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Fructose May Make You Fatter

Study: Simple Sugar Turns to Fat With “Surprising Speed”
By
WebMD Health News
Reviewed by Louise Chang, MD

July 31, 2008 -- Dieters know to limit their sugar intake, but new research suggests that not all sugars are equal when it comes to packing on the pounds.

Research from the University of Texas Southwestern Medical Center (UTSW) shows that the body turns fructose into fat more efficiently than it does other sugars.

"Our study shows for the first time the surprising speed with which humans make body fat from fructose," lead author Elizabeth Parks, PhD, of UTSW's Center for Human Nutrition.

The findings might be interpreted as confirmation that high-fructose corn syrup -- the much maligned sweetener added to many processed foods -- really does cause more weight gain than the other sugars we eat.

But it isn't that simple. Not by a long shot.

Sugars: Fructose, Glucose, and Sucrose

Parks and her research team studied the simple sugar fructose, not high-fructose corn syrup or sucrose, which are a mix of fructose and glucose.

Just about all the sugar we eat in foods, including those in fruits, contain some fructose and glucose.

"The goal was not to test the effects of high-fructose corn syrup," Parks tells WebMD. "The study didn't address that."

Instead, the researchers wanted to find out if fructose is more likely to lead to fat development than glucose.

They did this by feeding six healthy people breakfast drinks containing three different sugar combinations followed by a carefully controlled lunch over several weeks.

In one test, the breakfast drink contained 100% glucose, similar to the oral glucose test doctors give when they suspect diabetes. In another, the drink was half glucose and half fructose, and in the third, the drink was 25% glucose and 75% fructose.

The researchers measured the conversion of the sugars to fat in the liver and how the morning sugar meal influenced the metabolization of foods eaten later in the day.

They found that lipogenesis -- the process by which sugars are turned to body fat -- increased significantly when the breakfast drinks contained fructose.

In addition, the study suggested that when fructose is eaten with fat or before fat is consumed, the fat is more likely to be stored rather than burned, Parks says.

Parks explains that the liver tends to act like a traffic cop for glucose, determining whether glucose will be burned for energy or stored as fat. Fructose, on the other hand, seems to bypass the process.

"Fructose gets made into fat more quickly, and when that process is turned on there seems to be a signal that goes to the liver that says store all the other fats you are seeing," she says.

Fruit OK, Added Sugars Aren't

Although it would be almost impossible to avoid fructose without eliminating all sweet and sweetened foods from your diet, it is clear that not all foods containing fructose are equal, says nutritionist Lona Sandon, RD, who is with UTSW but did not work on the study.

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