supplement is not an antioxidant,'' Dr. Herbert said. ''It's a redox agent -- an antioxidant in some circumstances and a pro-oxidant in others.''
In contrast, vitamin C naturally present in food, he said, has no oxidizing effects.
Vitamin C supplements in large doses have been linked to genetic damage as far back as the mid-1970's. In a study then, Canadian researchers found that use of the vitamin in doses larger than in the British study, but not much larger than the amounts some people take to ward off colds and the flu, damaged genetic material in three systems: bacterial cells, human cells grown in test tubes, and live mice.
The lead author of the new study, Dr. Ian Podmore, said that at 500 milligrams, vitamin C did act as an antioxidant on one part of the DNA, the guanine bases. Oxidation of guanine to oxoguanine is what is usually measured to determine the degree of DNA damage through oxidation.
As expected, when the volunteers took a daily 500-milligram dose of vitamin C for six weeks, oxoguanine levels indeed declined, ''which is why vitamin C is generally thought to be an antioxidant,'' Dr. Podmore said.
But when they measured a second indicator of DNA oxidation, oxoadenine, the researchers found that it had risen rather than declined, ''indicating genetic damage to this DNA base,'' Dr. Podmore said.
A colleague, Dr. Joseph Lunec, said that at the 500-milligram level, vitamin C's ''protective effect dominated, but there was also a damaging effect.''
''There should be caution about taking too much vitamin C,'' Dr. Lunec said. ''The normal healthy individual would not need to take supplements of vitamin C.''
In the United States and Britain alike, the recommended daily intake of vitamin C for healthy adults is 60 milligrams, which can be easily obtained from foods -- by drinking about six ounces of orange juice, for example. Larger amounts are recommended for smokers and for pregnant and lactating women, but even these amounts can be readily obtained from foods.
Dr. Lunec took issue with the late Dr. Linus C. Pauling, the Nobel laureate chemist who took 12,000 milligrams of vitamin C daily and suggested that people could take as much of it as they wanted with no ill effect.
''We think that's not the case, to say the least,'' Dr. Lunec said. ''You can have too much of a good thing.''
The research team is now studying the effects of lower doses of vitamin C, ''to see if we can maximize the protective effect and minimize the damage,'' Dr. Lunec said. Given the new finding, he said, ''it would be unethical to test higher levels.''
''The vitamin C in supplements mobilizes harmless ferric iron stored in the body and converts it to harmful ferrous iron, which induces damage to the heart and other organs,'' Dr. Herbert said in an interview.
''Unlike the vitamin C naturally present in foods like orange juice, vitamin C as a supplement is not an antioxidant,'' Dr. Herbert said. ''It's a redox agent -- an antioxidant in some circumstances and a pro-oxidant in others.''
In contrast, vitamin C naturally present in food, he said, has no oxidizing effects.
Vitamin C supplements in large doses have been linked to genetic damage as far back as the mid-1970's. In a study then, Canadian researchers found that use of the vitamin in doses larger than in the British study, but not much larger than the amounts some people take to ward off colds and the flu, damaged genetic material in three systems: bacterial cells, human cells grown in test tubes, and live mice.
The lead author of the new study, Dr. Ian Podmore, said that at 500 milligrams, vitamin C did act as an antioxidant on one part of the DNA, the guanine bases. Oxidation of guanine to oxoguanine is what is usually measured to determine the degree of DNA damage through oxidation.
As expected, when the volunteers took a daily 500-milligram dose of vitamin C for six weeks, oxoguanine levels indeed declined, ''which is why vitamin C is generally thought to be an antioxidant,'' Dr. Podmore said.
But when they measured a second indicator of DNA oxidation, oxoadenine, the researchers found that it had risen rather than declined, ''indicating genetic damage to this DNA base,'' Dr. Podmore said.
A colleague, Dr. Joseph Lunec, said that at the 500-milligram level, vitamin C's ''protective effect dominated, but there was also a damaging effect.''
''There should be caution about taking too much vitamin C,'' Dr. Lunec said. ''The normal healthy individual would not need to take supplements of vitamin C.''
In the United States and Britain alike, the recommended daily intake of vitamin C for healthy adults is 60 milligrams, which can be easily obtained from foods -- by drinking about six ounces of orange juice, for example. Larger amounts are recommended for smokers and for pregnant and lactating women, but even these amounts can be readily obtained from foods.
Dr. Lunec took issue with the late Dr. Linus C. Pauling, the Nobel laureate chemist who took 12,000 milligrams of vitamin C daily and suggested that people could take as much of it as they wanted with no ill effect.
''We think that's not the case, to say the least,'' Dr. Lunec said. ''You can have too much of a good thing.''
The research team is now studying the effects of lower doses of vitamin C, ''to see if we can maximize the protective effect and minimize the damage,'' Dr. Lunec said. Given the new finding, he said, ''it would be unethical to test higher levels.''
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