Limiting fructose may boost weight loss, researcher reports

One of the reasons people on low-carbohydrate diets may lose weight is that they reduce their intake of fructose, a type of sugar that can be made into body fat quickly, according to a researcher at UT Southwestern Medical Center.

Dr. Elizabeth Parks, associate professor of clinical nutrition and lead author of a study appearing in a current issue of the Journal of Nutrition, said her team’s findings suggest that the right type of carbohydrates a person eats may be just as important in weight control as the number of calories a person eats.

Current health guidelines suggest that limiting processed carbohydrates, many of which contain high-fructose corn syrup, may help prevent weight gain, and the new data on fructose clearly support this recommendation.

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

“All three can be made into triglycerides, a form of body fat; however, once you start the process of fat synthesis from fructose, it’s hard to slow it down,” she said.

In humans, triglycerides are predominantly formed in the liver, which acts like a traffic cop to coordinate the use of dietary sugars. It is the liver’s job, when it encounters glucose, to decide whether the body needs to store the glucose as glycogen, burn it for energy or turn the glucose into triglycerides. When there’s a lot of glucose to process, it is put aside to process later.

Fructose, on the other hand, enters this metabolic pathway downstream, bypassing the traffic cop and flooding the metabolic pathway.

"It’s a less-controlled movement of fructose through these pathways that causes it to contribute to greater triglyceride synthesis," Dr. Parks said. "The bottom line of this study is that fructose very quickly gets made into fat in the body.”

Though fructose, a monosaccharide, or simple sugar, is naturally found in high levels in fruit, it is also added to many processed foods. Fructose is perhaps best known for its presence in the sweetener called high-fructose corn syrup or HFCS, which is typically 55 percent fructose and 45 percent glucose, similar to the mix that can be found in fruits. It has become the preferred sweetener for many food manufacturers because it is generally cheaper, sweeter and easier to blend into beverages than table sugar.

For the study, six healthy individuals performed three different tests in which they had to consume a fruit drink formulation. In one test, the breakfast drink was 100 percent glucose, similar to the liquid doctors give patients to test for diabetes — the oral glucose tolerance test. In the second test, they drank half glucose and half fructose, and in the third, they drank 25 percent glucose and 75 percent fructose. The tests were random and blinded, and the subjects ate a regular lunch about four hours later.