According to a new study from the School of Medical Sciences at Australia’s University of New South Wales, junk food can alter behavior by causing lasting changes in the brain’s reward circuiting, an alteration that triggers obesity.

Although the UNSW study was conducted on rats, the conclusions are likely applicable to humans because all mammals share similarities in the orbitofrontal cortex, which is involved in sensing and evaluating the pleasurable aspects of food. (Notice that we also see more overweight pets these days, too.)

Over time, animals (including humans) have evolved a simple mechanism that protects us from overeating: As we eat a particular food, the pleasure we get from eating it and the desire to eat more diminishes relative to other, uneaten foods. This phenomenon, called “sensory-specific satiety” reinforces the natural inclination to seek out a variety of foods, which promotes a healthy, balanced diet.

With junk food, this “shut-off valve” doesn’t work.

Nutrient-poor junk food, termed the “Cafeteria Diet” in this study, contains fatty acids that are not only associated with mitochondrial dysfunction, weight gain, and tissue inflammation, but also increased activity in the areas of the brain that process motivational control and reward behavior.

In the study, one group of rats were fed standard chow; the other, a diet of cookies, cakes, biscuits, and other junk food, for two weeks. They were then subjected to “Pavlovian conditioning tests” — in this case, a sound cue that informed the rats of the next serving.

Rats on a healthy diet that had already eaten ignored the sound cue, while the rats on junk food continued to the next serving even after being satiated. The junk-food rats lost the natural preference for novelty, an effect that lasted even after the rats returned to a normal diet.

In other words, eating junk food programs your brain to eat more junk.

“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,” said Dr. Amy Reichelt, lead author of the UNSW study.

“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,” said professor Margaret Morris, another UNSW team member.

The rats on a junk food diet had lost their natural preference for healthy foods, the team added.
 

Junk Food Mortality by the Numbers

The problem with junk food isn’t just that consuming it begets unnatural overeating; it’s also packed with unhealthful fats, toxic chemicals, hormones, mystery pharmaceuticals, fillers, and gross “natural flavors” like beaver-butt juice. Considering that eating all this garbage is practically like swilling Drano, junk food might be the most efficient depopulation strategy ever devised.

Obesity rates have doubled since 1980. In 2008, 500 million people were clinically obese, with body-mass indices greater than 30, while 1.4 billion were clinically overweight, with BMIs greater than 25. The United States is the epicenter of this crisis, as currently, two out of every three Americans are clinically overweight or obese.