Like many other substances with habit-forming qualities - both physical and psychological - fast food can trigger not just desire, but strong and urgent cravings that may lead to addictive behavior.
Additives and chemicals
Some research points to the idea that fast food is specifically designed to become habit-forming, while other information supports the idea that fast food restaurants simply use ingredients that are cheap. Either way, some of these substances can, indeed, be addictive. A milk protein called casein is often found in french fries and baked goods, for example, and it has been shown to trigger opioid receptors in the brain. MSG is also in many types of fast food - like taco meat, sauces, or burgers - which can surpress feelings of fullness, leading to overeating.
Sugar, fat and salt
Just as some of the additives and chemicals in fast food are addictive, a diet high in sugar, fat and salt can cause cravings for more. Sugar, in particular, spikes blood sugar and then causes a "crash," which triggers the craving for more sugar. Salt tolerance has also been shown to increase cravings - the more salt is consumed, the more you need in order to experience foods as tasting salty. Saturated fats can raise cholesterol levels and increase the risk for conditions like diabetes and obesity - the latter of which is characterized by the body's inability to receive hormonal signals about satiation and fullness. Over time, a tolerance develops and more high-fat, high-sugar foods are needed to experience the pleasurable feelings that comes from food.
Location
Fast food chains are strategically placed in low-income, highly populated areas. This doesn't mean there aren't fast food restaurants elsewhere, but low-income families and individuals often eat fast food because it's cheaper than buying groceries to make their own meals. This, of course, contributes to rising obesity rates. If a person wasn't "addicted" to fast food before, the simple act of being exposed to it over and over again in the community can encourage consumption.
Psychological dependence
Like any other type of drug or addictive substance, fast food can also cause a psychological dependence. If people are eating fast food because it brings about pleasurable feelings, this can become a habit - and eventually an addiction.
