A wedge of rich, dark chocolate cake iced in thick, sweet ganache beckons. Each bite melts onto the tongue, delivering a delightful rush and a feeling that all is well. Soon after the plate is empty, the mind wonders when the next piece may come along, inviting the question: Is sugar habit-forming?

In medical terms, a substance is addictive if it:

• induces a pleasant state or relieves distress,
• causes long-term chemical changes in the brain,
• leads to adaptive changes in the brain that trigger tolerance, physical dependence and uncontrollable cravings,
• causes dependence, so that abstaining is difficult and creates severe physical and mental reactions.

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Does sugar fit this profile?

Jan Ulbrecht, associate professor in biobehavioral health and medicine in the College of Health and Human Development, thinks not. "Since the human body does not become physically dependent on sugar the way it does on opiates like morphine and heroin, sugar is not addictive," he argues.

"Despite the anecdotal reports of people who claim to be addicted to sugar, and seemingly endless Web sites devoted to sugar addiction," says Cynthia Bartok, associate director for the Center for Childhood Obesity Research in Penn State's College of Health and Human Development, "modern science has not yet validated that idea."

"However, 'yet' may be the key word," Bartok adds. "It was once thought of as pseudoscience, but a whole field of research has sprung out of the idea that food components such as sugar or fat may have some similarities to addictive drugs."

Specifically, in analyzing how rats react to sugar consumption, scientists have found similarities to the response to drugs like heroin and cocaine When humans and rats eat sweets, their brain levels of dopamine—a neurotransmitter that regulates reward and is at the heart of many addictive behaviors—increases, notes Sue Grigson, an associate professor in neural and behavioral sciences at the Penn State College of Medicine.

A 2002 study at Princeton University found that rats deprived of food for 12 hours then given food and sugar-water tend to binge on the drink. This research has also shown that rats' brain systems change in response to sugar. When researchers blocked the brain's opioid system—which plays a role in the feeling of pleasure derived from a substance—the rats showed signs of withdrawal-like anxiety.

In yet another trial, rats given sugar-water after being without it for two weeks tended to drink more than ever before. All this behavior is consistent with the way rats—and humans—respond to other addictive substances.

"It appears that sweets under some circumstances can be addictive," concludes Grigson. More investigation is needed, she adds.

"The jury is still out," agrees Rebecca L. Corwin, an associate professor in nutritional sciences. Too much hype around sugar "addiction" might scare people away from sugar, which would be unnecessary, she adds. "Like all foods, sugar in moderation is perfectly okay."

In several of the noted rat studies, Corwin notes, the rats were given extremely large amounts of sugar, up to 46 percent of total caloric intake. Sugar makes up about 15 to 28 percent of total energy in the average American diet.

Corwin, who herself studies how rats react after bingeing on sugar and fat in order to better understand human eating disorders, also points out that sugar cravings are distinct from sugar addiction. What, then, explains those cravings?

For one thing, our brains depend on glucose to function properly, she says. Blood sugar at healthful levels feeds our metabolisms. "Our whole biology is geared towards the utilization of sugar as an energy source."

That doesn't mean living on candy bars is advisable, she adds, but a sweet treat now and then is not going to be habit-forming.

So the next time a piece of dark chocolate cake calls you by name, go ahead and indulge your craving, say the experts—as long as it's not an everyday occurrence.

Cynthia Bartok, Ph.D., is associate director for the Center for Childhood Obesity Research in Penn State's College of Health and Human Development. She can be contacted at cjb25@psu.edu. Rebecca L. Corwin, Ph.D., an associate professor in nutritional sciences, is at rxc13@psu.edu. Sue Grigson, Ph.D., is associate professor in neural and behavioral sciences at the Penn State College of Medicine and can be emailed at psg6@psu.edu. Jan Ulbrecht, MD, is associate professor in biobehavioral health and medicine in the College of Health and Human Development and co-director of the Penn State Diabetes Center. He can be reached at jsu1@psu.edu.