The new study could explain why excessive consumption of unhealthy ‘junk foods’ that are high in sugar salt and fat can change behaviour, weaken self-control and lead to overeating and obesity, say the team behind the paper.
Writing in Frontiers in Psychology, investigated with whether exposure to a ‘cafeteria’ style diet consisting of high fat foods has an impact on the ability of rats to learn about food-associated cues and the sensory properties of ingested foods – finding that such a diet not only makes rats fat, but also reduces their appetite for novel foods, a preference that normally drives them to seek a balanced diet.
“We observed that rats fed a cafeteria diet for 2 weeks showed impaired sensory-specific satiety following consumption of a high calorie solution. The deficit in expression of sensory-specific satiety was also present 1 week following the withdrawal of cafeteria foods,” explained the team, led by Professor Margaret Morris of UNSW Australia. “Thus, exposure to obesogenic diets may impact upon neurocircuitry involved in motivated control of behaviour.”
"The interesting thing about this finding is that if the same thing happens in humans, eating junk food may change our responses to signals associated with food rewards," added Morris. "It's like you've just had ice cream for lunch, yet you still go and eat more when you hear the ice cream van come by."
The team taught young male rats to associate each of two different sound cues with a particular flavour of sugar water – cherry and grape.
After this the team fed the rats on either a healthy control or a cafeteria style diet including pie, dumplings, cookies, and cake. Both sets of rats were then exposed to flavoured water before testing the trained response to cues for that flavour.
In healthy rats, raised on a control diet, Morris and her team found that rats stopped responding to cues linked to a flavour in which they have recently overindulged. This inborn mechanism, widespread in animals, protects against overeating and promotes a healthy, balanced diet, they suggested.
However, rats fed for two weeks on a diet that included daily access to cafeteria foods with 150% more calories caused significant changes in weight and also in feeding behaviour.
According to the researchers, the rats' weight increased by 10%, while they also became indifferent in their food choices and no longer avoided the sound advertising the over-familiar taste. This indicated that they had lost their natural preference for novelty, suggested the team – adding that the change in behaviour even lasted for some time after the rats returned to a healthy diet.
Morris and her colleagues suggested that this sort of ‘junk diet’ causes lasting changes in the reward circuit parts of the rats' brain, for example the orbitofrontal cortex, an area of the brain responsible for decision-making.
They added that their results may have implications for people's ability to limit their intake of certain kinds of foods, because the brain's reward circuitry is similar in all mammals.
"As the global obesity epidemic intensifies, advertisements may have a greater effect on people who are overweight and make snacks like chocolate bars harder to resist,” suggested Dr Amy Reichelt, first author of the paper.
Source: Frontiers in Psychology
Published online ahead of print, doi: 10.3389/fpsyg.2014.00852
“Cafeteria diet impairs expression of sensory-specific satiety and stimulus-outcome learning”
Authors: Amy C. Reichelt, Margaret J. Morris, R. F. Westbrook