High-calorie food can be just as addictive as smoking or the abuse of drugs suggests work published online this week in Nature Neuroscience. Although the findings cannot be directly transferred to human obesity, the study shows that overconsumption of high-calorie food can trigger addiction-like responses in the brain, and that junk food can turn rats into compulsive eaters in a laboratory setting. Addicts are known to show blunted activation of brain circuits responsible for reward in response to normally positive experiences. Paul Kenny and colleagues measured rats' sensitivity to rewarding experiences. When the researchers regularly offered rats a choice of high-calorie foods such as bacon, sausage, cake, and chocolate, in addition to their regular, healthier but less appetizing chow, the animals over-consumed calories and gained weight rapidly. Their reward sensitivity also plummeted, as has been shown before for addictive drugs. This blunting of reward response persisted for at least two weeks after the high-calorie food was no longer available.
A true addict, whether rat or human, will also compulsively consume their drug even when it is clearly detrimental to their own health. To test this, the team trained their rats to expect painful foot shocks when seeing a light signal. Although normal rats stop eating even the most delicious junk food when the light comes on, the obese rats used to a high-calorie diet just kept feeding.
The scientists also found decreased levels of a specific dopamine receptor in the overweight rats, as has been reported in humans addicted to drugs. Artificially decreasing levels of this dopamine receptor in another group of rats accelerated their loss of reward sensitivity when given access to the high-calorie food diet.