Junk food may seem like an addictive drug because it is. In rats, at least, too much fatty food raises the threshold for feelings of satisfaction, sparking a cycle of compulsive overeating.
In people, addictive drugs such as heroin and cocaine desensitise the brain by raising the threshold of 鈥渞eward鈥� activity that is needed to feel satisfied: more drug is needed to achieve the same effect.
and colleagues at the Scripps Research Institute in Jupiter, Florida, wondered if a diet rich in high-calorie, fatty food might also cause desensitisation and lead to obesity.
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They used electrodes to measure the sensitivity of rats鈥� brains to reward activity. Some ate normal rat food while others had limited or unlimited access to junk foods, tasty to both rats and humans. After 40 days, the brains of those that ate junk freely were less sensitive to reward activity than those in the other groups. They were also obese.
Compulsive eaters
To see if these rats would display compulsive eating in the face of negative consequences 鈥� a telltale sign of addiction 鈥� all the rats were taught that a flash of light led to a painful electric shock.
Rather than try to avoid the shock when the light came on, as the rats with limited or no access to junk food did, 鈥渁ddicted鈥� rats just kept on eating. 鈥淲e see the same thing in animals with extended access to cocaine,鈥� says Kenny. Like drug-addicted humans, the obese rats also had fewer receptors for the reward chemical dopamine.
The term 鈥渇ood addiction鈥� is slowly becoming accepted in the field of psychiatry, says , an addiction biologist at the University of Cincinnati in Ohio.
鈥淥nce we start to consider obesity and pathological overeating as a psychiatric illness we鈥檙e going to move a lot closer towards understanding how to come up with therapies or treatments,鈥� he says.
, a neuroscientist at Boston University, is not surprised that an excess of tasty food might 鈥渟hort-circuit鈥� the brain鈥檚 reward sensing system. 鈥淚n western societies, we are observing a sudden increase in food availability. We didn鈥檛 evolve to survive under these conditions,鈥� he says.
