Nature's Pharmacopeia: A World of Medicinal Plants
The ginkgo, also known as the maidenhair tree, originated in East Asia and is now grown throughout the world’s temperate zones. Ginkgo can reach a height of 35 meters or more, and some individuals are thought to be hundreds of years old.71 The plant has fan-shaped medium-green leaves, sometimes divided into two portions, that turn bright yellow before falling in the autumn. As a dioecious plant, ginkgo has separate male trees that release sperm cells and females that grow small olive-size seeds covered in a fleshy tissue (figure 14.9).72 Europeans first encountered ginkgo in East Asia during the sixteenth and seventeenth centuries and brought specimens back to the major botanic gardens by the early eighteenth century. Today, ginkgo is grown as an ornamental and shade tree around the world, particularly desired in urban landscapes, where it has proved resilient against the inconsistent precipitation, temperature changes, pests, and pollution levels of modern cities. Nearly all ginkgo employed in landscaping is male, as the female flesh-covered seeds release an unpleasant odor. Since ginkgo’s evolutionary origins predate the development of flowers and fruits, botanists classify it as a gymnosperm, along with the conifers and cycads. Ginkgo is unusual in being a monotypic taxon: the only extant species of its scientific genus, family, order, and class. The fossil record is rich in ginkgo relatives, however, leading some to dub Ginkgo biloba a “living fossil.”73
Ginkgo found its way into East Asian medicine and cuisine at least 1000 years ago, and its seeds became part of the traditional pharmacopeia. In Chinese medicine, prepared ginkgo seeds are thought to restrain Lung qi and promote the movement of dampness, therefore effective for breathing disorders, cough, vaginal discharge, and urinary complaints.74 (Lung is capitalized to distinguish the traditional Chinese physiological element from the Western anatomical structure.) The seeds, however, are toxic when fresh and in large quantity, and practitioners know to boil them well and prescribe them conservatively to avoid effects ranging from nausea to convulsions and death. Ginkgo seed is also commonly used as an ingredient in soups and porridges in contemporary East and Southeast Asia.75
FIGURE 14.9 Ginkgo: (left) leaves; (right) seeds.
FIGURE 14.10 A ginkgolide, from ginkgo leaves.
In contrast to the Asian traditions, in which ginkgo seeds played a role in medicine and food, the Europeans took an interest in ginkgo leaves for possible health-related effects. By the twentieth century, German chemists had developed a process to purify some of the oil-soluble and water-soluble chemical constituents of the leaves as a standardized extract. Ginkgo leaf extracts contain a complex assortment of flavonoids, flavonoid glycosides, and terpenoid compounds called ginkgolides (figure 14.10). Many ongoing research projects focus on the possible physiological effects of standardized extracts, in principle rendering the laboratory and clinical studies more comparable.76 Despite these efforts, many human trials using ginkgo leaf extracts have been criticized for inconsistent experimental design and dosing, and some commercial ginkgo extracts are unreliably prepared and labeled.77 Ginkgo extract is speculated to benefit brain function and has been recommended to improve vision and blood flow in peripheral arteries.78
Although some specific activities of ginkgo extract have been assessed in the laboratory, many of ginkgo’s possible physiological mechanisms remain unknown.79 Among the most widely lauded applications of medicinal ginkgo is in the prevention and treatment of cognitive decline (such as accompanies aging and neurodegenerative diseases) and the enhancement of normal cognitive functions. To test these possible medical applications, several research groups have assessed a large body of experimental literature for evidence of efficacy. With regard to the use of ginkgo extract to treat cognitive decline, the experimental evidence is mixed, with researchers at various sites using similar protocols but obtaining contradictory evidence. A meta-analysis concludes: “The evidence that Ginkgo biloba has predictable and clinically significant benefit for people with dementia or cognitive impairment is inconsistent and unreliable.”80 Recent large-scale studies on ginkgo extract as a preventive measure for age-related cognitive shortcomings—including declining memory, language ability, attention, and executive function—likewise failed to demonstrate significant differences over placebo control groups.81
Few human trials have investigated ginkgo’s possible effects in the eyes, where its extract is speculated to prevent or treat age-related deterioration of the retina.82 Ginkgo has also been suggested to treat tinnitus, the sensation of sound in the ears in the absence of any stimulus. A recent review of the literature revealed that ginkgo is generally ineffective for treatment of this condition.83 Finally, work has been initiated to study the possible effect of ginkgo on the symptoms of peripheral artery disease, which causes patients leg pain and is associated with blockage of the arteries. An analysis of a number of relatively small controlled trials revealed no significant difference between ginkgo treatment and placebo on measures of peripheral artery disease.84
Despite centuries of use as a medicinal and culinary plant in Asia and decades of research as a therapy for a variety of age-related ailments, ginkgo’s physiological activities remain enigmatic. Ongoing studies may add to the growing set of experimental outcomes questioning its utility as a drug or finally lend support to some of its long-ascribed properties.