They believe that changing the balance of chemicals in the brain by taking dietary supplements might boost intelligence.

Researchers have found a link between two brain chemicals and IQ.

Concentrations of the chemicals N-acetylaspartate (NAA) and choline are known to alter when people suffer brain diseases or injuries.

According to the new study, levels of the chemicals in healthy people account for much of the variation in people's IQ.

NAA is found only in neurons (nerve cells) and is thought to help them function properly, while choline is present in nerve cell membranes.

When large numbers of neurons are injured or killed, NAA levels drop and damaged cells release more choline.

In patients with brain injuries this shift is associated with a loss of thinking ability.

A study of 26 healthy volunteers with no history of brain disease or psychiatric illness showed a strong link between their performance in IQ tests and levels of the chemicals.

Low levels of choline and high levels of NAA were associated with high IQ.

Statistical analysis showed that the chemicals together could account for 45% of IQ variation.

Surprising finding

Neuroscientist William Brooks, of the University of New Mexico in Albuquerque, USA, who led the research, told New Scientist magazine: "We really didn't know what to expect, but we were surprised by what we found."

He speculated that in healthy people levels of NAA and choline reflected the rate at which neurons were being damaged and the amount of energy the brain was using to maintain them.

"If you have healthy neurons but you're struggling to keep them repaired, it makes sense you won't be as cognitively sound," he said.

However another possibility was that NAA and choline might directly enhance or inhibit the function of neurons.

If that was true, it might be possible to improve intellectual performance with dietary supplements, said Mr Brooks.

Howard Gardner, a psychologist at Harvard University, warned that IQ test performance could be affected by many factors, such as the subject's health and motivation.

"Beware the intelligence equals IQ equation," he said.