Scientists have discovered an enzyme in the brain that seems to drive sugar cravings, which could lead to new anti-obesity drugs.

A protein responsible for the brain's sweet tooth could point the way to new drugs that prevent obesity.

Scientists have discovered an enzyme that seems to drive sugar cravings in the hypothalamus, a brain region that regulates various functions including food intake.

The enzyme, glucokinase, was already known to be present in the liver and pancreas.

In tests on rats, boosting the protein's activity in the brain caused the animals to consume more glucose in preference over their normal food.

Reducing glucokinase activity led to the rats eating less glucose.

Glucose sugar is a component of carbohydrates and the main energy source for brain cells.

Lead scientist Dr James Gardiner, from Imperial College London, said our brains rely heavily on glucose for energy.

"It's clearly a very important nutrient, but in our evolutionary past it would have been hard to come by. So we have a deep-rooted preference for glucose-rich foods and seek them out."

The evidence suggests that glucokinase in the brain plays a key role in driving our desire for sugary food.

"This is the first time anyone has discovered a system in the brain that responds to a specific nutrient, rather than energy intake in general," Dr Gardiner said.

"It suggests that when you're thinking about diet, you have to think about different nutrients, not just count calories."

A drug targeting glucokinase or its biological pathway could potentially prevent obesity, the scientists believe.

The discovery also has implications for the way people diet.

"People are likely to have different levels of this enzyme, so different things will work for different people," said Dr Gardiner.

"For some people, eating more starchy foods at the start of a meal might be a way to feel full more quickly by targeting this system, meaning they eat less overall."

AAP.