Sydney researchers have found that fruit flies fed a steady diet of artificial sweeteners eventually started eating more and became more hyperactive.

Chronic use of artificial sweeteners changed pathways in the brains of fruit flies causing them to eat more, a new Australian study has found.

The findings should quash the assumption that artificial sweeteners, commonly used in diet soft drinks, do nothing to change the way the brain regulates appetite and taste perceptions, say the authors of the study published in the respected journal Cell Metabolism.

Researchers at the University of Sydney's Charles Perkins Centre and the Garvan Institute of Medical Research studied fruit flies exposed to a diet laced with synthetic sweetener sucralose for long periods of time.

Initially they didn't see much change but after five days the flies started eating more and became more hyperactive.

Further testing found they were much more sensitive to real sugar.

"It turns out that these animals were acting like they hadn't eaten, even though they had eaten," said co-author, Associate Professor Greg Neely.

The flies consumed 30 per cent more calories of naturally sweetened food.

Chronic consumption of this artificial sweetener actually increased the sweet intensity of real nutritive sugar, and this then increased the animal's overall motivation to eat more food.

"When sweetness versus energy is out of balance for a period of time, the brain recalibrates and increases total calories consumed," the authors said.

According to Prof Neely, the sweet taste sends signals to the brain that integrate with an energy sensor called AMPK. Together they work to change the message sent from reward neurons, such as dopamine.

"So basically when the sweetness was out of balance with the energy, those neurons switched and sent down a response that changed the motivation to feed and changed the sweet taste sensation," he said.

Billions of people worldwide consume artificial sweeteners, and are often prescribed them as a way to lose weight, despite little being known about the effects on the body, in particular the brain.

This is the first study to identify a complex neuronal network that responds to artificially sweetened food.

Care needs to be taken when extrapolating the findings to humans but the authors say this is important research.

"These findings further reinforce the idea that "sugar-free" varieties of processed food and drink may not be as inert as we anticipated.

"Artificial sweeteners can actually change how animals perceive the sweetness of their food, with a discrepancy between sweetness and energy levels prompting an increase in caloric consumption," Prof Herzog said.