People reluctant to give up smoking for fear they will get fat can take comfort from a report showing they will probably put on less than 10 extra pounds.

"From a health point of view, the weight gain is pretty moderate," said Dr. David F. Williamson, who directed the study. "Even this much weight gain is far healthier than continuing to smoke," he said."The last thing we want is to have people conclude from this article that this is a reason to continue to smoke," commented Dr. Robert Klesges of Memphis State University, who has done research on smoking and weight gain.

Smoking substantially increases the risk of lung cancer, heart attacks and emphysema. It is the single biggest preventable cause of death in the United States, killing 400,000 people annually.

The study, published in the New England Journal of Medicine, shows that the after-smoking weight gain is even steeper than some earlier studies estimated.

Quitters gained weight faster than smokers for at least seven years after they quit. By the end of the study, those who kicked the habit had returned to the weight of people who never smoked, but did not weigh more than they did.

"If you quit smoking, you won't look like the fattest person on the block," said Williamson. "You will look like somebody who has never smoked and is healthier."

Williamson, an epidemiologist at the Centers for Disease Control in Atlanta, based his conclusions on two surveys of 9,004 people conducted in the early 1970s and 1980s.

Among the findings:

- An average of five years after they stopped smoking, men put on six more pounds than those who continued to smoke, and women put on eight more pounds.

- Ten percent of the men and 13 percent of the women had major weight gains - about 30 pounds or more.

Williamson said that once people shake the grip of cigarettes, they can begin to concentrate on taking off the extra weight and keeping it off.

"If someone is heroic enough to quit smoking, then they can use that decision as a springboard to change two other aspects of their lives - diet and physical activity," he said. "Try walking 45 minutes every day and eating lots of fruits and vegetables."

Klesges said nicotine gum and phenylpropanolamine, a common ingredient of over-the-counter decongestants and diet pills, are being studied as ways to help quitters keep off the extra weight.

The 1990 surgeon general's report on smoking concludes that the average weight gain attributable to giving up smoking is four pounds.