When we are surrounded by sugary desserts and junk food, the balance is disturbed.

London - It is a truth universally acknowledged that however full you are, there’s always room for dessert.

And now there’s a scientific explanation – courtesy of Leon Trotsky’s great-granddaughter.

Dr Nora Volkow says that our junk food culture means the chemical signals produced by the stomach to say we are full can no longer override the brain’s pleasure centres.

Throughout evolution, these two systems have co-existed to control fullness. But today, when we are surrounded by sugary desserts and junk food, the balance is disturbed.

“That is what dessert is all about,” Volkow said. “They are bringing you a food that can overcome the satiety signals, so even though you are full, you eat it because of the pleasure it generates.”

Describing the stomach-based system, Volkow, director of the US National Institute on Drug Abuse in Maryland, said: “The feeling of satiety, which is a central component of this system, is triggered when the stomach is stretched beyond a certain threshold.

“At this point, chemical and neural signals are sent to the brain that instruct it to stop eating.

“This mechanism has evolved over hundreds of thousands of years of human evolution, and it has served us well in an environment that wasn’t particularly plentiful in terms of food, let alone foodstuffs with high-caloric content.”

The second, highly complex system, is in the brain and controls our motivation to eat and the pleasure food gives us.

The two systems constantly talk to each other and have long co-existed in harmony, keeping calorie intake, and so weight, in check. But today, when we are surrounded by desserts and junk food, the balance can be disturbed, leaving the signals from the full stomach unable to override the powerful wanting produced by the brain.

Those who want to exert more self-control should try saying “no” before temptation is placed in their way.

Volkow said: “If you know they are going to be bringing out these wonderful cakes, say to yourself, ‘I am not going to eat them,’ and you are much more likely to not eat them than if someone just put them in front of you.”

The researcher, whose great-grandfather was Russian revolutionary Leon Trotsky, added that food could be addictive, acting on the same brain pleasure centres as hard drugs, cigarettes and alcohol.

Perhaps not surprisingly, apples and lettuce are among the least addictive foods. Those most likely to have us hooked include salty fast food, sugary and fatty biscuits and pastries and ice cream.

She said solutions to the obesity crisis included subsidising the food industry to come up with recipes for healthy foods that tasted good.

British Association for Psychopharmacology president Barbara Sahakian, a Cambridge University professor who has run eating disorder clinics, said the thought of a different taste could reawaken appetite.

She said: “That’s the big dessert effect we all experience. Maybe we can’t even get through our whole meal, we eat three-quarters of the main course and can’t eat the rest of it.

“Then you sit for five or 10 minutes and they bring round the dessert trolley or menu and suddenly you feel like you can eat again.”

She advises pudding-lovers who don’t want to overeat to decide on dessert at the start of the meal, allowing them to tailor other courses with the last in mind. – Daily Mail