Cigarette smoking is providing a window into the study of other chronic habits -- one that could provide new clues to alcoholism and drug addiction.

How is it, for example, that some smokers are able to smoke just a few cigarettes regularly but never really get hooked? Why is it that when smokers quit, they crave sweet and starchy foods, a phenomenon that helps account for the average seven pounds that most smokers gain after giving up the habit? And why do certain situations more than others seem to condition people to crave a cigarette?

The answers to these and other questions about addiction are emerging from several laboratories throughout the country.

At the University of Pittsburgh, for example, Dr. Saul Shiffman, a clinical psychologist, studies chippers -- people who are able to smoke just a few cigarettes regularly but never become pack-a-day smokers.

Shiffman, who presented his findings last week at the American Psychological Association annual meeting in New York, estimates that about 4 to 5 percent of the 55 million Americans who smoke are chippers.

Unlike the pack-a-day smoker, chippers smoke no more than five cigarettes a day and usually smoke only about four days a week. Chippers are not smokers who have tried to quit and slipped back to their old habits but are people who can be satisfied smoking just a few cigarettes on a regular basis. Rather than smoking because they feel pressured, tense or are experiencing withdrawal from nicotine, chippers usually smoke because they are happy or are enjoying themselves, for instance at a party.

The fact that some smokers can remain chippers baffles smoking researchers. "We know how addicting tobacco is," Shiffman says. "These people shouldn't exist according to the theory {of smoking and nicotine,} and yet they do. It's very weird."

New research on chippers and heavy smokers is helping to clear up part of the mystery. For example, in a study of 18 chippers and 29 smokers, Shiffman found that chippers are far less likely to experience the strong withdrawal symptoms that heavy smokers feel when they go without a cigarette. In the study, both groups had to abstain from cigarettes overnight before coming into the laboratory for testing.

"Chippers came in calm and stayed calm {after not smoking overnight}," Shiffman said. "Smokers came in pretty uncomfortable. They were upset. They smoked a cigarette and then they got calm."

Chippers were also less likely than heavy smokers to have relatives who smoked. If a chipper's family member smoked, the odds were, Shiffman said, that the relative had successfully given up smoking.

Heavy smokers and chippers also differed on early smoking experience. Contrary to what Shiffman expected, chippers described their first cigarette as a fairly uneventful experience. "They reported experiencing fewer {bad} symptoms {of smoking} the first time they tried a cigarette than heavy smokers did," he said.

By comparison, heavy smokers reported that their first cigarette was an unpleasant experience, marked by such symptoms as nausea, headache, dizziness and coughing.

"These findings surprised me," Shiffman says. But the results are consistent with a study from Britain that found that nonsmokers who tried cigarettes as adolescents reported feeling the least number of unpleasant symptoms when compared to both chippers and heavy smokers.Defining the biological factors that seem to prevent chippers from becoming heavy smokers is a key goal of smoking research.

"If we could understand what it is that protects chippers from addiction, that might give us a big clue about what causes dependence in the first place and maybe how we could protect others," Shiffman says.

Finding the answer to cigarette chippers, Shiffman says, would be the equivalent of finding someone who is immune to a virus. "It might give us a big clue about what causes addiction in the first place, and how to prevent it," he says.

Additional clues to biological dynamics of addiction are also emerging from studies of how nicotine exerts its effects on the body.

At the Uniformed Services University of Health Sciences in Bethesda, Dr. Neil Grunberg, a health psychologist and addiction researcher, has found that nicotine alters the desire for sweet-tasting food and for carbohydrates.

Give up cigarette smoking, and the body craves sweet-tasting food. Evidence from animal studies suggests that nicotine acts through two separate metabolic regulating systems in the body. To affect the desire for sweet-tasting food, Grunberg said, nicotine seems to alter levels of the hormone insulin. (Insulin controls blood levels of the sugar glucose. As insulin levels rise, glucose is stored in cells throughout the body. When insulin levels drop, the cells release glucose into the blood for energy.) Nicotine tends to lower insulin levels. Studies also show that when nicotine is stopped, insulin levels rise, leading Grunberg and his colleagues to propose that the craving for sweets among recent ex-smokers may be triggered by this insulin-nicotine connection.

To affect the craving for carbohydrates or starchy foods -- Grunberg believes that nicotine may alter levels of serotonin, a substance that helps regulate mood. When serotonin levels plummet, so does mood. Raise serotonin and a person feels better -- less depressed. To test how nicotine may affect serotonin levels, Grunberg and his colleagues are embarking on additional studies.

Since serotonin and insulin pathways in the body are closely tied to appetite, Grunberg believes that nicotine, and perhaps other addictive drugs, "may actually affect similar systems in the body as food does."

"Think about it," Grunberg said. "When people feel hungry, they will search for food in much the same way that a smoker who just smoked the last cigarette in a pack will search for a cigarette."

Just as serotonin and nicotine blood levels seem to change for a smoker, they also are altered for the person who is hungry. But once a hungry individual finds food -- just as once the smoker can light up again -- the craving to eat goes away and insulin and serotonin levels return to normal.

By tapping into the body's system that controls hunger, Grunberg thinks that it may be possible to explain why nicotine -- and perhaps other addictive drugs as well -- are able to exert a such powerful hold on the body.

And although this theory still needs more research, Grunberg said that many alcohol and drug abuse programs already recognize the changes in appetite that often strike people attempting to kick their habits and advise their participants to eat sweet foods -- doughnuts, cookies and candy -- to help soften the craving for another drink or a fix.

In addition to biological factors, the setting where alcoholics, smokers and drug abusers engage in their habits may also play an important role in addiction.

At the National Institute of Drug Abuse Addiction Research Center in Baltimore, Dr. Jack Henningfield studies cocaine addicts in the laboratory. In this experimental setting, the addicts are allowed to give themselves cocaine, but to do so, they have an intravenous tube placed in their arms and must press an elaborate series of switches and buttons in the same pattern to have the drug administered.

Henningfield has found that the addicts give themselves a regular and constant dose.

But the habitual aspect of how people gave themselves the drug in this experimental setting also seems to be a factor. When Henningfield replaced the cocaine with saline -- a mixture of water and salt -- the addicts still pushed all the same buttons and flipped the switches to have their "fix" -- even though cocaine was no longer available.

These findings suggest, Henningfield says, that the place where a person engages in a habit is important in continuing the addiction. That is, the bar where an alcoholic has a drink, the coffee shop where a smoker enjoys a cigarette or being with the friends with whom a cocaine addict snorts a few lines all can trigger -- or condition -- the craving for the addictive substance.

"I think that this new research says that we have got to focus not only on the substance {being abused} but also on how different people respond to their particular addictive substance," says the University of Pittsburgh's Shiffman.