Ginkgo biloba extract and cognitive function

 

By Alan R. Gaby, M.D.

Ginkgo biloba extract (ginkgo) is a popular herbal remedy that is used to treat dementia, Alzheimer's disease, cardiovascular disease, erectile dysfunction, and certain other conditions. Each of these treatment indications is supported by scientific research. With regard to ginkgo and brain function, a review of nine double-blind trials (each lasting between 2 and 12 months) concluded that ginkgo is more effective than placebo in patients with dementia.¹

In contrast to the positive effects of ginkgo in patients with established dementia, a six-year study published in 2008 found that ginkgo did not prevent dementia or Alzheimer's disease in elderly volunteers who had normal cognitive function or mild cognitive impairment at baseline.² The data from that study have now been analyzed further, in order to determine whether ginkgo produced any subtle benefits that were not identified in the original report. No such benefits were found, and the new analysis concluded that ginkgo did not slow the rate of cognitive decline in elderly individuals with normal cognitive function or mild cognitive impairment.³ These new findings received widespread publicity, and have led some people to question whether ginkgo has any positive effect at all on brain function.

An important weakness of the new study was the high dropout rate. Only 60.3% of the participants were still taking their assigned tablets at the end of the trial. However, the reported results included all subjects who entered the trial, even those for whom final data were not available. Adherence to the study protocol may actually have been lower than that reported by the investigators. Previous research has shown that participants in clinical trials frequently engage in deceptive practices in order to hide the fact that they have not been taking their assigned medication.⁴ In the new ginkgo trial, adherence to the protocol was monitored by counting the number of unused pills each time the participant attended the clinic. It would have been easy for noncompliant subjects to avoid embarrassment by surreptitiously discarding most or all of their unused tablets. Considering that "compliance" was defined as taking 50% or more of the assigned medication, it would not be surprising if the lack of benefit observed in this study was due largely to poor adherence to the protocol.

However, even if the results of the new study are valid, they do not negate the findings from earlier research. The fact is that there never has been any strong evidence that long-term treatment with ginkgo will slow the normal age-related decline in cognitive function or prevent the development of Alzheimer's disease or other types of dementia. Scientists still have at best a rudimentary understanding of what causes dementia and how to prevent it or slow its progression. To be sure, there are a number of treatable causes of dementia, such as hypothyroidism or deficiencies of vitamin B12, folate, thiamine, or other vitamins. However, most cases of dementia are idiopathic, and an effective method of preventing it (with the exception of engaging in regular physical and mental exercise) remains elusive.

What ginkgo has been shown to do is to produce a modest improvement in cognitive function in people who already have Alzheimer's disease or other types of dementia. Possible mechanisms for this beneficial effect are an increase in cerebral blood flow and an increase in cellular metabolism in the brain irrespective of changes in blood flow. While it is not known how long ginkgo remains effective for patients with dementia, a double-blind study found that the benefits persisted for at least a year with continued treatment.⁵ By comparison, a prescription drug that is currently being promoted as a treatment for Alzheimer's disease (Aricept; donepezil hydrochloride) was studied for only six months.

Based on the available evidence, people with normal cognitive function or mild age-related cognitive impairment should not take ginkgo for the sole purpose of preventing dementia. However, treatment with ginkgo seems appropriate for individuals with clinically significant cognitive impairment. Most of the studies in which ginkgo was effective used 120 mg per day of an extract standardized to contain 24% ginkgo heterosides or 24% ginkgoflavone glycosides and 6% terpenes.

Editor’s Note: For more information about Ginkgo biloba, refer to the January 2010 issue of the Natural Medicine Journal in the Abstracts & Commentary section. Click here to view "Important Clarification of New Ginkgo biloba Study" by Mark Blumenthal.

Alan R. Gaby, MD, received his B.A. from Yale University, his M.S. in biochemistry from Emory University, and his M.D. from the University of Maryland. He is past president of the American Holistic Medical Association and Contributing Medical Editor of The Townsend Letter for Doctors and Patients. He provided expert testimony to the White House Commission on Complementary and Alternative Medicine regarding the cost-effectiveness of nutritional supplements. He is the author of Preventing and Reversing Osteoporosis and B6: The Natural Healer, andco-author of The Patient's Book of Natural Healing. He has compiled a collection of more than 25,000 scientific papers related to the field of nutritional medicine. He has appeared on the CBS Evening News and the Donahue Show.

REFERENCES

1. Ernst E, Pittler MH. Ginkgo biloba for dementia: a systematic review of double-blind, placebo-controlled trials. Clin Drug Investig 1999;17:301-308.
2. DeKosky ST, Williamson JD, Fitzpatrick AL, Kronmal RA, Ives DG, Saxton JA, et al. Ginkgo biloba for prevention of dementia: a randomized controlled trial. JAMA 2008;300:2253-2262.
3. Snitz BE, O'Meara ES, Carlson MC, Arnold AM, Ives DG, Rapp SR, et al. Ginkgo biloba for preventing cognitive decline in older adults: a randomized trial. JAMA 2009;302:2663-2670.
4. Simmons MS, Nides MA, Rand CS, Wise RA, Tashkin DP. Unpredictability of deception in compliance with physician-prescribed bronchodilator inhaler use in a clinical trial. Chest 2000;118:290-295.
5. Le Bars PL, Katz MM, Berman N, Itil TM, Freedman AM, Schatzberg AF. A placebo-controlled, double-blind, randomized trial of an extract of Ginkgo biloba for dementia. JAMA 1997;278:1327-1332.