When it comes to cellphones and brain health, the results have been mostly reassuring. The largest and longest studies done so far have no increased risk of tumours in cellphone users.

But now a new study in the Journal of the American Medical Association suggests there may be something else going on in the brains of cellphone users.

U.S. government researchers have found that just turning on a phone and keeping it near your head sets off biochemical functions in the brain.

In the first study of its kind, researchers with the National Institutes of Health found that 50 minutes of cellphone use led to an increase in brain glucose metabolism -- which is a marker of brain activity -- in the regions closest to the phone's antenna.

For the research, Dr. Nora D. Volkow, the director of the National Institute on Drug Abuse, led a team who took 47 healthy people and scanned their brains while they held cellphones to their ears.

They used PET imaging (positron emission tomography) to measure how their brains used glucose -- which is the primary fuel used by brain cells.

They measured the glucose metabolism when the phones were off, and found the levels normal. But when the phones were turned on and connected with the sound muted, there was a seven per cent boost in glucose use in the area next to the ear.

Only the areas of the brain closest to the phone's antenna were activated -- the orbitofrontal cortex and temporal pole. Areas that were far away saw no effect.

"It tells us acute exposure to cell phone will actually activate the brain and make it consume more glucose," Volkow told CTV News

"We showed with the study that indeed, the human brain is sensitive to the [radiofrequency-modulated electromagnetic fields] emitted by cellphones," says Volkow.

So what does it mean that there is more activity in the areas of the brain near the cellphone?

The authors can't say. They're not even clear that this increased brain activity is necessarily harmful in either the long or the short term. They can only say the finding was unexpected and intriguing and deserves further study.

One worry is that brain tumours use glucose to grow so an increase in glucose activity may not be good. But previous studies have been inconclusive about whether cellphone users face an increased brain tumour risk.

Another worry is that as cell phone use increases among kids, their smaller, still-developing brains may face an increased risk from the radiofrequency emissions. But again, studies need to be done to investigate.

Until those studies are done, Dr. Rolando F. Del Maestro of the Montreal Neurological Institute says this new data should prompt people to use cellphones with caution.

"In people who are using cell phones a lot, this suggests to me you would be wise to use a hands-free system, if you possibly can. It just makes logical sense that if something is changing brain metabolism, you probably don't want that near your ear," he said.

With a report from CTV's medical specialist Avis Favaro and producer Elizabeth St. Philip