Medary.com

Specifically, fructose (as in "high-fructose corn syrup") appears to cause more weight gain (in mice) than other sugar sweeteners like glucose, and that weight gain is not explained by calories alone.

All the mice began the study at an average weight of 39 grams. Those consuming the fructose-sweetened water showed significant weight gain over the course of the study, with an average final weight of 48 grams--compared with averages below 44 grams for the other groups--and had about 90 percent more body fat than the mice that consumed water only.

Total caloric intake was lower in the mice that consumed the fructose-sweetened water than in the other groups, except for the control animals provided with water only.

"We were surprised to see that mice actually ate less when exposed to fructose-sweetened beverages, and therefore didn't consume more overall calories," said Dr. Tschöp. "Nevertheless, they gained significantly more body fat within a few weeks."

I'm always astonished that it seems to come as a surprise to most dieticians and physicians that not all calories are the same. There's this little thing called biochemistry. This is the intricate system of chemical reactions which determines how the body processes food. It's driven by enzymes created by the body, and each person's body creates and processes enzymes at slightly different rates (just like everybody has a unique fingerprint). This means that everybody's biochemistry is a bit different. This is why one diet does not fit all. Some diets (i.e. controlled carbohydrate diets) directly address manipulating the body's biochemistry. Others ("portion control" starvation diets and "low-fat" diets) don't.