If you can't resist eating an entire bar of chocolate in one go, the solution could be simple.

Unwrap it and break it into all of its individual chunks before tucking in.

Research shows that people eat less when presented with several small pieces of food rather than one large one even when the two contain the same number of calories.

It isn't entirely clear why this is so, but scientists believe it could be down to an optical illusion, with the brain and belly fooled into thinking that a bar of chocolate broken into pieces is bigger than one that remains whole.

Similarly, a muffin may suddenly seem more filling if cut into quarters before taking a bite.

US researchers watched more than 300 college students as they ate bagels.

Some were given a bagel that had been cut in four, while others were given a whole one.

Twenty minutes later, the volunteers were given another meal and told they could eat as much or as little of it as they liked.

Those whose bagel had been cut into pieces ate less of it, and also less of the later meal.

Another explanation is that cut-up food is eaten more slowly, allowing the body to realise it is full before the person has overeaten.

An experiment on rats backed up the results.

They were let loose in a maze where one direction led to 30 small food pellets and the other led to a single large pellet that was just as nutritious.

The rats were more likely to choose the route with 30 pellets and ran more quickly towards it.

Devina Wahera, lead author of the report by researchers at Arizona State University, told the Society for the Study of Ingestive Behavior's annual conference: 'Cutting up energy-dense foods into smaller pieces may be beneficial to dieters who wish to make their meal more satiating while also maintaining portion control.'