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.
Fruit has fructose, but it also has fiber and nutrients, which makes it an important part of the diet, whether you are trying to lose weight or not, she says.
"The health benefits of eating fruit far outweigh the slight increase in fat production that might occur as the result of eating something with fructose in it," Sandon says.
She points out that the breakfast drinks served to the study participants had as much as 65 grams of fructose.
"By comparison, a cup of strawberries has 4 grams of fructose and an apple has about 11," she says.
And what about high-fructose corn syrup?
Audrae Erickson, president of the Corn Refiners Association, sees no need to worry.
"Like sugar, honey and some fruit juices, high fructose corn syrup contains almost equal portions of fructose and glucose. Glucose has been shown to have a tempering effect on specific metabolic effects of fructose," Erickson says in a statement.
"New research continues to confirm that high fructose corn syrup is no different from other sweeteners. It has the same number of calories as sugar and is handled similarly by the body."
But Sandon says there is some evidence that high fructose corn syrup breaks down differently in the body than other sugars.
She adds that people who want to lose weight should limit all added sugars, not just one kind.
"That's really a no-brainer," she says. "I've never had a client who has become overweight eating too much fruit or too many vegetables, but I have had plenty who ate too many foods with added sugar and fat."
Madelyn Fernstrom, PhD, CNS, agrees that demonizing one type of sugar misses the point.
"Everything in moderation," she says. "We are blaming individual sugars or individual fats when we should be focusing on calories. If someone drinks a 64-ounce soda, who cares if it is high-fructose corn syrup or cane sugar? It's still about 800 calories."
The study was partially funded by unrestricted research funds from the Sugar Association and the Cargill Higher Education Fund (Cargill is a manufacturer of high-fructose corn syrup).