Researchers say vitamin E might slow the progression of mild-to-moderate Alzheimer's disease - the first time any treatment has been shown to alter the course of dementia at that stage.
In a study of more than 600 older veterans, high doses of the vitamin delayed the decline in daily living skills, such as making meals, getting dressed and holding a conversation, by about six months over a two-year period.
The benefit was equivalent to keeping one major skill that otherwise would have been lost, such as being able to bathe without help. For some people, that could mean living independently rather than needing a nursing home.
Vitamin E did not preserve thinking abilities, though, and it did no good for patients who took it with another Alzheimer's medication. But those taking vitamin E alone required less help from caregivers - about two fewer hours each day than some others in the study.
"It's not a miracle or, obviously, a cure," said study leader Dr Maurice Dysken of the Minneapolis VA Health Care System. "The best we can do at this point is slow down the rate of progression."
The US Department of Veterans Affairs sponsored the study, published on Tuesday by the Journal of the American Medical Association.
No one should rush out and buy vitamin E, several doctors warned. It failed to prevent healthy people from developing dementia or to help those with mild impairment ("pre-Alzheimer's") in other studies, and one suggested it might even be harmful.
Still, many experts cheered the new results after so many recent flops of once-promising drugs.
"This is truly a breakthrough paper and constitutes what we have been working toward for nearly three decades: the first truly disease-modifying intervention for Alzheimer's," said Dr Sam Gandy of Mount Sinai School of Medicine in New York. "I am very enthusiastic about the results."
About 35 million people worldwide have dementia, and Alzheimer's is the most common type. In the US, about 5 million have Alzheimer's. There is no cure and current medicines just temporarily ease symptoms.
Researchers don't know how vitamin E might help, but it is an antioxidant, like those found in red wine, grapes and some teas. Antioxidants help protect cells from damage that can contribute to other diseases, says the federal Office on Dietary Supplements.
Many foods contain vitamin E, such as nuts, seeds, grains, leafy greens and vegetable oils. There are many forms, and the study tested a synthetic version of one - alpha-tocopherol - at a pharmaceutical grade and strength, 2000 international units a day.