If new research about fast food proves true, avid Big Mac and Whopper eaters aren't just junk food (search) lovers -- they're junkies.

A new study involving lab rats has found that foods high in fat, salt and sugar might be physically addictive -- activating the same areas of the brain that respond when certain drugs are used.

"The combination of fat with sugar or fat with salt seems to have a very particular neurochemical effect on the brain," Ann Kelley, a professor at the University of Wisconsin (search) who co-authored the unpublished study, said on the Fox News Channel. "What that does is release certain chemicals that are similar to drugs, like heroin and morphine."

But some nutrition experts are skeptical of the claims.

"I've never seen anything that convinces me people get addicted to certain foods," said Ruth Kava, director of nutrition at the American Council on Science and Health (search). "Certainly if your blood sugar is really low, you're going to have physiological symptoms, but I don't know that it counts as an addiction."

The Wisconsin research is complete, but hasn't yet been published. However, early results have been widely circulated beginning with references to it in New Scientist magazine and in a BBC documentary Big Mac Under Attack, which aired this month.

During the study, Kelley and co-author Matthew Will gave the rats a diet high in fat, sugar and salt. They found that the brain's pleasure chemicals were activated when the rodents ate the foods, and the animals had what the researchers likened to withdrawal symptoms when the fatty foods were taken away.

"Just that taste of the fat will immediately release these substances into the brain and actually make us have an emotional response to the food," Kelley said.

Other studies conducted at Princeton University by Dr. Bartley G. Hoebel and colleagues, one of whom appeared in the June 2002 issue of Obesity Research, examined sugar addiction. The Obesity  study found that "repeated, excessive intake of sugar" in lab rats "caused behavioral and neurochemical signs of opioid withdrawal," a state which was "similar to withdrawal from morphine or nicotine."

Hoebel said the Wisconsin research is not evidence that fast food and fat are addictive, and emphasized that the study was done on rats, not people. That point is also the basis for others' criticism of the Wisconsin study.

"Can you take one animal study and extrapolate to humans? I don't think so," Kava said. "One study doesn't prove anything."

Self-proclaimed fast food addict Jim Smith, 41, of Indiana -- who is obese and had to cut back on his favorite Wendy's cheeseburgers and other junk food after he nearly died of congestive heart failure -- has no problem believing the implication of the Wisconsin research. Smith said he used to eat two to three hamburgers a day, but now consumes at most one or two a week.

"It's hard. I miss it," Smith said. "I've often wondered if it was addictive. I can't just have one -- I have to have two. Same with french fries."

But a spokesman from the National Restaurant Association (search) said that just because chemicals are released into the brain when certain foods are seen or tasted doesn't mean the body is addicted to them.

"To leap from finding trace amounts of chemicals in the brain to 'that is addiction' is a big river that we don't have a bridge across yet," said Steven Grover, the association's vice president of health and safety regulatory affairs. "An addiction is a much stronger compulsion."

Grover acknowledged that compulsive over-eating is a real problem -- but said it's a psychological, behavioral issue, not one of physical addiction.

In any case, it's likely that the Wisconsin study implying fast food is addictive will be the basis of a number of obesity lawsuits against the fast food industry.

Lawyer John Banzhaf III, famous for his battles against the tobacco industry, has already used some of the Wisconsin study as a foundation for suits he's threatening to bring against six fast-food giants.

He wants McDonald's, Burger King, Wendy's, Taco Bell, Pizza Hut and Kentucky Fried Chicken to post signs in their restaurant windows warning customers that research has shown the consumption of fatty, salty, sugary foods causes symptoms in animals associated with addiction.

Kelley's hope is that her study will lead to programs and medications for those who have eating compulsion problems -- especially with fast food.

"The over-consumption of this food leads to problems like obesity," she said. "Maybe we can develop treatments that will help us curb our appetite for these substances."