In the second installment in our Psychology of Food series, Angela Mulholland looks into how sweet, salty and fatty foods can change the way we eat and think.

A little over a month ago, a group of rats living in Florida became mini celebrities.

They were ordinary in every way except these rats had developed a deep love for sausages, fries, cheesecake, candy bars -- just about any junk food you could send their way. In fact, the rats were so hooked, they led the research team studying them to conclude that junk food could become as addictive as heroin.

The rats had spent weeks eating only unhealthy foods and had grown to love them so much, they were even willing to withstand electrical shocks to get at them. Even more surprising was that when the junk food was suddenly taken away from them and replaced with healthy food, the rodents refused to eat altogether.

"The change in their diet preference was so great that they basically starved themselves for two weeks after they were cut off from junk food," recounted the lead researcher, neuropsychopharmacologist Paul J. Kenny of the Scripps Research Institute.

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Kenny's team had other evidence that junk food could become addictive. They noted that when they studied the rats' brains, they showed distinct addiction-like changes, with altered D2 dopamine receptors, like those seen in heroin addicts who had developed a tolerance for the drug.

Like addicts, the more junk food the rats ate, the more they overloaded their brains' reward centre, and so the more food the rats wanted. The researchers concluded that junk food could become as addictive as drugs.

Media outlets pounced on the story. And while it was a study that had many people talking, it also made waves in the medical world among experts who study the root causes of obesity.

These animals preferred to eat nothing than to eat healthy. 

-- Dr. Valerie Taylor


Dr. Valerie Taylor, an assistant professor in psychiatry and behavioural neuroscience at McMaster University in Hamilton, Ont. says the study offered groundbreaking evidence that certain foods can override the circuitry of the brain.

"That study has done a lot to get people to think a little differently about the whole concept of addiction and whether it's possible to become addicted to food. So the science is starting to catch up with what those of us who work clinically have seen for a while," she told by phone.

"These animals preferred to eat nothing than to eat healthy. They'd been programmed that way, which is fairly significant. And it goes to show why some people really have difficulty sticking to a diet. It can be impossible."

Food manufacturers add 'hedonic value'

Dr. David Kessler knows all too well how hard it is to resist junk food. After struggling for years with his weight, the former commissioner at the U.S. Food and Drug Administration researched and wrote a bestseller, called the "The End of Overeating."

In the book, released last year, Kessler argued that food makers and fast food outlets are fully aware that humans can't get enough of fatty, salty and super-sweet foods. That's why they've learned to layer fat, salt and sugar into their foods -- sometimes all at the same time (think: egg, bacon and pancake breakfast sandwiches.)

"Combined in the right way, they make a product indulgent, high in "hedonic value," Kessler wrote.

Why we crave salt, sugar and fat isn't clear, though perhaps it has an evolutionary basis, a holdover from our caveman days when our bodies learned to seek out high-calorie food that would build up fat stores so we could survive until we killed the next water buffalo.

What science does know is that fat, salt and sugar stimulate the brain's reward system. These tastes light up the brain's pleasure centre -- the dopamine pathway -- the same brain area that lights up in people addicted to alcohol or drugs. And, as Kessler explains, the more we activate this neural circuitry in our brains, the more we crave those foods, even when we're not physically hungry.

"Chronic exposure to highly palatable foods changes our brains, conditioning us to seek continued stimulation. Over time a powerful drive for sugar, fat and salt competes with our conscious capacity to say no," Kessler wrote.

Addicted to the 'food experience'

It's not just the food itself that becomes addictive, Kessler argued; it's the whole experience of eating it that we become conditioned to seek out.

"What the food industry did was to take fat, sugar and salt, put it on every corner, make it available 24/7, make it socially acceptable to eat any time," Kessler wrote. "We've added the emotional gloss of advertisement, we've made the food into entertainment, and we're living the consequences."

With all those forces in our brains compelling us to seek out junk food, it's no wonder so many of us confess to being "chocoholics" or junk food junkies.

But Dr. Yoni Freedhoff, the director of the BMI clinic in Ottawa, doesn't think there is anything unique in the chemistry of junk food that explains why people overeat. He notes french fries and chocolate bars are hardly new, having been mass produced for decades, yet the obesity crisis has only exploded in the last generation.

"What's changed is the environment," Freedhoff told by phone.

‘"Dr. Kessler might say that there have been changes to foods to make them more palatable, and indeed, that is possible -- why not? The food industry is designed to sell food, so I would hope the research they do would make food more palatable.

"But the pace of the world has changed. People aren't eating the way they used to."

Freedhoff believes that when people eat in disorganized ways -- by skipping breakfast, snacking on junk food, not stopping to cook their own meals – they become more likely to turn to easily accessible and highly processed foods.

Though many of these foods will fill our bellies, loaded as they are with calories, they are quickly-digested carbohydrates and low in satisfying nutrients, such as protein and fibre. So what often happens, Freedhoff says, is that people overeat these foods in the evening, sometimes consuming a full day's calories in a matter of hours. They then wake up, skip breakfast, eat little during the day, until they become so desperately hungry, they seek out more fast food or junk food, and the cycle repeats itself.

He says many people who eat in a disorganized way say they don't feel like they have any control over their eating. And, in fact, they're probably right.

"When they eat with that pattern of eating, they aren't in control, so they do feel like that they are at the mercy of food. The feeling that food is ruling their lives is real to them," he says.

"But in these cases, if you can get them eating better and more organized, you can often turn it off. Most of those people, when we suggest to them that perhaps it's just a matter of getting them to eat in a different pattern, they never believe it's going to make a difference. But then they're pleasantly surprised when it often does."

Disorganized eating largely to blame

Freedhoff says he disagrees with Kessler that the way to beat junk food addiction is simply to avoid it at altogether.

"We don't ever tell patients to avoid the food they love. To expect them to go through the rest of their lives avoiding foods they love because they're scared of them is a very unreasonable expectation… It sets you up to feeling incredibly guilty when you fail and it's a recipe to fall completely off the horse.

He maintains that if people eat full, satisfying meals properly throughout the day, and they have plenty of protein, they'll feel full and can then indulge a little in the treats they love, knowing they will be able to control themselves.

"I believe in eating the smallest amount of the bad stuff that you need to be happy. And the only way to answer that question properly is to have eaten properly throughout the day… If your body is not telling you to eat, you're left with your brain. And if your brain says, ‘You know what? I don't need that much.' you'll be able to listen."

So while those cheesecake and sausage loving rats in Florida might remain slaves to salt, sugar and fat, there's hope that humans don't have to.

Are you struggling with food addiction? Check out our resources and links on food addiction and portion control for help.