A recent study showed that junk food not only makes rats fat, but also reduces their appetite for novel foods, a preference that normally drives them to seek a balanced diet.
The team of researchers, led by Professor Margaret Morris, Head of Pharmacology from the School of Medical Sciences, UNSW Australia, taught rats to associate each of two different sound signals with a particular flavor of sugar water - cherry and grape.
The study explained how excessive consumption of junk food could change the behavior, weaken self-control and lead to overeating and obesity. Healthy rats, raised on a healthy diet, stopped responding to signals linked to a flavor in which they have recently overindulged. This inborn mechanism, widespread in animals, protects against overeating and promotes a healthy, balanced diet.
After being on cafeteria foods, that included pie, dumplings, cookies, and cake containing 150% more calories for two weeks, the rats showed an increase in weight by 10% and their behavior changed dramatically. They became indifferent in their food choices and no longer avoided the sound advertising the over-familiar taste. This indicated that they lost their natural preference for novelty, which lasted for some time after the rats returned to a healthy diet.
The researchers speculate that junk food causes lasting impressions in the reward circuit parts of the rats' brain, for example, the orbitofrontal cortex, an area of the brain responsible for decision-making. They say these 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.
Professor Morris in a statement said , "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. 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."
Estimates by The World Health Organisation show that more than 10 per cent of the world's adult population is obese. It has been recorded by the WHO that at least 2.8 million people die each year from of being overweight or obese, which are the the major factors leading chronic diseases like diabetes, heart disease and cancer.
Lead author of the research paper Dr Amy Reichelt said, "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."
The findings were reported in the journal Frontiers in Psychology