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People of retirement age who took supplements of both vitamin E and C daily saw their risk of Alzheimer's disease plummet by almost 80 per cent, a new study shows.

However, the researchers saw no appreciable decrease in the devastating neurological disease if people took vitamin E or vitamin C alone, or if they took a multivitamin.

Peter Zandi, a professor of mental hygiene at the school of public health of Johns Hopkins University in Baltimore, Md., said it appears that when the two vitamins are taken in high doses they work together to protect neurons.

Both vitamin E and C are antioxidants, meaning they protect cells (including those in the brain) against free radicals. Neurons are particularly sensitive to damage by free radicals, and neuron damage is believed to be at least partly responsible for the development of Alzheimer's.

"These results are extremely exciting," Dr. Zandi said.

"The use of these antioxidant vitamins may offer an attractive strategy for the prevention of Alzheimer disease."

But Dr. Zandi stressed that a lot more research needs to be done before drawing firm conclusions about the effectiveness of vitamin E and C supplements.

The research, published in today's edition of the medical journal Archives of Neurology, was based on the findings of a large, continuing research project known as the Cache County Study.

Researchers have been tracking 4,740 residents of Cache County, Utah, since 1995 to learn more about dementia.

As part of that research, participants, all of them over the age of 65, were asked about their use of vitamin supplements.

About 17 per cent reported taking vitamin E or C supplements. Another 20 per cent took multivitamins.

Dr. Zandi said it appears the high dose of vitamin available in individual supplements is what provided the additional protection.

In the study, 2,828 people took no vitamin supplement at all. Over the period of the study, 138 of those people developed Alzheimer's. Another group, consisting of 336 people, took the vitamin C and vitamin E combination.

Of those, three showed symptoms of Alzheimer's. Therefore, the risk was lessened by 78 per cent in the second group compared with the first group.

The current recommended daily allowance for vitamin E is 22 international units, and for vitamin C it is 90 milligrams.

These levels tend to be included in multivitamins.

Individual supplements, which can be purchased in drug stores and supermarkets without a prescription, typically contain many times those amounts.

Vitamin E is sold in pills and capsules containing up to 1,000 IU and vitamin C pills or drops contain 500 to 1,000 milligrams per dose.

Vitamin supplements -- and vitamins E and C -- have been touted for years as effective against a host of conditions, including cardiovascular disease and some forms of cancer, but the evidence is inconsistent.

Last year, the U.S. Preventive Services Task Force, a blue-ribbon panel that examines the effectiveness of prevention tools, said claims that antioxidants prevent disease had no sound scientific basis. But, at the same time, it said there is no reason to warn consumers against taking vitamin supplements because they may be filling gaps in their diet, and the pills are unlikely to do any harm.

The difficulty in determining the effectiveness of vitamin supplements is that they are just one tiny part of a person's diet and that people who take supplements tend to have healthier lifestyles to begin with, which confounds the scientific results.

In the Alzheimer's study, for example, those who took vitamin E and C supplements were principally younger, more educated women who reported better overall health than other participants.

While they tried to adjust for those factors, the researchers concede that other factors are at play in determining who is at risk for Alzheimer's.

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