This is an interesting article in today's Times (in the UK):-

Oh, just one more ... How chocolate really goes to your head

Chocolate triggers the production of a chemical that encourages Lori Eanes/The Food Passionates/Corbis

Chocoholics, take note. Scientists have shown that chocolate is irresistible for a good reason: it triggers the production of a natural, opium-like chemical in the brain that encourages over-eating.

A study published yesterday in the scientific journal Current Biology traces the urge to eat sweet and fatty treats to an area of the brain not previously thought to be involved.

“This means that the brain has more extensive systems to make individuals want to over-consume rewards than previously thought,” said Alexandra DiFeliceantonio, who helped to carry out the study at the University of Michigan.

Ms DiFeliceantonio’s team made the discovery in rats when they injected a part of the brain called the neostriatum with enkephalin, an opium-like chemical produced naturally in this brain region.

“We found that the injected rats gorged on an amount of M&M chocolates more than double that what they normally would,” said Ms DiFeliceantonio. “Most rats ate over 17 grams of M&Ms, equal to about 5 per cent of their body weight. That level of consumption is equal to an average person eating about 3.6 kilograms of M&Ms in a single hour, and is clearly overriding normal satiety signals.”

To see whether the extreme eating behaviour they saw with an injection of enkephalin could occur naturally, the researchers next studied whether the chemical was produced when the rats were presented with a large amount of M&Ms.

“We used a technique called micro-dialysis to collect chemicals produced in the neostriatum region,” Ms DiFeliceantonio said, “and we saw a huge surge of enkephalin when the chocolates were given to the animals and while they were eating them. This is the first study to show an opium-like surge in the brain in response to a treat.”

The findings reveal a surprising extension of the neostriatum’s role, as this brain region was thought to be primarily involved in the control of movement. To confirm that the enkephalin surge was not being triggered by movements made by the rats while munching on the M&Ms, micro-dialysis was carried out when the rats were gnawing on wood, grooming, or walking. However, no surge in enkephalin was found during such activities.

Further studies by the researchers indicated that the enkephalin surge does not make rats like the chocolates more but rather that these brain chemicals increase their desire and impulse to eat them.

Ms DiFeliceantonio said: “Our findings are interesting in light of previous imaging studies in humans which showed that this region of the brain lit up when obese binge-eaters were shown food cues and cocaine addicts were shown those relating to the drug.

“What we have discovered about the underlying basis of this response could therefore be of future importance in designing new ways to diagnose and treat binge-eating disorders, drug addiction and other compulsive behaviours.”