Researchers believe they have identified a molecule that could be targeted to treat mental impairment in people with Down's syndrome.
Down's syndrome is caused by an extra chromosome
A team at the Institute of Psychiatry at King's College London found people with Down's syndrome have higher levels of myo-inositol in their brains.
They also found increased levels of this molecule are associated with reduced intellectual ability.
The study is published in Archives of General Psychiatry.
The researchers also suspect that high levels of myo-inositol could play a role in predisposing people with Down syndrome to early-onset Alzheimer's disease.
The molecule is known to promote the formation of amyloid plaques - a hallmark of Alzheimer's.
Once they reach the age of 40, almost all people with Down's syndrome show the brain characteristics of Alzheimer's disease - though they do not all go on to develop dementia.
The combination of pre-existing mental retardation with an increasing overlying dementia is difficult to treat, and expensive to manage.
Lead researcher Professor Declan Murphy said: "We have shown in this study that adults with Down's syndrome have a significantly higher concentration of myo-inositol in the hippocampal region of their brains, and this increase is associated with a reduced cognitive ability.
"We are now carrying out more studies to see if we can reduce the concentration of myo-inositol in the brains of people with Down's.
"We hope that if we can do this, it will be a new way of treating this devastating disorder."
Down's syndrome is the most common genetic cause of mental impairment.
It is caused when a child has three copies of chromosome 21, rather than the usual two.
The latest research has shown that one of the genes on chromosome 21 controls production of a protein that pumps the molecule myo-inositol into the brain.
The increased levels of myo-inositol in the brains of people with Down's syndrome could be explained by the fact that these people have an extra copy of the gene that makes this pump.
Not a cure
The Down's Syndrome Association said it welcomed any research that may have a beneficial effect on the lives of people with the condition.
In a statement, the charity said: "We are very pleased that scientists are producing results that help us to understand the reasons behind Down's syndrome's associated learning disability.
"However, the Institute of Psychiatry's research does not herald a 'cure' for the condition, and any treatment available is still a long way in the future."
The researchers used sophisticated scan technology to measure the concentrations of myo-inositol in the brains of 38 adults with Down's syndrome and 42 healthy controls.