Prolonged fructose consumption may contribute to the development of metabolic syndrome, a combination of medical disorders that, when occurring together, increases the risk of developing cardiovascular disease and diabetes.
The findings, just published online in the journal Nutrition & Metabolism, are derived from a UC Davis study that investigated the relative effects of fructose or glucose consumption on 32 older, overweight or obese men and women who consumed glucose- or fructose-sweetened beverages, which provided 25 percent of their energy requirements for 10 weeks.
Consumption of fructose, but not glucose, impacted various parameters associated with metabolic syndrome, including increased circulating concentrations of uric acid, which is known to be higher in people with metabolic syndrome, increased GGT activity, which is an indicator of liver dysfunction, and production of a type of protein known as RBP-4, associated with increased insulin resistance. No previous studies have investigated the effects of glucose or fructose consumption on circulating levels of RBP-4.
The study design involved three phases, including a two-week inpatient baseline period, an eight-week outpatient intervention period, and a two-week inpatient intervention period.
During baseline, subjects resided in the UC Davis Clinical and Translational Science Center's Clinical Research Center (CCRC) for two weeks before beginning the outpatient intervention, consuming either fructose- or glucose-sweetened beverages. They returned to the CCRC for the final two weeks of intervention.
Fasting and 24-hour blood collections were performed at baseline and following 10 weeks of intervention for measurement of plasma concentrations of uric acid, RBP-4 and liver enzyme activities.
The study's first results, published in 2009, showed that visceral adipose volume (fat inside the abdominal cavity) was significantly increased only in subjects consuming fructose, along with increases in several circulating lipids and a decrease in insulin sensitivity, although both groups exhibited similar weight gain.
Senior author Peter Havel, a UC Davis professor with joint appointments in the Department of Molecular Biosciences in the School of Veterinary Medicine and the Department of Nutrition, is currently the principal investigator for a follow-up study comparing impacts of glucose, fructose and high-fructose corn syrup in younger patients. His colleague and collaborator, Kimber Stanhope, directed and coordinated the clinical research study.
Glucose and fructose are both simple sugars, and equal parts of each is the recipe for table sugar (sucrose). The pure glucose and fructose that were used to sweeten the beverages in this study are not found in nature. Most fruits and honey contain comparable amounts of glucose, fructose and sucrose. Grains such as wheat, oats, corn and barley contain large amounts of glucose (and negligible amounts of fructose), but the glucose is packaged as long chains that are called starch or complex carbohydrate.
Co-author Lars Berglund, director of the UC Davis Clinical and Translational Science Center, points out that fruit juices typically feature more concentrated forms of these sugars, while fruits and vegetable contain fiber and other beneficial components.
"It's healthier to eat apples than to drink apple juice," said Berglund.
Other authors include former graduate students Chad Cox, James Graham, Bonnie Hatcher, Steven Griffen and John McGahan, all of UC Davis; Jean Marc Schwarz of Touro University, Vallejo, Calif.; Andrew Bremer of Vanderbilt University, Nashville; and Nancy Keim of UC Davis and the U.S. Dept. of Agriculture, Davis, Calif.
This research was supported with funding from National Institutes of Health (RO1 HL-075675), the NIH's National Center for Research Resources (UL1 RR024146), and the NIH Roadmap for Medical Research. Havel's laboratory receives support from the NIH (HL-091333, DK-097307, and DK-095980) and a Multicampus Award (#142691) from the University of California, Office of the President, and Keim's research is supported by intramural U.S. Department of Agriculture-Agricultural Research Service (ARS) Current Research Information System (CRIS) grant 5306-51530-016-00D.