Researchers 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.
"Our study shows how fat mass increases as a direct consequence of soft drink consumption," said Dr. Tschöp.
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).
Consumption of sweetened foods and beverages containing sucrose and high-fructose corn syrup?particularly carbonated soft drinks and some juices and cereals--has been thought to be a leading cause of obesity. A widely used sweetener derived from corn, high-fructose corn syrup is similar to sucrose (table sugar) in its composition, about half glucose and half fructose.
Dr. Tschöp's lab used novel body composition analyzers that use magnetic resonance technology to carefully monitor body fat in mice.
All the mice began the study at an average weight of 39 grams. Those consuming the fructose-sweetened water showed significant weight gain over the course of the study, with an average final weight of 48 grams--compared with averages below 44 grams for the other groups--and had about 90 percent more body fat than the mice that consumed water only.
Total caloric intake was lower in the mice that consumed the fructose-sweetened water than in the other groups, except for the control animals provided with water only.
"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.
This study was conducted at both UC and the German Institute of Human Nutrition, in collaboration with the University of California, Davis.
Other coauthors were Hella Juergens, Wiltrud Haass, Annette Schürmann, Corinna Koebnick and Hans-Georg Joost, all of the German Institute of Human Nutrition, Potsdam-Rehbruecke; Tamara Castañeda, University of Cincinnati; Frank Dombrowski, Otto-von-Guericke-University, Magdeburg, Germany; Bärbel Otto, Innenstadt University Hospital, Munich; Andrea Nawrocki and Philipp Scherer, both of Albert Einstein College of Medicine, Bronx, New York; and Jochen Spranger and Michael Ristow, both of Charité University Medicine, Berlin.