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Effects of Ginkgo Biloba on Cognitive Functions

Ginkgo is derived from the leaves of the ginkgo tree, one of the oldest species of trees, and has been used for thousands of years by the Chinese as an herbal remedy for a variety of ailments.

The typical daily dose of ginkgo biloba is 120 milligrams of dried extract in two or three oral doses. The extract contains several flavonoids, a large group of natural plant products that are characterized by a specific chemical structure containing a series of carbon rings. Ginkgo extract also contains some biflavonoids, a related group of compounds, and two different types of terpenes, a class of naturally occurring chemicals that includes the active ingredients in catnip and marijuana.

Today ginkgo biloba is perhaps the most widely used herbal treatment aimed at augmenting cognitive functions — that is, improving memory, learning, alertness, mood and so on. In Europe, ginkgo is an important part of mainstream medicine, with sales accounting for more than 1 percent of all pharmaceutical purchases. In the U.S. alone, $310 million dollars worth was sold in 1998. But is its popularity based on folklore or on experimental findings?

While dozens of investigations have examined the cognitive effects of ginkgo in humans, many of the research reports are in non-English publications or in journals with very restricted distribution, making assessment of the findings difficult. The great majority of studies have involved subjects with mild to moderate mental impairment, usually a diagnosis of early Alzheimer’s. Most of the experiments that show evidence of cognitive enhancement in Alzheimer’s patients have used a standardized ginkgo extract known as EGb 761.

Ginkgo biloba and Alzheimer’s

In 1998 Barry S. Oken of Oregon Health Sciences University and his colleagues considered more than 50 studies involving subjects with mental impairment and selected four that met a conservative set of criteria, including sufficient characterization of the Alzheimer’s diagnosis, use of a standardized ginkgo extract, and a placebo-controlled, double-blind design (in which neither the subjects nor the investigators know until the end whether a given patient is receiving the extract or the placebo). Each of these studies showed that the Alzheimer’s patients who received ginkgo performed better on various cognitive tests than did patients who received a placebo. Improvements were evident in standardized tests measuring attention, short-term memory and reaction time; the average extent of improvement resulting from ginkgo treatment was 10 to 20 percent.

Oken and his colleagues reported that ginkgo’s effect was comparable to that of the drug donepezil, which is currently the treatment of choice for Alzheimer’s.

Despite these apparently encouraging findings, though, another recent, large and well-controlled trial of EGb 761 (sponsored by its manufacturer, Dr. Willmar Schwabe Pharmaceuticals in Karlsruhe, Germany) involving patients with a mild or moderate stage of dementia reported no “systematic and clinically meaningful effect of ginkgo” on any of the cognitive tests employed.

Ginkgo biloba for age-related memory loss

Researchers at the UCLA Neuropsychiatric Institute examined the impact of ginkgo biloba, compared to a placebo, in 10 patients, aged 45 to 75, who did not have dementia but complained of mild age-related memory loss. Over a period of six months four subjects received 120 mg of ginkgo biloba twice daily, and six received a placebo or inactive substance such as a sugar pill.

Researchers used cognitive tests to measure verbal recall and PET to measure brain metabolism before and after the treatment regimen. When compared with the group that received a placebo, they found significant improvement in verbal recall among the group of people who received ginkgo biloba. However, actual changes in brain metabolism, measured by PET for the first time, did not differ significantly between the study’s two volunteer groups.

It should be noted that ginkgo has also been shown to impair performance. For example, in a small study of elderly people with mild to moderate memory impairment, Gurcharan S. Rai of Whittington Hospital in London and his team found that after 24 weeks of treatment, patients who took ginkgo could not recall digits as well as patients who received a placebo.
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