A chemical in green tea has been shown to improve cognitive ability in people with Down's syndrome, scientists and doctors said on Tuesday.

In a year-long clinical trial, the treatment led to improved scores on memory and behaviour tests, they reported in a study, published in the The Lancet Neurology.

The positive impact remained six months after the trial ended.

Brain scans revealed that the compound, called epigallocatechin gallate, altered the way neurons in the brain connect with one another.

"This is the first time that a treatment has shown efficacy in the cognitive improvement of persons with this syndrome," said Mara Dierssen, senior author of the study and a researcher at the Centre for Genomic Regulation in Barcelona.

While significant, she added in a statement, the results should not be interpreted as a "cure".