When rats raised on regular chow were suddenly given unrestricted access to a high-fat diet, they lost complete control over their eating. Not even mild foot shocks kept them from compulsively feasting on chocolate bars, cream-stuffed cakes, sausage, frosting and other highly palatable human foods. Within 40 days, their body weight had increased 25 per cent.
The rats not only got fat, they also showed addiction-like changes in brain reward circuits - the same changes that have been reported in humans addicted to drugs.
Specifically, the obese rats showed lower levels of a receptor in the brain called the dopamine D2 receptor. The D2 receptor responds to dopamine, the chemical associated with feelings of reward. The brain releases bursts of dopamine when we eat food that tastes good.
The more junk food the rats ate, the more they overloaded the brain's reward circuitries until they essentially crashed. As the pleasure centres in the brain became more and more blase, and less responsive, the rats quickly turned into compulsive overeaters. They were motivated to keep eating to get their fix.
"They're in a state of reward deficit, so that they're now even more motivated to obtain rewarding food, perpetuating this vicious cycle even further," said study co-author Paul Kenny, an associate professor at Scripps Research Institute in Florida.
The findings, published Sunday in an advance online edition of the journal Nature Neuroscience, could have profound implications for the millions of Canadians struggling to control their eating.
"What this is telling you is that, if you persist in eating food that you know is bad for you, there is a chance that you will develop a habit, and you will keep on going back to that food unless you make a really strong, conscious effort to stop it," Kenny said
"It's incumbent upon people to make sure that they're more respectful and aware of what they're eating. Just be aware that there are dangers and risks associated. Enjoy (high-fat) food but make sure it's occasionally and very-well controlled. Don't overindulge repeatedly, because there could be repercussions."
Dr. Valerie Taylor, an assistant professor in psychiatry and behavioural neuroscience at McMaster University in Hamilton, said the study is a validation "that some people are simply more vulnerable to the whole concept of being addicted to food."
"The fact that we're now in this high-temptation environment further serves to exacerbate that."
Taylor said the study provides "very strong evidence supporting what a lot of us who work in the field have seen clinically - that, for some people, it's more than just simply willpower. There's something else going on."
According to the latest estimates from Statistics Canada, 37 per cent of the adult population age 20 to 69 - 7.9 million people - are overweight. Another 24 per cent - 5.3 million - are obese.
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Written by: Sharon Kirkey