Pretty much every ingredient behind a fast food counter is designed to make you supersize your meal. For instance, trans fat inflames the gut, potentially impairing the body's ability to produce appetite-controlling neurotransmitters such as dopamine and serotonin, Dr. Decotiis says. Meanwhile, the GI tract absorbs high fructose corn syrup (commonly found in buns, condiments, and desserts) quickly, causing insulin spikes and even bigger hunger pangs. Lastly, fast food's huge helpings of salt can spur dehydration. And with symptoms that closely mimic those of hunger, it's easy for dehydration to trick you into thinking you need to go back for seconds.