The study comes after soft drinks manufacturers face growing pressure to put health warning labels on their products and remove vending machines from schools.
Researchers at the University of Cincinnati allowed mice to freely consume either water, fructose sweetened water or soft drinks. They found increased body fat in the mice that drank the fructose-sweetened water and soft drinks - despite that fact that these animals decreased the amount of calories they consumed from solid food.
This, said author Matthias Tschöp, MD, associate professor in UC's psychiatry department and a member of the Obesity Research Center at UC's Genome Research Institute, suggests that the total amount of calories consumed when fructose is added to diets may not be the only explanation for weight gain.
Instead, he said, consuming fructose appears to affect metabolic rate in a way that favors fat storage.
This could provide ammunition to campaigners demanding industry action to tackle the growing obesity crisis.
"Hardly any kids are getting enough calcium, vitamins, fiber, vegetables, or fruit," said Lucy Nolan, executive director of pressure group End Hunger Connecticut! "The more soda you drink, the less of those you get.
"If school systems spent half as much time trying to get more fruits and vegetables into schools as they did trying to keep soda contracts, our kids would be much better off."
Dr. Tschöp's lab used novel body composition analyzers that use magnetic resonance technology to carefully monitor body fat in mice.
"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."
Results from an earlier study in humans led by Peter Havel, DVM, PhD, an endocrinology researcher at the University of California, Davis, and coauthored by Dr. Tschöp, found that several hormones involved in the regulation of body weight, including leptin, insulin and ghrelin, do not respond to fructose as they do to other types of carbohydrates, such as glucose.
Based on that study and their new data, the researchers now also believe that another factor contributing to the increased fat storage is that the liver metabolizes fructose differently than it does other carbohydrates.
"Similar to dietary fat, fructose doesn't appear to fully trigger the hormonal systems involved in the long-term control of food intake and energy metabolism," said coauthor Dr. Havel.
The researchers say that further studies in humans are needed to determine if high-fructose corn syrup in soft drinks is directly responsible for the current increase in human obesity.
"Our study shows how fat mass increases as a direct consequence of soft drink consumption," said Dr. Tschöp.
Not everyone is pointing the figure of blame at the soft drink makers however. In Connecticut, governor Jodi Rell recently vetoed a nutrition bill that would have outlawed soft drinks and junk food in schools.
And ABA president Susan Neely claimed that asking the FDA to put warning labels on soft drinks, or any food products that contain caloric sweeteners, would be highly patronizing to consumers.
The research appears in the July 2005 issue of Obesity Research, the official journal of the North American Association for the Study of Obesity (NAASO).