It may be possible to reduce the effects of Alzheimer's disease by taking the right combination of vitamins, US research suggests.
Alzheimer's causes damage to the brain
Scientists have found vitamins E and C may protect the ageing brain - but only if taken together.
They both mop up destructive molecules, called free radicals, released by the body's metabolic processes.
Researchers at Johns Hopkins University in Maryland announced their findings in the journal Archives of Neurology.
Brain cells, known as neurons, are thought to be particularly sensitive to damage caused by free radicals.
Lead researcher Dr Peter Zandi said: "These results are extremely exciting.
"Our study suggests that the regular use of vitamin E in nutritional supplement doses, especially in combination with vitamin C, may reduce the risk of developing Alzheimer's disease."
Dr Zandi's team examined data on 4,740 people aged 65 years or older. Of these 304 showed signs of Alzheimer's disease.
Approximately 17% of the study participants reported taking vitamin E or C supplements.
Another 20% used multivitamins, but without a high dosage of vitamin E or C.
The researchers found that taking a combination of vitamin E and C seemed to have a protective effect.
People taking both vitamins were 78% less likely to show signs of Alzheimer's than those not taking the combination.
They found no benefit from taking either of the vitamins in isolation, or from taking multivitamins alone.
But there was some evidence of a protective effect from combining vitamin E with the lower doses of vitamin C found in multivitamin supplements.
Multivitamins typically contain the recommended daily allowance of vitamin E (22 IU or 15 mg) and vitamin C (75-90 mg), while individual supplements contain doses up to 1,000 IU of vitamin E and 500-1,000 mg or more of vitamin C.
Dr Zandi said further trials were needed before drawing firm conclusions about the protective effects of the two vitaimins.
He told BBC News Online it was unclear why the two vitamins had to be taken together to have any beneficial effect in reducing the damage caused by free radicals - known as oxidative stress.
It was possible that it was simply a dosage effect - taking two vitamins instead of just one meant more was circulating around the body.
However, he said: "There is also evidence of a synergistic effect between the activities of vitamin E and C.
"Vitamin E is lipid-soluble and thus sticks around in fat tissues of the body a relatively long time.
"In contrast, vitamin C is water-soluble and is rapidly excreted from the body.
"Vitamin C may act to recharge the antioxidant capacities of vitamin E so that the vitamin E can continue doing its job of soaking up free radicals and reducing oxidative stress."
Harriet Millward, of Alzheimer's Research Trust, said: "Oxidative damage probably occurs to the brain in the early stages of Alzheimer's disease and therefore antioxidants in food, and possibly in supplements, could help prevent Alzheimer's.
"Randomised prevention trials are now needed because it is always possible that in a retrospective study such as this, people taking vitamins might be more health conscious than those not taking vitamins, and therefore the difference might be due to factors other than the vitamins."
Dr Susanne Sorensen, of the Alzheimer's Society, said: "The doses of vitamins vitamin C and E that made a difference in this study are quite extreme.
"It may be difficult to find populations outside the US taking equivalent doses on a regular basis to verify this evidence."
It is thought that Alzheimer's disease will become an increasing problem as an increasing proportion of the population lives for longer.