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.
- AFP