Why have rates of obesity skyrocketed in America and worldwide? Overeating and lack of exercise are major causes, but there may be other factors as well.

In recent years, some experts have pointed the finger of blame at high-fructose corn syrup, a sweetener that has replaced cane and beet sugar in many foods and beverages. According to this argument, high-fructose corn syrup reacts with the body's metabolism in a unique way that causes weight gain.

So, what is high-fructose corn syrup, the newest villain in the battle of the bulge?

What is it?

High-fructose corn syrup is a concentrated liquid made from corn starch, with added amounts of two substances -- fructose (the sugar found naturally in fruit) and glucose (another simple sugar). These substances help make high-fructose corn syrup about 75 percent sweeter than table sugar (sucrose), which is made of fructose and glucose.

Today, the average American consumes about 2.5 times as much added sugar as recommended by federal dietary guidelines. Nearly half of those sugars come from high-fructose corn syrup, according to the American Dietetic Association. This sweetener is used because it has several advantages over sugar, including being sweeter, less expensive and more easily mixed into foods and beverages.

Soft drinks and fruit drinks such as lemonade are a major source of high-fructose corn syrup. The sweetener is also found in cookies, gums, jams and jellies, and baked goods. Even healthful foods, such as low-fat yogurt and marinara sauce, may have significant amounts of high-fructose corn syrup.