The study looked at more than 600 older patients at 14 VA medical centers across the country. Dr. Maurice Dysken from the Minneapolis VA health care system co-authored the study.
In the study, some patients took vitamin E alone, the FDA-approved drug memantine alone, or a combination. Another group was given placebos.
In the report, which was sponsored by the U.S. Department of Veterans Affairs and published Tuesday by the Journal of the American Medical Association, researchers found vitamin E slowed the rate of Alzheimer's disease progression 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.
After a little more than two years of follow-up, those on vitamin E alone had a 19 percent lower annual rate of decline in daily living skills compared to the placebo group. Memantine made no difference.
Vitamin E did not preserve thinking abilities, though, and it did no good for patients who took it with another Alzheimer's medication.
"Memantine did not have a significant effect in slowing the rate of progression compared to placebo," Dysken explained.
Also, there was a significant benefit to the caregivers. Dysken says vitamin E slowed the process, saving about two hours of care-giving time each day compared to those on the medication.
"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."
Now, researchers want to know if vitamin E therapy can also help in patients with milder forms of the disease.
"For people who are in the early stage of Alzheimer's disease, I think any delay in the rate of progression is meaningful and important," Dysken said.
The study was five years long. Patients were followed for six months up to four years. Study authors say both vitamin E and memantine are considered safe.
However, 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.
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, 2,000 international units a day.
The Associated Press contributed to this report.