October 23 (HealthDay News) -- Consuming too much fructose -- a sugar found in foods ranging from cookies to candies and soda -- can block the appetite-controlling hormone leptin from doing its job and increase the risk of obesity, a University of Florida study of rats suggests.

Leptin resistance has long been linked to obesity, and a number of studies have shown that overconsumption of fructose may be an important factor in the United States' obesity epidemic. This new study is the first to link fructose and leptin resistance.

The researchers found that rats became leptin-resistant after being fed a high-fructose diet for six months. There were no visible signs that leptin resistance was occurring, but the fructose-fed rats gained considerably more weight than rats that never received fructose when both groups of rats were switched to a high-fat diet.

The study was published in the American Journal of Physiology -- Regulatory, Integrative and Comparative Physiology.

"Leptin resistance is a condition that leads to obesity in rats when coupled with a high-fat diet. The surprising finding here was that increasing the amount of fructose in the diet without increasing the amount of calories led to leptin resistance and later exacerbated obesity when paired with a high-fat diet," study senior author Philip J. Scarpace, a professor of pharmacology and therapeutics in the College of Medicine, said in a university news release.

"[Fructose] blocks leptin action most likely by blocking leptin entry into the brain," study author Alexandra Shapiro, an assistant scientist in the department of pharmacology and therapeutics, explained in the news release.

"If these findings are applicable to humans, then there could be consequences of eating a diet high in fructose, but only if you also consume an excessive amount of calories," Scarpace said.

SOURCE: University of Florida, news release, October 2008