Flanking boulevards and dotted around urban parks across America, the ginkgo tree - also known as the maidenhair tree - is the greenery of choice for American town planners. Not only is it tall, beautiful (with an abundance of fan-shaped leaves) and a shrewd municipal investment (life expectancy: 1,000 years), but it can cope with just about every environmental hazard that nature, industry or al-Qaida wishes to throw at it. Resistant to insects, fungi, viruses, pollution and even nuclear radiation, the ginkgo was the only plant to survive the atomic bombing of Hiroshima. It has existed unchanged in China for 250m years - from the time of the dinosaurs - and was described by Darwin as a "living fossil".
Given this robustness and longevity, it is hardly surprising that ginkgo biloba (to use the tree's full botanical name) is believed to imbue humans with precisely those qualities. In traditional Chinese medicine, the leaves and nuts have been used for 5,000 years to "benefit the brain" and treat conditions ranging from asthma and coughs to chilblains, stomach bugs and hangovers. In the West, ginkgo biloba extract (GBE), made from the dried leaf, is said to boost circulation, slow the effects of Alzheimer's disease, improve mental function, memory and reasoning, and protect nerve cells against the effects of ageing. Ginkgo is perceived as the wonder drug of the wrinklies - prescribed by 120,000 doctors worldwide.It is the best-selling herbal extract in America.
It is also the world's most widely researched herb. Hundreds of clinical trials have looked at everything from its antioxidant qualities to its effects on erectile dysfunction. One of the most conclusive studies was in 1997, when neurologists at the New York Institute for Medical Research found that ginkgo could improve mental function in patients with Alzheimer's and dementia caused by strokes. Intrials involving 309 patients, 27% of those taking GBE had improved, compared with 14% in a placebo group. The researchers concluded that ginkgo could stabilise or enhance mental function in a third of patients mildly impaired by Alzheimer's.
Meanwhile, scientists in Germany reported that ginkgo biloba could aid circulation and ease the pain of walking in patients with "intermittent claudication" - painful blockages of the arteries, also known as "smoker's legs".
All these effects are due to two groups of components: flavone glycosides and terpene lactones. The first are a type of bioflavanoid - crystalline compounds found in many plants, known for beneficial actions such as fighting viruses and reducing inflammation. In the case of ginkgo, the bioflavanoids act as antioxidants, limiting the damage done to the central nervous system by free radicals - compounds associated with ageing. Bioflavanoids also inhibit "platelet aggregation", or stickiness of the blood, and control the tone and elasticity of blood vessels.
The second component group, terpene lactones, increase blood flow to the brain and protect nerve cells. Research with animals suggests that one such compound - bilobalide - rebuilds damaged nerve cells. Together, these two groups of chemicals appear to have a positive effect on many conditions, from bronchial constriction, diabetes and kidney problems, to migraine, multiple sclerosis and depression. However, since ginkgo thins and lubricates the blood, it may be unsafe for those taking aspirin, warfarin or other anticoagulants for circulatory disorders.
In Europe, it is normal to prescribe smaller doses (40mg daily, as opposed to 120mg) to patients in this category. While the dried-leaf extract is considered safe at recommended doses, excessive use can cause nausea, vomiting, diarrhoea, skin problems, dizziness and restlessness. Unlike some herbs, which should be taken only for short periods, GBE only works when taken for four weeks or more, and may take 12 weeks to have an effect.
Confusingly, ginkgo biloba nuts (rather than the leaves) are mildly toxic and should not be taken in excess over long periods. However, they are a Japanese delicacy - particularly in steamed egg custard.
· Next time: ginseng
