The definition of addiction can be applied to many substances, including amphetamine, heroin, cocaine, nicotine, caffeine and... fast food. Research shows that fast food effects on the brain are similar to what happens with narcotics.

The addiction isn't quite as strong and the social consequences aren't nearly as bad, but the addiction is real for sure and does have terrible consequences for our health and wellbeing.
A lot of us may know a person who has started to get physical warning signs from eating unhealthy, such as increased weight and chronic fatigue. That same person might have received some cautionary words from the doctor, but is still unable to quit eating fast food. The person might give it a shot, and fail, maybe give it a few more shots and fail again, and eventually give up.
This person will most likely end up obese, with raving metabolic syndrome and type II diabetes. Never, at any point, was that same person able to summon the willpower to quit eating fast food. That is because fast food effects include causing neuro-chemical addiction in the brain.
Fast food effects on the brain
Two things that are often found in fast food are sugar and wheat, the two things that seem to be key players in causing the epidemic of diseases of civilization. There are actual research studies showing how these foods cause changes in brain chemistry, involving neuro-transmitter receptors for opiates and dopamine.
When sugar is fed to rats, their brains experience neuro-chemical and behavioral changes that are similar to them consuming narcotics. These changes are specifically found in opiate and dopamine receptors in an area of the brain called the nucleus accumbens.
Another study in rats discovered that down regulation of dopamine receptors is correlated with development of obesity, and this is similar to what happens with reward homeostasis in heroin and cocaine addicts. The rats ate such things as chocolate, frosting and cheesecake.
Gluten proteins in wheat form opioid peptides in the digestive tract Opioid peptides are short sequences of amino acids that are able to pass through the blood-brain barrier and cause stimulation of opioid receptors in the brain.
Substances we know that can do this as well include heroin and morphine, along with natural peptides such as the endorphins that we release automatically after exercise.
There are some studies showing that glutens in wheat can contribute to development of schizophrenia and autism in susceptible people, but it is a known fact that gluten proteins can degenerate the body's intestinal lining. These peptides are thought to be able to enter the bloodstream through this mechanism.
(Source: EzineArticles)