This post is rewritten and expanded from a blog comment I made here.
In human biology, the part of the brain that controls appetite is called the hypothalamus, and it responds to four different hormones: insulin, leptin, CCK, and ghrelin. The first three signal when one has had enough to eat and the last inhibits the function of the first three. With normal foodstuffs, the satiety signal appears readily enough. However, food manufacturers through the application of science can now outwit our hormones so that, for instance, a 32-oz Big Gulp no longer seems like a lot of liquid to drink because the normal “I’m full” signal never reaches the brain even though the liquid reaches the bladder. Fructose and high-fructose corn syrup (more commonly known as HFCS, which has roughly equal parts fructose and glucose), major ingredients in soda, in particular fail to stimulate production of these hormones and are accordingly regarded as toxins or poisons by many dieticians. It might sound conspiratorial to suggest that food makers have purposely substituted HFCS for other sweeteners precisely to forestall the feeling of fullness, but then, no one believed the conspiracy to load cigarettes with addictive nicotine for a long while, either.