People on low-carb diets lose weight in part because they get less fructose, a type of sugar that can be made into body fat quickly, according to a researcher at UT Southwestern Medical Center.

The lead author of the study, Dr. Elizabeth Parks, said it shows that the type of carbs someone eats can be as important as the amount.

"Our study shows for the first time the surprising speed with which humans make body fat from fructose," Parks said. Fructose, glucose and sucrose, which is a mixture of fructose and glucose, are all forms of sugar but are metabolized differently.

Though fructose is naturally found in high levels in fruit, it is also added to many processed foods, especially in the form of high-fructose corn syrup, which is found in many processed foods, including those that would not be thought of as sweets (See slideshow).

For the study, six healthy people performed three different tests involving drinking various mixes of glucose and fructose. Researchers found that fructose turned into body fat much more quickly, and that having it for breakfast changed how the body handled fats at lunch.

"The carbohydrates came into the body as sugars, the liver took the molecules apart like tinker toys, and put them back together to build fats. All this happened within four hours after the fructose drink. As a result, when the next meal was eaten, the lunch fat was more likely to be stored than burned," Parks said.

Parks said that people trying to lose weight shouldn't eliminate fruit from their diets but that limiting processed foods containing the sugar may help.