Sufferers taking the placebo continued to show decreased levels of Abeta40 and did not improve in cognitive tests. Although researchers warn that the trial is too small to advise people to start taking the supplement, they say the results are "very interesting" and further work should be done.
"Number one, we found that resveratrol was safe in older people with Alzheimer's disease and number two, it looked like it may have had a beneficial effect on biomarkers and disease progression," said Dr Scott Turner, director of the memory disorders programme at Georgetown University Medical Centre, in Washington DC.
The researchers chose resveratrol because it activates proteins called sirtuins, which are also switched on by calorie restriction. Animal studies have found that most age-related diseases, including dementia, can be prevented or delayed by cutting calories to two thirds of the recommended level.
Resveratrol is also released by plants in response to injury and is thought to have antioxidant properties.
Dr Doug Brown, director of research at the Alzheimer's Society, said: "While this is an interesting study, it did not investigate whether resveratrol has any effect on memory or improving other symptoms of Alzheimer's disease.
"Before we can say this has potential as a treatment option we need to see larger and longer trials in people that specifically look at whether it can improve the lives of those with the condition".
The research was published in the journal Neurology.