Eating food with high levels of fat and carbohydrates activates an ingrained reward system in human brains which releases dopamine, a new study has found.
The study, which was published earlier this month by Germany's Max Planck Institute for Metabolism Research, found that foods which combine both high fat and carbohydrate levels in particular further reinforce the observed neuronal effect.
The researchers, who were from the institute and Yale University in the United States, pointed out that the combination of high fat and carbohydrate levels in foods rarely occurs in nature.
Unprocessed food is either high in fats, as exemplified by nuts, or high in carbohydrates, as exemplified by potatoes. A notable exemption to the rule is the breast milk consumed by all mammals.
Researcher Marc Tittgemeyer explained that humans had consequently most likely evolved to "react intensely" to nutrition which was both high in fat and carbohydrates, experiencing a pleasant dopamine rush, because mother milk was necessary for the survival of infants.
The researchers invited 40 participants to play a computer simulation game where they would bid money to secure a reward of food with different calorie qualities.
At the same time, magnetic resonance imaging (MRI) technology was used to observe how the food on offer triggered brain activity in the players of the game.
Both in terms of the money offered in the game and the MRI results obtained, participants showed a strong conscious and neuronal preference for food which was simultaneously heavy in fat and carbohydrates.
Dr Tittgemeyer warned, however, that what might once have been a useful survival mechanism has now become a serious health risk to human societies which enjoy an abundance of food, including naturally rare variants with high fat and carbohydrate levels. The result was a rapidly growing incidence of obesity and related diseases.
"We did not evolve to say 'no' all the time. As a consequence, we usually do not stop eating when we are already satiated," he said.
The findings suggest that the reward signal sent to the brain when fatty and carbohydrate-heavy food is consumed is more powerful than the sensation of satiation experienced during eating.
The institute hopes to build on this insight in the development of future therapies to treat obesity.