High fructose corn syrup is a product made from corn that contains a very high concentration of fructose sugar, making it super sweet. It is used in many processed foods like pop, cereals, and baked goods. High fructose corn syrup saves manufacturers money because less HFCS is needed to sweeten foods when compared with regular granular sugar.
In several studies with rats done at the University of Florida, high fructose corn syrup was shown to affect the hormone leptin by disabling it. Leptin is a digestive hormone that tells people when they are full, and thereby encourages them to stop eating. Unlike table sugar, high fructose corn syrup delays or removes that 'full' feeling, paving the way for high fat, high carb eating without any normal bodily feedback to stop.
When these studies came out (between 2004 and 2008) they got lots of press in highly respected nutritional journals and popular science magazines for laymen. The corn industry and the food industry launched an ad campaign that stresses that high fructose corn syrup is chemically identical to sugar, and at the same time, got the FDA to change food labeling to remove the words 'high fructose corn syrup' from the ingredients list on food packaging.
The commercials promoting high fructose corn syrup are deceptive, since it isn't the chemical composition of high fructose corn syrup that causes the problem, it's the concentrated nature of the product and its EFFECTS on the human body. HF corn syrup changes human metabolism (and rat metabolism), causing diabetes and making obesity easier to 'achieve' by removing the natural set point for fullness. Worse, it happens 'silently'--with no sign that it is happening and no warning until the pre-diabetic condition kicks in.