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After years of conflicting studies on gingko biloba's benefits for memory loss, a definitive clinical trial finally concludes that the extract does not prevent brain decline.

Reporting Nov. 18 in the Journal of the American Medical Association, Dr. Steven DeKosky, dean of the school of medicine at University of Virginia, found that taking 120 mg of gingko biloba twice a day did not prevent the development of dementia in a group of 1,545 seniors 75 years and older; there was no difference in rates of dementia among the intervention group and a similar group of 1,524 participants who took identical placebos. This was the largest and longest investigation into the effects of gingko biloba, and the first study to explore whether a supplement could prevent Alzheimer's disease in healthy volunteers. (See TIME's A-Z Health Guide.)

"This study puts a period on the debate," says Dr. Bill Thies, vice president of medical and scientific relations for the Alzheimer's Association. "These results allow the Association to say with some confidence that gingko doesn't work in spite of what anybody trying to sell it says."

It is understandable that so many people would try almost anything, including popping gingko supplements — on which Americans spend more than $100 million annually — in the hopes of holding off the slow and agonizing mental decline that characterizes dementia. The claimed benefits of gingko have mostly been based on the supplement's antioxidant effects, which have been shown in lab studies — but not in patients — to gnaw away at the fatty plaques that infiltrate the Alzheimer's brain and destroy nerve cells. Studies in patients have involved only small groups, making those results interesting but hardly definitive.

DeKosky, who was at the University of Pittsburgh between 2000 and 2008, when the trial was conducted, decided to address the uncertainties by designing a randomized, controlled, double-blinded trial — the most rigorous study of gingko biloba to date. The end result is scientific confidence that the findings are both reliable and reproducible: At the end of six years of follow up, 523 of the more than 3,000 healthy subjects had developed dementia — 277 of those patients had taken gingko, and 246 had received the placebo. Of these cases, 92% were likely in the first phase of Alzheimer's, researchers say. "These results show that gingko doesn't slow down entry into the disease," says DeKosky.

Despite the study's negative findings, the trial is important to Alzheimer's research for an altogether different reason — researchers had been doubtful that a large, placebo-controlled trial could be conducted on such a large and elderly population for such a long period of time. In fact, DeKosky had recruited double the number of people who participated in the study — the volunteers, plus a study partner for each volunteer who ensured that participants would be available for follow up testing. "There have not been many prevention trials in Alzheimer's," says Thies. "This study says that you can run a study in this population and carry it out to its end. That's important as we look to the future to test drugs that could change the course of the disease." And hopefully someday, perhaps even prevent it from developing at all.

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