Junk food could be addictive and stimulate cravings in much the same way as an illicit drug, a brain scanning study shows.
The findings suggest that limiting "high-glycaemic" carbohydrates that cause blood sugar to surge could help curb over-eating and obesity.
Scientists investigated how food intake is regulated by the brain's "pleasure centres" which are known to be linked to addiction.
Functional magnetic resonance imaging (fMRI) was used to observe the brain activity of overweight and obese volunteers for four hours after a meal.
This crucial period has been shown to influence behaviour the next time a meal is eaten.
A dozen overweight men were given test meals designed as milk shakes that tasted the same and were matched for calories.
One milk shake contained rapidly digested high-glycaemic index (GI) carbohydrates, often found in junk food, while the other was made with low GI carbohydrates.
Consumption of the high GI drink was followed by an initial surge in blood sugar levels followed by a sharp crash four hours later.
The sudden decrease in blood sugar was associated with excessive hunger and intense activation of the nucleus accumbens, a brain region central to addictive behaviour.
Lead scientist Dr David Ludwig, director of the New Balance Foundation Obesity Prevention Centre in Boston, US, said: "Beyond reward and craving, this part of the brain is also linked to substance abuse and dependence, which raises the question as to whether certain foods might be addictive.
"These findings suggest that limiting high-glycaemic index carbohydrates like white bread and potatoes could help obese individuals reduce cravings and control the urge to over-eat."
Alhough the study only looked at obese people, further studies could clarify whether avoiding processed carbs could help lean people avoid packing on weight, Dr Ludwig told medical website LiveScience.
The authors hypothesize that the sharp drop in blood sugar spurred by cookies, chips or cake may not only stimulate hunger, but also make the idea of eating seem more rewarding and pleasurable to the brain.
There's a strong evolutionary reason to have food linked to pleasure, Dr Ludwig said.
"The problem is when the modern, highly processed carbohydrates can hijack this evolutionarily based reward system, putting it, in effect, into overdrive," Dr Ludwig said.
"To put it simply, we didn't evolve to eat the low-fat Twinkie for breakfast."
The research is reported in the American Journal of Clinical Nutrition.
Good Carbs V Bad Carbs
The body typically breaks down carbohydrates into blood sugar, or glucose. Whole grains and carbohydrates in vegetables take a long time to break down, raising blood sugar slowly, whereas refined carbs such as sugar, corn syrup and white flour cause sharp spikes in blood sugar.
Research suggests that refined carbohydrates are tied to pleasurable feelings, but those studies typically compared enormously different foods: cheesecake versus steamed broccoli, for instance. That made it difficult to discern whether the carbohydrates, or some other part of the food, caused the cravings.
- PAA, www.nzherald.co.nz