Ginkgo biloba - one of the most popular herbal health supplements and even suggested as a potential treatment for Alzheimer's disease - does nothing for the memory of healthy older people, according to research published today.

An extract boiled from the leaves of the ginkgo tree is marketed worldwide as a memory enhancer. A team from Williams College, Massachusetts, and from the Memory Clinic in Bennington, Vermont, report in the Journal of the American Medical Association that they tested the product on 230 physically and mentally healthy volunteers aged 60 or more. One group took ginkgo, one a placebo.

The volunteers underwent 14 tests of learning, memory, attention and concentration.

The manufacturers claim the beneficial effects of the herbal supplement should have been detectable after four weeks. Six weeks on, the 230 volunteers retook the tests.

There was no discernible difference between the performances of the two groups.

"The results indicate that when taken following the manufacturer's instructions, ginkgo provides no measurable benefit in memory, attention or concentration," said Paul Solomon, of Williams College.