High-fructose corn syrup (HCSF) is a naturally occurring substance (albeit a chemically produced one) which derives from corn. Its primary function is that of a cost-efficient alternative to sugar. 

Studies done by the American Journal of Clinical Nutrition have shown a link between high-fructose corn syrup and obesity. Another recent study at Princeton University concluded that laboratory rats given high-fructose corn syrup gained twice the amount of weight as rats given identical amounts of regular sugar. Researchers concluded that the brain reacts to high-fructose corn syrup in a way which causes inefficient levels of appetite suppression. Avoiding products which contain high-fructose corn syrup is a strategy toward avoiding obesity that has been proven successful. 

• Read nutrition labels and avoid HFCS whenever possible.
• Choose alternatives to junk food, like fruit or foods sweetened naturally.
• Avoid processed foods in general. The more processed a food, the more additives it will have and the more likely any sweetness will be derived from HFCS.