Paul Kenney and his colleagues at the Scripts Institute in Florida took three groups of rats and fed them all a balanced, healthy diet. However, one of the three groups was given access to junk food (Ding-Dongs, cheesecake, bacon, etc.) for one hour per day, and another group was given unlimited access.
The unlimited access group soon shunned the healthy food and quickly gained lots of weight. These rats were found to have low levels of dopamine, which is also low in humans addicted to drugs. They also were no longer deterred by the threat of a mild shock!
PositiveTip: Free access to highly palatable, high-fat foods can trigger addictive responses in rats--and probably people as well! It's a good idea to skip junk foods.